Titanium Alloys: An Atlas of Structures and Fracture Features [1st edition] 9780849350108, 0849350107

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TITANIUM ALLOYS An Atlas of Structures and Fracture Features

TITANIUM ALLOYS An Atlas of Structures and Fracture Features

Vydehi Arun Joshi

Boca Raton London New York

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.

Published in 2006 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2006 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-10: 0-8493-5010-7 (Hardcover) International Standard Book Number-13: 978-0-8493-5010-8 (Hardcover) Library of Congress Card Number 2005022915 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Joshi, Vydehi Arun. Titanium alloys : an atlas of structures and fracture features / Vydehi Arun Joshi. p. cm. Includes bibliographical references and index. ISBN 0-8493-5010-7 (9780849350108) 1. Titanium alloys--Fracture. I. Title. TA480.T54J67 2006 620.1'893226--dc22

2005022915

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com Taylor & Francis Group is the Academic Division of T&F Informa plc.

and the CRC Press Web site at http://www.crcpress.com

Dedication To my husband Arun S. Joshi; my brother Dr. N. Narakanti Rao; and my sisters Shanta, Saroja, and Neeraja.

Foreword Titanium and its alloys find application in aerospace, the chemical and power industries, transportation, armament, and sports, but it is primarily the former that has driven the development of this material. This wonderful book documents the fractography of a set of aeronautical-grade titanium alloys that failed in the laboratory under a wide variety of controlled testing conditions. The compositions span the range of different classes of titanium alloys — alpha, alpha/beta, and beta — as well as the titanium aluminides that have been researched at the Defence Metallurgical Research Laboratory (DMRL) in India over the years or that have been produced at Mishra Dhatu Nigam Limited (Midhani). The book, therefore, covers the breadth and the range of titanium alloys that are used in India, as well as internationally. However, the book is also more than a simple fractographic handbook, since it documents the underlying microstructure and test conditions that produced the fracture, together with a brief description of the special fractographic features associated with each combination of alloy, microstructure, and test condition. It is unique in that there is no document in the literature today that captures the fractographic features of a wide range of titanium alloys together with the conditions

that produced failure. It is, therefore, a book that will be used by researchers as well as those engaged in failure analysis of titanium components in the aeronautical industry. The author combines wide experience in the use of scanning electron microscopy in fracture analysis with an eye for detail that has led to several award-winning micrographs and fractographs over the years. The book, therefore, combines scientific detail with aesthetic appeal. As someone who has been involved in developing the understanding of physical metallurgy of titanium and its alloys, I am very happy to see the publication of this edition. It is important that the database that constitutes this book be updated periodically as new alloys and applications of titanium emerge. I am sure that the DMRL and Midhani will be at the forefront of these efforts in India, and I would love to see a Web edition of the book that reaches a wide audience and that could be updated in a regular manner. Dipankar Banerjee, Ph.D. Chief Controller Research & Development (AMS) Defence Research & Development Organisation New Delhi, India

The Author Vydehi Arun Joshi graduated from Osmania University in 1967 and joined the Defence Metallurgical Research Laboratory (DMRL) in the same year. She has been working at DMRL for the past 38 years. She completed her postgraduate studies and holds an M.Tech. degree from the Institute of Technology, Banaras Hindu University, Varanasi. After a decade of service in the Optical Metallography Group, she switched to the field of transmission and scanning electron

microscopy, where she still works as a scientist. She has wide experience in microstructural characterization and fractography of different metals and alloys (titanium, nickelbased superalloys, aluminum, steels, and so forth) pertaining to various DMRL projects. She has also won numerous prizes in the metallography contests that are held annually by the Indian Institute of Metals, and she has several national and international publications to her credit.

Acknowledgments Over the past few decades, I have been carrying out the scanning electron microscopy of different metals and alloys as a part of my regular work at the Defence Metallurgical Research Laboratory (DMRL), Hyderabad, India. During this period I have had occasion to interact with many scientists who have specialized in different areas of metallurgy. The foremost among them is Dr. A.K. Gogia, scientist G, project director of the DMRL Project Office (Materials), Kanchanbagh, Hyderabad, with whom I have had constant interaction while carrying out the scanning electron microscopy of titanium alloys. In fact, the idea of taking up the compilation work for this atlas was initiated at his suggestion. I am exceedingly grateful to him for the continuous technical support and advice that he has provided me ever since, particularly in the interpretation of the fractographs. Although Dr. Gogia has not formally coauthored this book, he has virtually fulfilled that role. He has also permitted me to make abundant use of his doctoral thesis and other publications, and I am very happy to acknowledge his contribution. In this context, I thank the Journal of Metallurgical and Materials Transactions and the authors — A.K. Gogia, D. Banerjee, and T.K. Nandy — of the paper entitled “Structure, tensile deformation and fracture of Ti3Al-Nb alloy” for according me permission to use the SEM micrographs. It was only after taking up the actual work of compilation that I realized the enormous magnitude of the activities involved and the difficulties in executing the task. I was extremely fortunate in getting the support of my colleague Dr. K. Satyaprasad in this arduous stage of the project. His constant support and presence while scanning and storing the entire set of photographs, titles, and captions and then rendering them computer compatible for publication, not only lightened my burden but also made this tedious work pleasant and enjoyable. When a sufficient number of fractographs and microstructures were ready, they were bound together for our personal in-house use. Dr. D. Banerjee, distinguished scientist and chief controller of Research & Development, AMS, New Delhi, made me realize that this reference work could be exhibited in a larger and wider gallery, and could serve a more worthwhile purpose, if descriptive text were appended to each photograph and the resulting work then brought out in book form. I am

extremely grateful to Dr. Banerjee for this suggestion and for his subsequent interest and follow-up. I particularly wish to thank him for readily agreeing to write the foreword to this book. I am grateful to Dr. A.M. Sriramamurthy, director, Defence Metallurgical Research Laboratory, Hyderabad, for granting me permission to publish this book and also for allowing me to use the laboratory infrastructure and facilities. I am extremely thankful to the team that carried out the failure investigation of a high-pressure compressor blade, namely, Dr. A.K. Gogia, Dr. K. Muraleedharan, Dr. D. Banerjee, and the Metallography and Electron Probe Micro Analysis Groups of DMRL and Mishra Dhatu Nigam Limited (Midhani), Hyderabad, for the case study reported in Chapter 9 of this book. I am also thankful to Dr. A. Venugopal Reddy, regional director, Regional Centre for Military Airworthiness (Materials), Hyderabad, for sharing his publishing experiences with me. I have benefited greatly from his advice, and I vividly recall my long association with him and remember the bygone days when he inducted me into the science of fractography. A work of this nature could never have been possible without the active cooperation of scientists and other executives who are working on titanium alloys and in allied fields. I wish to thank Mr. S.N. Jha and Dr. T.V.L. Narasimha Rao of the Aeronautical Material Testing Laboratory, Hyderabad & Midhani, Hyderabad, for providing fractured test specimens. Likewise, I am thankful to Dr. K. Muraleedharan, Dr. T.K. Nandy, T. Raghu, Amit Bhattacharjee, G.S. Sharma, A.G. Paradkar, Dr. A.K. Singh, Dr. P.K. Sagar, H. Mishra, and S.M. Gupta for providing me with fractographs and specimens for scanning electron microscopy. I am also indebted to Dr. P. Ghosal, Dr. R. Balamuralikrishnan, and D.V. Sridhara Rao for helping me prepare the disk book. Thanks are also due to the Metallography Group for the support given. Lastly, I wish to thank my husband, Arun S. Joshi, for his patience while I worked late hours, after which he drove me safely home in one piece, despite the sustained efforts of the local city drivers to test his braking skills. A thrilling narration of our narrow escapades might well be the subject of my next book!

Preface Titanium is a newcomer among the metals that have gained widespread industrial importance. Alloys of titanium have found a niche market even in the aerospace sector, where material requirements are very demanding. The reason for this proliferation of applications is its excellent blend of low density and high strength, superior corrosion resistance, and strength at moderately high temperatures. The properties of any material depend upon its microstructure, which in turn is defined by its composition and processing history. Hence, in the alloy development stage, it becomes mandatory to test the material and study the microstructure and fracture features of the tested specimens. During this development, a comprehensive compilation of micrographs of different alloys of one metal can help in understanding the related experimental work. This atlas is intended to fulfill this need for titanium alloys by serving as a ready reference source of detailed fractographic and microstructural analyses. Chapter 1 provides an introduction to fractography, with typical fractographs of ductile, fatigue, intergranular, and cleavage fractures in general and of titanium alloys in particular. Chapter 2 covers the physical metallurgy of titanium alloys and the evolution of their microstructures, while Chapter 3 presents the compositions of some commercially used titanium alloys. Chapters 4 through 8 are

compilations of more than 300 photographic illustrations, with accompanying descriptions, of the microstructures and fracture features of α-, α+β-, and β-titanium alloys and Ti3Al- and TiAl-based titanium aluminides that were tested under various conditions. The concluding chapter (Chapter 9) of this atlas deals with the case study of a failed titanium blade. A CD with the included images accompanies this atlas. Thus, this compilation — arguably the largest collection of microstructures and fractographs of titanium alloys ever assembled within a single book — provides exhaustive information for engineers and researchers working in these areas. This atlas is an outgrowth of the alloy development work carried out by the Titanium Alloy Group and the Electron Microscopy Group in the Defence Metallurgical Research Laboratory, Hyderabad, India. I would like to thank all of those who have helped in the making of this book. I hope that this combined effort will help in understanding the rich insights into the microstructure and fracture features of titanium alloys that have been gained by optical and scanning electron microscopic observations. Vydehi Arun Joshi Hyderabad, India

Abbreviations AC bcc BSE EPMA FC fcc hcp OQ ppm

Air cooled Body-centered cubic Backscattered electron Electron-probe microanalysis Furnace cooled Face-centered cubic Hexagonal close packed Oil quenched Parts per million

RT SE SEM ST STA TEM WQ

Room temperature Secondary electron Scanning electron microscope Solution treated Solution treated and aged Transmission electron microscope Water quenched

Contents Chapter 1 1.1 1.2 1.3 1.4

Dimple Rupture .........................................................................................................................................................1 Cleavage.....................................................................................................................................................................1 Fatigue........................................................................................................................................................................2 Intergranular...............................................................................................................................................................3

Chapter 2 2.1 2.2 2.3 2.4

2.5

Introduction to Fractography.........................................................................................................................1

Physical Metallurgy of Titanium Alloys .......................................................................................................7

Introduction................................................................................................................................................................7 Application of Titanium Alloys.................................................................................................................................7 Effect of Alloying Elements ......................................................................................................................................9 Types of Titanium Alloys ..........................................................................................................................................9 2.4.1 Alpha (α) Alloys..........................................................................................................................................10 2.4.2 Near α Alloys ..............................................................................................................................................10 2.4.3 α+β Alloys...................................................................................................................................................10 2.4.4 Metastable β Alloys.....................................................................................................................................10 2.4.5 Beta Alloys...................................................................................................................................................10 2.4.6 Titanium Aluminides ...................................................................................................................................10 The Microstructure of Titanium Alloys ..................................................................................................................10 2.5.1 Conventional Titanium Alloys.....................................................................................................................11 2.5.2 Titanium Aluminides ...................................................................................................................................15

Chapter 3

Chemical Compositions...............................................................................................................................17

Chapter 4

Alpha Alloys ................................................................................................................................................19

Chapter 5

Near-Alpha Alloys .......................................................................................................................................23

Chapter 6

Alpha + Beta Alloys ....................................................................................................................................59

Chapter 7

Beta Alloys...................................................................................................................................................97

Chapter 8

Titanium Aluminides .................................................................................................................................111

8.1 8.2

Ti3Al-Based Alloys ................................................................................................................................................111 TiAl-Based Alloys .................................................................................................................................................111

Chapter 9 9.1 9.2

9.3

Case Study: Failure Investigation Report of IMI 550 High-Pressure Compressor (HPC-I) Aero Engine Blade ....................................................................................................................................203

Introduction............................................................................................................................................................203 Investigation...........................................................................................................................................................203 9.2.1 Chemical Analysis .....................................................................................................................................203 9.2.2 Microstructure............................................................................................................................................203 9.2.3 Fractography ..............................................................................................................................................203 9.2.4 Stress-Concentration Effects of a Notch...................................................................................................204 9.2.5 Analysis of the Deposits............................................................................................................................205 Conclusion .............................................................................................................................................................205

References.......................................................................................................................................................................219 Index ...............................................................................................................................................................................221

1 Introduction to Fractography Materials fracture either by transgranular (through grains) or intergranular (along grain boundaries) fracture paths. There are basically four principal modes of fracture: 1. 2. 3. 4.

Dimple rupture Cleavage Fatigue Intergranular

The detailed features of the above modes of fracture are given below.

1.1 DIMPLE RUPTURE Most of the structural alloys fail by a mechanism known as microvoid coalescence when fractured under continually increasing loads. Microvoids nucleate at the interfaces between matrix and inclusions, second-phase particles, grain boundaries, or imperfections such as microcracks and microporosity. As the load increases, microvoids grow and coalesce and eventually fracture. This mode of fracture is called dimple rupture. The shape and depth of the dimples or microvoids can be related to the size of and spacing between initiating particles, to the applied stress (tension, shear, or torsion), and to the fracture toughness of the specimen. When nucleation sites are few and widely spaced, the microvoids grow to large dimples. Small dimples are formed when numerous nucleation sites are activated and individual microvoid growth is limited. Some very ductile materials have deep conical dimples. The increase in free surface resulting from microvoid nucleation can be great. Because the growth of free surface occurs by plastic deformation, strain markings are occasionally evident on the walls of some large dimples. These markings include serpentine glide, ripples, and stretching. A typical ductile fracture of titanium alloy is shown in Figure 1.1.

1.2 CLEAVAGE This type of fracture occurs on well-defined crystallographic planes and is a low-energy fracture. Generally metals with body-centered cubic (bcc) and hexagonal close-packed (hcp) crystal structure only fracture by

cleavage mechanism. However, even metals with facecentered cubic (fcc) crystal structure like Al also have been observed to cleave on contact with mercury. In brass, cleavage by stress corrosion cracking is observed. Cleavage may not indicate the relative ductility of the material. It describes only the fracture mechanism. A cleaved fracture surface shows features like cleavage steps, river markings, feather markings, herringbone structures, and tongues, since the materials are polycrystalline and contain imperfections, precipitates, inclusions, etc. Flat featureless cleavage surfaces are very rarely seen. River marking is one of the main cleavage features and is usually observed within a grain. This is a step between a cleavage crack-segment on the cleavage planes. The branches of the river pattern join in the crack-propagation direction and can thus be used to find the fracture direction. A typical cleavage fracture in titanium alloy is shown in Figure 1.2. Feather markings resemble a chevron pattern in that they point back in the direction of local crack. They are an array of very fine cleavage steps on a cleavage facet. Herringbone structure forms as a result of the interaction of an advancing cleavage crack with deformation twins. This is generally seen in bcc materials. Tongues are formed by the local deviation of a cleavage plane crack as it intersects a boundary between a deformation twin and the matrix. Wallner lines are occasionally seen on brittle phases. They are parallel cleavage steps creating a rippled pattern. They cross each other and are different from fatigue striations, which do not cross each other. They are believed to result from the interaction of a simultaneously propagating crack front and an elastic shock wave in the material. Quasicleavage is a mechanism involving a mixture of both microvoid coalescence and cleavage. In quasicleavage, there is no apparent boundary between a cleavage facet and a dimpled area bordering the facet. This mode of fracture is common in high-strength materials. 1

2

Titanium Alloys: An Atlas of Structures and Fracture Features

10 μm FIGURE 1.1 Typical ductile fracture in a Ti alloy showing fine equiaxed dimples.

1.3 FATIGUE This type of fracture occurs by damage from cyclic stresses, i.e., fatigue. The crack growth due to fatigue leaves clear fractographic evidence known as fatigue striations. Each fatigue striation has been shown to be the result of a single stress cycle. Stage 1 is the initial stage of fatigue fracture and is attributed to slip-plane fracture from repeated reversing of the operative slip system. Fatigue striations are not generally seen during the first stage. Striations are formed during stage 2 fatigue cracking. Fatigue fracture in a titanium alloy is shown in Figure 1.3.

High-cycle fatigue generally has closely spaced, welldefined fatigue striations. In low-cycle fatigue, striations appear to be broad and widely spaced and often discontinuous. Large second-phase particles and inclusions can change the local crack growth rate and resulting fatiguestriation spacing. A fatigue crack approaching a particle can briefly retard if the particle remains intact or accelerate if the particle cleaves. Small particles have little effect on the striation spacing or crack growth.

3

Introduction to Fractography

100 μm FIGURE 1.2 Typical transcrystalline cleavage fracture in titanium aluminide.

Tire cracks are fracture features associated with highstress, low-cycle fatigue. These are seen on steep slopes of the fracture surface. Tire cracks are caused either by mechanical damage to the fracture caused by repeated impact and the relative motion of two mating surfaces or by loose particles caught between mating surfaces. They are not fatigue striations, but they indicate fatigue. Fatigue striations usually bow outward in the direction of local crack propagation. Fatigue striations are best seen on the fracture surface of moderately hard alloys.

1.4 INTERGRANULAR As the name implies, intergranular fracture occurs by grain-boundary separation, i.e., between grains. Intergranular fracture is very clearly distinguishable from other types of fracture. The causes for this type of fracture are the presence of weak or brittle grain boundary phases as well as environmental or mechanical factors, such as stress corrosion, hydrogen damage, or a triaxial state of stress. Elevated-temperature creep-to-rupture fractures are often intergranular.

4

Titanium Alloys: An Atlas of Structures and Fracture Features

1 μm FIGURE 1.3 Typical fatigue fracture showing striations.

Sometimes a small layer of microvoid coalescence is seen at the grain interfaces. Some of these fracture features have a “rock candy” appearance. A typical intercrystalline fracture in a titanium alloy is shown in Figure 1.4. The failure of a component in service is not always due to one type of fracture mode; it could be the result of

a mixed-mode fracture, i.e., the operation of two or more intermingled mechanisms of fracture. The shape, size, cross section of the component, and the conditions prevailing during failure have an effect on the fracture features and the mode of fracture.

5

Introduction to Fractography

1 μm FIGURE 1.4 Typical intercrystalline fracture in a Ti alloy.

Metallurgy 2 Physical of Titanium Alloys 2.1 INTRODUCTION Titanium metal was first discovered by the English chemist William Gregor in 1971 in the black magnetic sand ilmenite, and the metal was named “titanium” after the titans of Greek mythology, a symbol of power and strength [1]. Titanium is the fourth-most-abundant metal in the Earth’s crust, the other three being aluminum, iron, and magnesium. Titanium has low density and high strength, good corrosion and erosion resistance to different media, good oxidation resistance, and moderate strength at high temperatures, making it attractive for industrial applications. It has a number of features that distinguish it from other light metals and that make its physical metallurgy both complex and interesting. The yield strength, fracture toughness, and creep properties of titanium alloys can be increased tremendously. These alloys can also be tailored to achieve a desired combination of properties by changing the alloying and processing parameters. A change in the alloy composition and processing modifies the microstructure. This change is due to the phase transformation of various equilibrium and nonequilibrium phases present in the alloy system. The resultant mechanical properties depend on the nature of deformation, fracture of the microstructural constituents, and the interaction between constituents. The physical and electronic properties of the titanium atom, because of its position in the periodic table, make it suitable for alloying with other elements to produce a wide range of alloys. Titanium has allotropic phase transformation from high-temperature β phase with body-centered cubic structure to the room-temperature α phase having a closely packed hexagonal crystal structure. There is a strong dependence of the transformation temperature on the alloy composition, and a variety of phase transformations are possible. All these allow a wide variety of microstructures, which can be optimized by controlling the thermomechanical processing.

2.2 APPLICATION OF TITANIUM ALLOYS Titanium and its alloys are used for aerospace, chemical, general engineering, and biomedical applications because they show an astonishing range of mechanical properties

(Figure 2.1). The unique high strength-to-weight ratio, easy formability, and fatigue resistance led to the introduction of titanium in aerospace applications like rocket engine parts, fuel tank, gas bottles, etc. It is also used in the airframe structures, such as landing-gear beams, hydraulic tubings, wing boxes, spacers, bolts, etc. Titanium alloys are used in fan-jet engines for which large front fans are required. The high specific strength of titanium along with the metallurgical stability at high temperatures and low creep rates make it favorable for jet engine components like blades and discs in the low and intermediate sections of compressors. The next important area of application of titanium alloys is chemical and general engineering. The outstanding corrosion resistance of titanium in many environments is the prime reason for its use in these industries. For low-stress applications, commercially pure (CP) titanium is generally used, and for high-strength applications Ti-6Al-4V or Ti-13Nb-13Zr alloys are used. In the petrochemical industries, CP titanium grades and Ta- or Pd-containing alloys are utilized for outstanding corrosion resistance. Titanium alloys are used in marine and offshore applications for their excellent corrosion resistance in seawater and in sour hydrocarbon atmospheres. Figure 2.2 illustrates typical applications of titanium alloys. In the field of biomedical applications, titanium is used for prosthetic devices for bone and joint implants, heart valves, and dental implants. These are made from CP titanium, Ti-6Al-4V, or recently developed alloys such as Ti-6Al-7Nb. In the automobile sector, titanium engine valves have been used by Toyota in Japan. Titanium products like springs are also used in racing cars and motorcycles. A more recent application of titanium is in architecture, a concept first used in Japan. The Guggenheim Museum in Bilbao, Spain, is the most spectacular titanium building. Besides these applications, titanium is also used in sports equipment, such as spikes for running shoes used by sprinters, tennis rackets, mountain crampons, ice axes, bicycle frames, etc. Titanium is also finding increasing use in jewelry and fashion industries. 7

8

Titanium Alloys: An Atlas of Structures and Fracture Features

1400 Ti 6 24

1200

6 IM

I5

800

A Ti

79

6-2

-4 I 55 0

IM

Ti3-2

lA y llo

600

Al-2

.5 S

n

s

0.2 % Y.S. MPa

1000

Ti 2 15 BET AI II Ti 102-3 Ti6-4 IMI 6 IMI 85 829 Ti-S

A Ti lA y llo

400

s

CP

Ti

200

20

40 60 Fracture toughness, KICMPa√m

80

100

FIGURE 2.1 Range of yield strength and toughness in titanium alloys at room temperature.

Characteristics of Titanium Alloys

Corrosion resistance

Low-density high-strength easy formability

Fatigue resistance

Creep strength Oxidation resistance Microstructural stability

Chemical and Process Industry - Heat exchangers - Reaction vessles - Tanks, pumps - Valves, tubes - Anodes for electrolysis cells - Consumer goods and jewelry

Marine Applications - Submarine hulls - Propellers, pumps - Deep drilling pipes Biomedical Applications - Bone and joint implants - Heart valves - Dental implants

Airframe Structures - Landing gear beams - Hydraulic tubings, wing boxes - Spacers, bolts, etc. Automotive Applications - Springs, fasteners, piston valves - Rocket engine parts - Fuel tanks, gas bottles

Applications of Titanium Alloys

FIGURE 2.2 General characteristics and typical applications of titanium alloys.

Jet Engines - Fan discs and blades - Compressor discs and blades - Casings, after burner cowlings - Flange rings, spacers, bolts, etc.

9

Physical Metallurgy of Titanium Alloys

TABLE 2.1 Physical Properties of Unalloyed Titanium Property

Value

Atomic number Atomic weight Crystal structure: α-hcp

22 47.9

β-bcc Density Compressibility Coeff. of thermal expansion at 20˚C Thermal conductivity Specific heat α to β transus Latent heat of transformation Heat of fusion Melting point Heat of vaporization Boiling point Electrical resistivity: High purity Commercial purity Modulus of elasticity

c = 4.6832 ± 0.0004 Å a = 2.9504 ± 0.0004 Å c/a = 1.5873 a = 3.28 ± 0.003 Å 4.54 g/cm3 0.8 × 10−6 cm2/kg 8.4 × 10−6 cm/cm/K 0.041 Cal/cm/s/K 0.125 Cal/K/g 882˚C (1155.5K) 1050 Cal/mole 5020 Cal/mole 1668˚C (1941K) 112,500 Cal/mole 3260˚C (3533K) 42 µΩ-cm 55 µΩ-cm 11.6 × 1011 dyne/cm2

2.3 EFFECT OF ALLOYING ELEMENTS Titanium is one of the transition metals and has an atomic number of 22 and atomic weight of 47.90. Table 2.1 summarizes the important physical properties [2]. Titanium exists in two allotropic modifications, a high-temperature β that is stable between 882˚C and its melting point of 1668˚C. The α modification of titanium exists at temperatures below 882˚C. Titanium has an incomplete shell in its electronic structure, which enables the formation of solid solutions with most substitutional elements having a size factor within ±20%. Elements like carbon, oxygen, etc. form interstitials. The stabilization of α or β phase depends on the number of electrons per atom of the alloying element (or the group number). Alloying elements with an electron/atom (e/a) ratio of less than 4 stabilize the α phase, and elements having a ratio greater than 4 stabilize the β phase. Elements with an e/a ratio of 4 are neutral [3]. Kornilov [4] classified the elements in the periodic chart into four major groups, depending on their interaction with titanium. 1. Continuous solid-solution-forming elements with α or β titanium: Zirconium and hafnium have an outer-shell electronic configuration identical to that of titanium. The structure is also isomorphic to titanium. Thus the phase diagrams with these elements show continuous

α and β solid solutions. Vanadium, niobium, tantalum, and molybdenum are isomorphic to β-titanium and form a continuous solid solution with the β allotrope of titanium. These elements have limited solubility in α phase. 2. Limited solid-solution-forming elements with α and β titanium: Chromium, manganese, iron, cobalt, nickel, and copper undergo eutectoid transformation and lower the β transus. With the increase in the group number, the maximum solubility in β titanium decreases and eutectoid temperature increases. Aluminum, gallium, and indium show a peritectoid reaction and raise the β transus. These elements have higher solubility in α titanium. 3. Ionic and covalent compound-forming elements: Fluorine, chlorine, bromine, iodine, sulfur, selenium, tellurium, and phosphorous form ionic and covalent compounds with titanium. They do not go into solid solution in α or β titanium. 4. Elements not interacting with titanium: Except beryllium, which has limited solubility in β titanium, no other alkali or alkaline earth metal interacts with titanium. Boron, carbon, oxygen, nitrogen, and hydrogen form interstitial solid solutions because of the large size difference between the atoms of titanium and these elements. There is a difference in solubility of these elements in α and β titanium. Hydrogen is more soluble in β phase and reacts eutectoidally.

2.4 TYPES OF TITANIUM ALLOYS Titanium has two allotropic modifications: α, which has a closely packed hexagonal structure, and β, having a body-centered cubic structure. Various elements forming solid solution with titanium are classified on the basis of their effect on the solubility of α or β phases. Elements stabilizing α phase are known as α stabilizers (Al, Ga, O, N, C), and elements stabilizing β phase are known as β stabilizers (V, Mo, Nb, Fe, Cr, Ni, etc.). Some of the elements like Sn and Zr are neutral, as they do not stabilize either α or β phase, though they enter into solid solution with titanium. Aluminum is the only α stabilizer of commercial importance and forms a constituent of most of the commercial titanium alloys. The aluminum content is normally restricted to 7% or aluminum equivalent to 9% in the commercial titanium alloys to avoid precipitation of Ti3Al phase, which leads to severe embrittlement. Aluminum equivalent (Rosenberg criterion [5]) = Al + Sn/3 + Zr/6 + 10(O + C + N) (2.1)

10

Recent developments in Ti alloys exploit the hightemperature properties of intermetallics Ti3Al (α2) and TiAl (γ). Molybdenum, vanadium, niobium, and tantalum (isomorphous with titanium), which are the β-stabilizing elements, are generally the preferred alloying additions in commercial Ti alloys. Iron and chromium are also added in limited amounts, although they are eutectoid-forming β stabilizers. Based on the alloying additions and phases present in the microstructures, Ti alloys are classified as follows.

2.4.1 ALPHA (α) ALLOYS These are single-phase alloys, solid solution strengthened by the addition of α stabilizers or neutral alloying elements. Alpha alloys have good stability and good hightemperature properties but are not amenable to heat treatment for microstructural property modifications.

2.4.2 NEAR α ALLOYS Small additions (1 to 2%) of β stabilizers improve the strength and workability and are a good compromise between the higher strength of α+β alloys and the creep resistance of simple α alloys. The most widely used commercial high-temperature Ti alloys for aero-engine application belong to this class. They are primarily α alloys containing some amount of retained β in the final microstructure.

2.4.3 α+β ALLOYS These alloys contain larger amounts of beta stabilizers (4 to 6%). Beta alloys can be heat-treated to develop a variety of microstructures and mechanical property combinations. Ti-6Al-4V, the most widely used alloy, belongs to this class.

2.4.4 METASTABLE β ALLOYS In alloys containing 10 to 15% of β stabilizers, β phase is retained at room temperature in a metastable condition. This metastable β can be aged to form very fine Widmanstätten α in the matrix of enriched β. These alloys have high strength, toughness, excellent hardenability, and forgeability over a wide range of temperatures. These alloys contain small amounts of α-stabilizing elements as strengthening agents. Beta and near- β alloys offer increased fracture toughness over α+β alloys at a given strength level. They are finding increasing use in aircraft structural applications.

2.4.5 BETA ALLOYS Very large addition (30%) of β stabilizers results in retaining β as a stable phase at room temperature. Beta alloys

Titanium Alloys: An Atlas of Structures and Fracture Features

resemble refractory metals in their high densities and poor ductility and are used for highly specialized burn-resistance and corrosion-resistance applications.

2.4.6 TITANIUM ALUMINIDES A number of attractive intermetallic alloys have been developed with useful ductility and strength. These alloys are based on the intermetallic compounds Ti3Al (α2) and TiAl (γ). The alloys based on Ti3Al usually contain a large amount of Nb additions and other β stabilizers (10–25 at .%, 20–45 wt.%). They consist of β phase or B2 phase to improve their ductility. TiAl is also alloyed with β stabilizers such as Nb, V, Mn, and Cr in limited amounts. Recently, alloys based on compound Ti2AlNb (O-phase) are also under evaluation. All these alloys possess excellent high-temperature properties and good oxidation resistance but have poor ductility and fracture toughness. This has limited their use in commercial applications. Commercial titanium alloys use a very narrow composition range of α stabilizers (as dictated by the Rosenberg criterion in Equation 2.1). This has been a restraining factor in the development of titanium alloys. The situation may change if the alloys based on Ti3Al and TiAl find commercial application. The range of compositions available in titanium alloys may then expand substantially.

2.5 THE MICROSTRUCTURE OF TITANIUM ALLOYS Titanium alloys can exhibit a wide variety of microstructures depending upon alloy chemistry, processing, and heat treatment. This is made possible because titanium and its alloys exhibit a broad range of phase transformations. Some of these transformations are related to the α to β allotropic transformations, while others are precipitation reactions that involve the formation of metastable transition phases and equilibrium that occur during the decomposition of the metastable α or β phases. The symbols and definitions of various phases are listed in Table 2.2 [2]. The latter transformation usually occurs in more highly alloyed situations, and the resulting microstructures are very complex in such alloys. The phase transformations and resultant microstructures in alloys based on ordered intermetallics Ti3Al (α2), Ti2AlNb (O), and TiAl (γ), while being similar to conventional titanium alloys in many aspects, are still more complex. In addition to the microstructural variations evolving through various phase transformations, morphological changes of the constituent phases due to other metallurgical processes such as recrystallization, spheroidization, etc. can lead to many more types of structural variations. Some of the major microstructural variations usually generated in titanium alloys are presented in this section.

11

Physical Metallurgy of Titanium Alloys

β

TABLE 2.2 Phases Observed in Titanium Alloys Phase Symbol

Description

α

Low-temperature allotropic form of titanium with an hcp structure; exists below β transus High-temperature allotropic form with bcc structure; exists at low temperature as metastable or stable phase in highly enriched alloys Ti3Al; exists over a wide range of Al content; has an ordered hexagonal structure DO19 Ordered bcc phase with CsCl structure; Ti2AlNb ordered version of high-temperature bcc allotrope; can exist at low temperature as a metastable phase Ti2AlNb with orthorhombic structure; can exist over a broad range of Al and Nb content TiAl with L10 structure; extends over a wide range of Al content Nonequilibrium phase due to martensitic transformation; hcp structure Martensite with orthorhombic structure A high-pressure allotrope of titanium with hexagonal structure; occurs as a transition phase Several intermetallic precipitates can occur, depending upon the alloy (TiZr)5Si3; Ti2Cu are prominent precipitates bcc phases of different composition than the matrix; occurs as a result of phase separation in β-stabilized alloys

β

α2 B2/β2

O γ α′ α″ ω Intermetallic precipitates

Temperature

B1/β′

β MS α ωS β stabilisers

FIGURE 2.3 Pseudo binary schematic phase diagram of α+β titanium alloys.

2.5.1 CONVENTIONAL TITANIUM ALLOYS A central point in the evolution of microstructures in titanium alloys is the α→β transformation temperature, generally referred to as the β transus temperature, since it separates the single-phase β field from the two-phase α+β

Temperature

β transus

α+β

Martensite Time

FIGURE 2.4 TTT curve of a typical α+β titanium alloy.

field. A schematic pseudo binary phase diagram and a time, temperature, and transformation (TTT) diagram [6] for titanium alloy are shown in Figure 2.3 and Figure 2.4, respectively, to illustrate the fact. Titanium alloys, when heat-treated above the β transus (specific temperature is a function of the alloy chemistry), are in single-phase β. On cooling through the β transition temperature, β can transform to various equilibrium or nonequilibrium phases, depending upon the cooling rate and alloying content. On faster cooling (like water quenching or oil quenching), the β phase can transform martensitically (Figure 2.5) to α′ (hcp) or α″ (orthorhombic); with increasing β stabilizing element, there is an increasing tendency to form α″ in preference to α′. On slower cooling, β can transform by nucleation and growth to Widmanstätten α phase (Figure 2.6). The morphology of Widmanstätten α phase may change from a colony of similarly aligned α laths to a basket-weave arrangement with an increase in cooling rate or alloying content. Moreover, lamellar structure becomes finer as the cooling rate is increased. On slower cooling, α phase is also present on prior β grain boundaries. In addition to the transformation products (α, α′, α″), the microstructure may retain small amounts of β phase, depending upon the alloying content. The amount of retained β in the microstructure on cooling from β goes on increasing as the solute content is increased. The α′ or α″ martensite decomposes upon subsequent aging to precipitate fine β, which leads to useful increments in strength. In some alloys, additional intermetallic phases such as silicides may form upon aging of martensites. In highly β stabilized alloys, the β phase may be retained completely as a metastable phase on fast cooling. However, on slow cooling, the α phase can precipitate out at the grain boundaries and within the grain, the amount of α being a function of cooling rate and β-stabilizer content. In some compositions, athermal ω may form in

12

Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 2.5 Ti-6Al-4V, β heat-treated at 1020˚C/20min/WQ. Optical micrograph shows martensitic structure with prior β boundaries.

FIGURE 2.6 Ti-6Al-4V, β heat-treated at 1020˚C/20min/FC. Optical micrograph shows Widmanstätten α structure with α phase present on prior β grain boundaries.

Physical Metallurgy of Titanium Alloys

13

FIGURE 2.7 Ti-6Al-4V, α+β heat-treated at 960˚C/1h/WQ. Optical micrograph shows equiaxed α and transformed β microstructure.

the β phase during quenching. The athermal ω phase forms as very fine precipitates (2 to 5 nm). The metastable β phase decomposes upon subsequent aging to precipitate fine α phase. The aging leads to a significant increase in strength, while ductility registers a decline. However, strength and ductility combination in these alloys can be optimized by selecting the appropriate combination of aging temperature and time. Two other decomposition reactions in metastable β may occur at low temperatures: ω formation in lean β alloys and a phaseseparation reaction ω phase → β1+β2 in richer alloys. The formation of ω phase is considered undesirable because its presence can cause severe embrittlement of the alloy concerned and should be avoided by controlling the aging condition. β-phase-separation reaction has not received much attention because it is not considered in commercial alloys. Both ω phase reaction and β separation reaction may affect morphology and distribution of α phase in some alloys, since α phase may form indirectly from either the ω or β1 phases. The microstructure resulting from the solution treatment above the β transus and transformation of β phase are generally referred as “transformed β” or β heat-treated structure, irrespective of the finer details of the microstructure. In addition to the α-phase morphologies that result from martensitic transformation or nucleation and growth of the α phase (generally termed as secondary α), thermomechanical processing at temperatures in the two-phase α+β region has an important effect on α-phase morphology. Hot working below the β transus (in α+β-phase field) results in

recrystallization of α phase to equiaxed morphology (referred to as primary α). The aspect ratio of primary α phase is determined by temperature, strain rate, and extent of hot working in the two-phase region. Solution heat treatment of α+β-worked alloys permits control over the final duplex microstructure. The relative volume fraction of primary α and transformed β can be controlled by solution-treatment temperature in the two-phase field and cooling rate from the solution-treated temperature. The effect of cooling rate on the microstructure from a given solution-treatment temperature is shown in Figure 2.7, Figure 2.8, and Figure 2.9. The β phase present at the solutiontreated temperature undergoes transformation to α/α′/α″, depending upon the cooling rate and β-phase chemistry, as described earlier. These types of microstructures are commonly known as α+β structures or equiaxed α + transformed-β structures. The α+β structures exhibit much finer β grain size than β heat-treated structures. Due to anomalously high diffusion rate in the β phase, solution-treated times for β heat treatments are generally very short. In α+β heat treatments, β grain growth is restricted by the presence of second phase (α) at the solution-treatment temperatures. Apart from the distribution of α and β phases in the microstructures as discussed above, there are other structural features that occur on a much finer scale. Precipitation of α2 (Ti3Al) in the α phase of some alloys after prolonged thermal exposure and precipitation of other intermetallics, such as Ti2Cu (in Ti-Cu alloy) and silicides, are examples of fine-scale microstructural features. Precipitation

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Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 2.8 Ti-6Al-4V, α+β heat-treated at 960˚C/1h/AC. Optical micrograph shows equiaxed α and transformed β microstructure.

FIGURE 2.9 Ti-6Al-4V, α+β heat-treated at 960˚C/1h/FC. Optical micrograph shows equiaxed α and transformed β microstructure. The volume fraction of α increases with a decrease in cooling rate, and transformed β becomes coarser.

Physical Metallurgy of Titanium Alloys

of α phase in fact imposes an upper limit on α stabilizer content in commercial titanium alloys, as described earlier.

2.5.2 TITANIUM ALUMINIDES The microstructural evolution in titanium aluminide alloys exhibits startling similarities to conventional titanium alloys. Apart from the fact that α and β undergo ordering transformations to α2 (Ti3Al)/O (Ti2AlNb) and B2 structures, the morphological changes on transformation from β or B2 phases in Ti3Al/Ti2AlNb-based alloys to α2/O phases are very similar to those observed in β → α transformation in conventional titanium alloys. The Ti3Al/ Ti2AlNb-based alloys can also be processed and heattreated below the β/B2 transus temperature to achieve equiaxed α2/O + transformed β (B2) structures. Also the arrangement of α2/O laths, on cooling from the β/B2 heattreatment temperature, changes from basket-weave to colony structure as in the conventional titanium alloys. However, much more complex microstructures, especially in the finer scale, can be generated in these alloys due to the retention of B2 phase upon quenching and subsequent decomposition to α2/O laths in a variety of transformations and α2 → O transformations. The alloys based on TiAl (γ) consist of α2 and γ phases as alternate lamellae in the microstructure. Similar to other titanium alloys, the morphology of the γ phase can be modified to equiaxed shape by thermal and/or thermomechanical processing, and a mixture of equiaxed γ + lamellae (α2+γ) can be achieved in the microstructure. Hot working at temperatures below the α transus generally

15

results in a fine-grained microstructure. Postworking heat treatment in a single-phase α field results in fully lamellar structures, while heat treatment in the two-phase α+γ field results in a mixture of equiaxed and lamellar γ structure. Therefore, the α transus temperature is of particular importance in these alloys and has the same significance as the β transus temperature in conventional titanium alloys. The various microstructures have a strong influence on the deformation and fracture behavior and consequently on the mechanical properties of titanium alloys. Finer microstructural features lead to increased strength and ductility. They also retard crack nucleation. Coarse microstructures, on the other hand, are more resistant to creep and fatigue-crack growth. Equiaxed structures in general exhibit high ductility and high fatigue strength, while lamellar structures possess high fracture strength and show superior resistance to creep and fatigue-crack growth. Bimodal structures combine the advantages of lamellar and equiaxed structures and show a balanced profile of properties. These general observations regarding the structure–property relationship not only apply to conventional titanium alloys, but also hold true for titanium aluminide alloys. The influence of microstructure on fracture features in titanium alloys will be easily perceived as you glance through the fractographs in this atlas. The fractographs presented are for different titanium alloys, including titanium aluminides in a variety of microstructural conditions.

3 Chemical Compositions TABLE 3.1 Typical Composition of Titanium Alloys (At. %) Alloy

Al

Nb

V

Mo

Ta

Zr

Sn

Si

Mn

Cr

Fe

Other Elements

Ti

α Alloys IMI260 IMI317

— 5.0

— —

— —

— —

— —

— —

— 2.5

— —

— —

— —

— —

0.2 Pd —

Bal. Bal.

Near-α Alloys Ti-811 Ti-6242 IMI679 TIMETAL685 TIMETAL834

8.0 6.0 2.25 6.0 6.0

— — — — 0.7

1.0 — — — —

1.0 2.0 1.0 0.5 0.5

— — — — —

— 4.0 5.0 5.0 3.5

— 2.0 11.0 — 4.0

— — — 0.25 0.35

— — — — —

— — — — —

— — — — —

— — — — 0.06 C

Bal. Bal. Bal. Bal. Bal.

α+β Alloys IMI318 Ti-662 IMI550 IMI680 Ti-6246

6.0 6.0 4.0 2.25 6.0

— — — — —

4.0 6.0 — — —

— — 4.0 4.0 6.0

— — — — —

— — — — 4.0

— 2.0 2.0 11.0 2.0

— — 0.5 0.2 —

— — — — —

–– — — — —

— 0.7 — — —

— — — — —

Bal. Bal. Bal. Bal. Bal.

Metastable β Alloys Timet LCB Ti-10-2-3 BetaIII TIMETAL 21S

1.5 3.0 — 3.0

— — — 2.6

— 10.0 — —

6.8 — 11.5 15.0

— — — —

— — 6.0 —

— — 4.5 —

— — — 0.2

— — — —

— — — —

4.5 2.0 — —

— — — —

Bal. Bal. Bal. Bal.

— — —

— — —

35.0 — —

— 40.0 30.0

— — —

— — —

— — —

— — —

— — —

15.0 — —

— — —

— — —

Bal. Bal. Bal.

24.0 25.0 25.0 25.0

10.0 8.0 10.0 17.0

— — 3.0 —

— 2.0 1.0 1.0

— 2.0 — —

— — — —

— — — —

— — — —

— — — —

— — — —

— — — —

— — — —

Bal. Bal. Bal. Bal.

Ti2AlNb (Orthorhombic) 22.0 22.0

27.0 24.0

— —

— —

— —

— —

— —

— —

— —

— —

— —

— —

Bal. Bal.

— 2.0 4.0 4.0 2.0

1.0 — 2.0 — —

— — — 1.0 —

— — — — —

— — — — —

— — — — —

— — — — —

— — — — 2.0

— 2.0 — — —

— — — — —

— — — — —

Bal. Bal. Bal. Bal. Bal.

β Alloys Alloy C

Ti3Al (α2) Alloys Near α2 α2+O

TiAl (γ) Alloys 48.0 48.0 48.0 48.0 48.0

17

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Titanium Alloys: An Atlas of Structures and Fracture Features

TABLE 3.2 Chemical Composition of Alloys in This Book (wt.%) Alloy Commercial (ASTM-2) OT4-1 IMI 685 IMI 834 Ti-64 VT9 Ti-10-2-3 β-Ti alloy Ti-24Al-11Nb Ti-24Al-15Nb Ti-24Al-20Nb Ti-24Al-11Nb-4Ta Ti-27Al-14Nb-1Mo Ti-25Al-15Nb Ti-24Al-27Nb Ti-47Al-2Nb-2Cr Ti-48Al-4Nb-1Mo

Al

V

Mo

Nb

Ta

Si

Zr

Sn

Cu

Fe

C

Mn

Cr

Ti

— 1.5 6.0 5.5 6.0 6.5 3.0 1.65 13.5 13.0 12.5 12.2 14.8 13.6 11.8 32.5 32.3

— — — — 4.0 — 10.0 9.8 — — — — — — — — —

— — 0.5 0.5 — 3.2 — — — — — — 2.0 — — — 2.4

— — — 1.0 — — — — 21.3 28.0 36.0 19.2 26.5 28.2 45.6 5.0 9.3

— — — — — — — — — — — 13.6 — — — — —

— 0.15 0.25 0.35 — 0.25 — — — — — — — — — — —

— 0.3 5.0 4.0 — 1.8 — — — — — — — — — — —

— — — 4.0 0.1 — — — — — — — — — — — —

— — — — 0.1 0.05 — — — — — — — — — — —

0.3 0.3 — 0.5 0.3 0.06 2.0 4.95 — — — — — — — — —

0.1 — — 0.6 0.08 0.01 — 0.03 — — — — — — — — —

— 1.2 — — — — — — — — — — — — — — —

— — — — — — — — — — — — — — — 2.5 —

Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal. Bal.

4 Alpha Alloys Alpha titanium alloys are single-phased alloys. Alpha stabilizers like aluminum and oxygen stabilize the α phase. Tin and zirconium are neutral elements and have solid solubility in both the α and β phases; they also strengthen the α phase, along with aluminum and oxygen. Alpha alloys have high stability and good high-temperature properties, but they cannot be heat-treated for modification of microstructure for improving their properties.

Commercially pure (CP) titanium and Ti-5Al-2.5Sn are the most important alloys of this type. CP titanium (ASTM grades 1–4) is usually hot rolled, forged, and heattreated in the single α-phase field. Typical processing temperature for CP titanium is 800˚C. Typical heat treatment is 675˚C/1h/AC. Alpha alloys are mainly used in the chemical and process-engineering industries, where corrosion resistance and deformability are the main concern. Microstructure and fractographs of CP titanium (ASTM grade 2) are presented in this section.

FIGURE 4.1 Commercial titanium, 675°C/1h/AC tensile tested at room temperature. Optical micrograph shows equiaxed α grains. 19

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Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 4.2 Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. Low-magnification scanning electron microscope (SEM) fractograph shows the fracture surface.

FIGURE 4.3 Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph of Figure 4.2 shows overload fracture with fine dimples.

Alpha Alloys

21

FIGURE 4.4 Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. SEM fractograph of a different area of Figure 4.2 shows overload fracture with fine dimples and a few conical dimples.

FIGURE 4.5 Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph of the conical dimples shows serpentine glide (stretch-like regions) surrounded by fine dimples.

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Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 4.6 Commercial titanium, 675˚C/1h/AC, tensile tested at room temperature. High-magnification SEM fractograph of a different area shows the conical dimple.

5 Near-Alpha Alloys These alloys contain 1 to 2 wt.% of β stabilizers, which are added to improve their strength and workability. The α phase is predominant in these alloys, which are a good compromise between high-strength α+β alloys and creepresistant α alloys. OT4-1 (Ti-1.5Al-1.2Mn-0.15Si-0.3Zr), a Russian alloy, is normally used in α+β treated condition. This alloy is primarily used in structural components for applications up to 300˚C. IMI685 (Ti-6Al-5Zr-0.5Mo-0.3Si) is another alloy that is aimed at higher-temperature applications in jet engines (up to 520˚C). This alloy is mainly used in the β heat-treated condition. The alloy is usually processed in the β- or high in the α+β-phase field. This chapter presents the microstructure and fractography of heat-treated alloy IMI685, tensile tested at room temperature and at 520˚C. The fracture features of the room-temperature-tested specimen shows predominantly cleavage and dimples at the colony and lath boundaries, whereas the specimen tested at 520˚C shows ductile fracture features with dimples. IMI834 (Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si), the most advanced alloy of this class, can be used up to

550˚C. SEM fractographs of this alloy are also included in this chapter. This alloy is typically heat-treated high in the α+β-phase field to achieve 5 to 10% primary α, and it offers a good combination of fatigue and creep resistance. The solution-treatment temperature that determines the primary α volume fraction has a strong influence on the properties. A study of IMI834 alloy, heat-treated at different temperatures and creep-tested at 220 MPa and 650˚C, shows the variation of dimple size with the increase in the heat-treatment temperatures. The creep life increased from 22 h to 220 h when the solution-treatment temperature was increased from 970˚C (70% α) to 1080˚C (0% α). This can be attributed to the increase in the volume fraction of transformed β phase, consisting of acicular α, with the increase in heat-treatment temperature. Fractography of the sample heat-treated at 1080°C/2h/AC + 700°C/2h/AC clearly shows coarse β grains at low magnification, while ductile dimples are seen at higher magnification. A small area showing cleavage facets was also observed at the center of the specimen.

23

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Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 5.1 Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC. Optical micrograph shows fine α grains with small amounts of β at the grain boundaries.

FIGURE 5.2 Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC, sheet specimen, tensile tested at room temperature. Low-magnification scanning electron microscope (SEM) fractograph shows the general fracture appearance.

Near-Alpha Alloys

25

FIGURE 5.3 Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC, sheet specimen, tensile tested at room temperature. SEM fractograph shows overload fracture features with fine dimples.

FIGURE 5.4 Ti-1.5Al-1.2Mn-0.15Si-0.3Zr, 650˚C/0.5h/AC, sheet specimen, tensile tested at room temperature. High-magnification SEM fractograph shows fine equiaxed dimples and a few conical dimples with serpentine glide (arrows).

26

Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 5.5 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC. Optical micrograph shows transformed β structure with prior β grain boundaries.

FIGURE 5.6 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. Macrograph shows rough-faceted fracture features.

Near-Alpha Alloys

27

FIGURE 5.7 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. SEM fractograph shows transcrystalline fracture features and tear ridges (arrow marked).

FIGURE 5.8 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. SEM fractograph shows fracture along the laths. Fine dimples and microvoids (arrows) are also seen in between the laths. This could be due to the fracture of the thin layer of the β phase in between the α laths.

28

Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 5.9 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows transcystalline cracks along the colony boundaries. Dimples and tear ridges are also seen.

FIGURE 5.10 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. Optical micrograph of the grip of the tensile-tested specimen shows transformed β structure with prior β grain boundaries.

Near-Alpha Alloys

29

FIGURE 5.11 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. Macrograph shows ductile features. High-magnification SEM fractographs of regions A and B are shown in Figure 5.13 through Figure 5.16.

FIGURE 5.12 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. High-magnification SEM fractograph of Figure 5.11 shows ductile dimples and deep secondary cracks. A few coarse shallow dimples are also seen in the center of the fractograph.

30

Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 5.13 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. SEM fractograph of region A in Figure 5.11 shows dimples of different sizes.

FIGURE 5.14 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. High-magnification SEM fractograph of region A shows large shallow dimples and voids.

Near-Alpha Alloys

31

FIGURE 5.15 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. SEM fractograph of region B in Figure 5.11 shows mixed-size dimples and tear ridges.

FIGURE 5.16 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, tensile tested at 520˚C. High-magnification SEM fractograph of region B shows large shallow dimples and voids. The dimple size is coarser as compared with that of region A.

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Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 5.17 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC. Optical micrograph shows acicular α within prior β grains.

FIGURE 5.18 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. Lowmagnification SEM fractograph shows the general fracture appearance.

Near-Alpha Alloys

33

FIGURE 5.19 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. SEM fractograph shows the origin of the fracture.

FIGURE 5.20 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. Highmagnification SEM fractograph shows fatigue striations.

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Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 5.21 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. Lowmagnification SEM fractograph shows final overload fracture with dimples.

FIGURE 5.22 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 550˚C/24h/AC, HCF tested at 310˚C at a stress of 784 MPa. Highmagnification SEM fractograph of Figure 5.21 shows final overload fracture with dimples and tear ridges.

Near-Alpha Alloys

35

FIGURE 5.23 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC + creep tested at 520˚C for 100 h followed by tensile testing. Optical micrograph of the grip of the tested sample shows Widmanstätten α within prior β grains.

FIGURE 5.24 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing. SEM fractograph shows rough fracture and prior β boundaries.

36

Titanium Alloys: An Atlas of Structures and Fracture Features

FIGURE 5.25 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing. SEM fractograph shows transgranular fracture along the colonies.

FIGURE 5.26 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing. High-magnification SEM fractograph shows cleavage facets and fine dimples at the lath boundaries. Tear ridges are also seen.

Near-Alpha Alloys

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FIGURE 5.27 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing. High-magnification SEM fractograph of another region of Figure 5.24 shows cleavage facets. Fine dimples at the lath boundaries are due to the fracture of the β phase.

FIGURE 5.28 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing. SEM fractograph of a different region of the fracture surface shows the effect of Widmanstätten microstructure (Figure 5.23) on the fracture.

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FIGURE 5.29 Ti-6Al-5Zr-0.5Mo-0.25Si, 1050˚C/1h/OQ + 600˚C/24h/AC, creep tested at 520˚C for 100 h followed by tensile testing. High-magnification SEM fractograph of Figure 5.28 shows microvoids along the lath and colony boundaries.

FIGURE 5.30 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/AC + 700˚C/2h/AC. Optical micrograph shows primary α and transformed β.

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FIGURE 5.31 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, tensile tested at room temperature. SEM fractograph shows rough fracture surface.

FIGURE 5.32 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, tensile tested at room temperature. Lowmagnification SEM fractograph shows transcrystalline fracture features. Prior β boundaries are delineated.

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FIGURE 5.33 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, tensile tested at room temperature. Highmagnification SEM fractograph shows cleavage features and dimples. Tear ridges are also seen (arrow).

FIGURE 5.34 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, notch tensile tested. SEM fractograph shows smooth fracture surface.

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FIGURE 5.35 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, notch tensile tested. SEM fractograph shows cleavage fracture features, dimples, and secondary cracks. The effect of the microstructure can be seen on the fracture.

FIGURE 5.36 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, notch tensile tested. High-magnification SEM fractograph shows cleavage facets with river pattern. Dimples surrounding the cleavage facets are also seen.

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FIGURE 5.37 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at a stress of 475 MPa. SEM macrograph shows the fracture surface.

FIGURE 5.38 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at a stress of 475 MPa. High-magnification SEM of Figure 5.37 shows origin.

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FIGURE 5.39 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at a stress of 475 MPa. SEM fractograph shows inclusion at the origin at higher magnification. Cleavage-like fracture features are also seen.

FIGURE 5.40 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at a stress of 475 MPa. SEM fractograph shows fatigue striations and secondary cracks at higher magnification. Fissures at the roots of fatigue striation are also seen.

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FIGURE 5.41 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, low-cycle fatigue tested at 550˚C at a stress of 475 MPa. SEM fractograph shows fatigue striations in a different region away from the origin. Secondary cracks are also seen.

FIGURE 5.42 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of 550 MPa. SEM macrograph shows smooth fracture surface. Origin is indicated by arrow.

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FIGURE 5.43 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of 550 MPa. Low-magnification SEM fractograph of the origin shows secondary cracks and cleavage facets.

FIGURE 5.44 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of 550 MPa. High-magnification SEM fractograph shows the origin and secondary cracks with cleavage facets.

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FIGURE 5.45 SEM fractograph of Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at RT a stress of 550 MPa. High-magnification view of a different region showing patches of fatigue striations separated by tear ridges (A). Cleavage facets (B) are also seen at places. Grains are favorably-oriented to the stress axis fracture by cleavage and those oriented to relax the load by cyclic relaxation fracture by fatigue.

FIGURE 5.46 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of 550 MPa. SEM fractograph of a different area shows cleavage facets and dimples.

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FIGURE 5.47 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, high-cycle fatigue tested at a stress of 550 MPa. SEM fractograph shows fatigue striations and fine dimples.

FIGURE 5.48 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. SEM macrograph shows rough fracture surface.

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FIGURE 5.49 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. Low-magnification SEM fractograph shows fine dimples with voids and secondary cracks.

FIGURE 5.50 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. SEM fractograph shows a mixture of fine and coarse dimples and large voids. These could be due to the α-phase pullout.

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FIGURE 5.51 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1030˚C/2h/OQ + 700˚C/2h/AC, stress ruptured. High-magnification SEM fractograph shows coarse, deep dimples surrounded by fine dimples.

FIGURE 5.52 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 970˚C/2h/AC + 700˚C/2h/AC. Optical micrograph shows primary α and transformed β.

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FIGURE 5.53 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1000˚C/2h/AC + 700˚C/2h/AC. Optical micrograph shows primary α and transformed β.

FIGURE 5.54 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1045˚C/2h/AC + 700˚C/2h/AC. Optical micrograph shows primary α and transformed β. The percentage of primary α decreased with increasing solution treatment temperature.

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FIGURE 5.55 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/AC + 700˚C/2h/AC. Optical micrograph shows fully transformed β microstructure.

FIGURE 5.56 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 970˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 22-h life. SEM macrograph shows general fracture appearance.

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FIGURE 5.57 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 970˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 22-h life. SEM fractograph shows ductile fracture features with voids and secondary cracks.

FIGURE 5.58 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 970˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 22-h life. High-magnification SEM fractograph shows dimples and voids. Tear ridges are also seen.

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FIGURE 5.59 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1000˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 32.5-h life. SEM macrograph shows rough fracture features.

FIGURE 5.60 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1000˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 32.5-h life. SEM fractograph shows ductile fracture features with voids and secondary cracks.

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FIGURE 5.61 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1000˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 32.5-h life. High-magnification SEM fractograph shows dimples and large voids. Tear ridges are also seen.

FIGURE 5.62 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1045˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 54-h life. SEM macrograph shows rough fracture features.

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FIGURE 5.63 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1045˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 54-h life. SEM fractograph shows ductile fracture features with voids and secondary cracks.

FIGURE 5.64 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1045˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 54-h life. High-magnification SEM fractograph shows slightly coarse dimples and large voids and secondary cracks. The fracture features of samples heat-treated at temperatures from 970˚C to 1045˚C are similar.

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FIGURE 5.65 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 220-h life. SEM macrograph shows coarse, granular fracture features with prior beta boundaries. The general appearance of the fracture surface is different compared with that of Figure 5.56, Figure 5.59, and Figure 5.62.

FIGURE 5.66 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 220-h life. SEM fractograph shows a mixture of coarse and fine dimples.

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FIGURE 5.67 Ti-5.5Al-4Zr-4Sn-1Nb-0.6C-0.5Mo-0.35Si, 1080˚C/2h/OQ + 700˚C/2h/AC, creep tested at 650˚C, 220 MPa, 220-h life. High-magnification SEM fractograph shows large dimples and a few cleavage facets (marked A). Creep life increased with the increase in solution treatment temperature. At this heat-treatment temperature, the fracture seems to be changing to mixed mode.

6 Alpha + Beta Alloys* Alloys containing 4 to 6% of beta stabilizers are called α+β alloys. Examples of this class of alloys are Ti-6Al4V, Ti-6Al-6V-2Sn, etc. These alloys can be heat-treated to develop a variety of microstructures and mechanical properties. Ti-6Al-4V, the commonly used alloy of this class, is primarily used in α+β condition. This alloy is used in the annealed condition or in the solution-treated and aged (STA) condition. Fractographs of this alloy in the STA condition, tensile tested at room temperature, 200°C, and 300°C, are shown in this chapter. Ti-6Al-4V is a very welltested alloy and has a good balance of properties, and hence it is used in the aerospace industry. VT9 (Ti-6.5Al-3.2Mo-1.8Zr-0.25Si) [7] is also an α+β alloy of Russian origin. This alloy can be used in the α+β or β heat-treated condition. Fractographs showing the fracture features of low-oxygen-content (≈600 ppm) VT9 alloy tensile tested at room temperature and 500°C are

also shown in this chapter. The dimples in the 500°Ctested specimen are coarser and more equiaxed compared with the room-temperature-tested sample. The fracture features are similar in the alloy containing higher oxygen (1300 ppm). The room-temperature impact-tested specimen showed similar features in both the low- and highoxygen-content specimens. However, the dimple size of the higher-oxygen-content specimen was slightly finer. The fracture features of the −70°C impact-tested specimen of the high-oxygen-content sample (not shown in this book) were also similar to the low-oxygen-content sample. The α+β heat-treated tensile-tested (500°C) specimen appears to be more ductile, with prominent necking showing cup-and-cone fracture with coarse equiaxed dimples compared with the β heat-treated specimen tested at similar conditions. Other α+β alloys (Ti-6246, Ti-6222, Ti17, etc.) are developed for high-temperature applications in gas-turbine engines up to 400°C.

FIGURE 6.1 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC. Optical micrograph of tensile-tested specimen shows primary α and transformed β. * The micrographs shown in Figure 6.23 to Figure 6.73 are all from DMR Technical Report from Banerjee, D., Saha, R.L., Mukherjee, D., and Muraleedharan, K., Structure and Properties of Ti-6.5Al-3Mo-1.8Zr-0.25Si Alloy, DMRTR 8983, Defence Metallurgical Research Laboratory, Hyderabad, India, 1989. With permission.

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FIGURE 6.2 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. Low-magnification fractograph showing cup-and-cone fracture with a prominent shear lip. The high magnification photographs of regions A and B are shown in Figure 6.3 and Figure 6.4, respectively.

FIGURE 6.3 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. SEM fractograph shows dimples in the central region (A) at high magnification.

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FIGURE 6.4 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. SEM fractograph of shear-lip region (B) shows equiaxed dimples.

FIGURE 6.5 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at room temperature. SEM fractograph of another area of shear lip shows elongated dimples at high magnification.

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FIGURE 6.6 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. Scanning electron microscope (SEM) macrograph shows cup-and-cone fracture with a pronounced shear lip.

FIGURE 6.7 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. The central region of the SEM fractograph of Figure 6.6 shows a mixture of coarse and fine dimples.

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FIGURE 6.8 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. SEM fractograph shows dimples at higher magnification. The dimple size seems to be coarser as compared with room-temperature-tested specimen.

FIGURE 6.9 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. Low-magnification SEM fractograph of the shearlip region shows equiaxed dimples.

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FIGURE 6.10 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 200˚C. High-magnification SEM fractograph of the shearlip region shows elongated dimples.

FIGURE 6.11 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 300˚C. SEM macrograph shows classical cup-and-cone fracture with a prominent shear lip.

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FIGURE 6.12 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 300˚C. SEM fractograph of the central region of Figure 6.11 shows a mixture of coarse and fine dimples at higher magnification. The dimple size seems to be increasing with increasing test temperature, indicating greater ductility.

FIGURE 6.13 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, tensile tested at 300˚C. SEM fractograph of the shear-lip region of Figure 6.11 shows slightly elongated dimples at higher magnifications.

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FIGURE 6.14 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. Low-magnification SEM fractograph shows smooth fracture surface and the origin (arrow).

FIGURE 6.15 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. High-magnification SEM fractograph shows fatigue striations at higher magnification away from the origin.

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FIGURE 6.16 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. SEM fractograph showing a different area of Figure 6.14 reveals fatigue striations and numerous secondary cracks. Fissures at the roots of some fatigue striations are also seen.

FIGURE 6.17 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–800 MPa. The final fracture of the same specimen (Figure 6.14) shows dimpled overload fracture features.

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FIGURE 6.18 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. SEM fractograph shows the fracture surface and origin (arrow).

FIGURE 6.19 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. Low-magnification SEM fractograph of Figure 6.18 shows origin.

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FIGURE 6.20 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. SEM fractograph shows fatigue striations at higher magnification away from the origin.

FIGURE 6.21 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. High-magnification SEM fractograph of a different area of the specimen shows fatigue striations with secondary cracks. Fissures (arrow) at the roots of some striations are also seen.

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FIGURE 6.22 Ti-6Al-4V, 960˚C/1h/WQ + 535˚C/6h/AC, high-cycle fatigue tested at 430-MPa stress. SEM fractograph of the final overload fracture of the specimen shows dimples and tear ridges (arrow).

FIGURE 6.23 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC. Optical micrograph of low-oxygencontent (600 ppm) alloy shows primary α and transformed β. The transformed β microstructure is coarse with prominent acicularity.

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FIGURE 6.24 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC. Optical micrograph of high-oxygencontent (1300 ppm) alloy shows α+β microstructure. The percentage of primary α is greater in high-oxygen-content alloy compared with low-oxygen-content alloy (Figure 6.23).

FIGURE 6.25 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature tensile tested. Low-magnification SEM fractograph shows surface of a tensile specimen. The fracture consists of a flat fibrous central region and a shear lip.

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FIGURE 6.26 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature tensile tested. SEM fractograph of central region of the specimen in Figure 6.25 shows dimples at low magnification. Tear ridges are also seen.

FIGURE 6.27 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature tensile tested. High-magnification SEM fractograph of the central region of Figure 6.25 shows dimples.

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FIGURE 6.28 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, tensile tested at 500˚C. Low-magnification SEM fractograph shows classical cup-and-cone fracture with pronounced necking and lip formation.

FIGURE 6.29 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, tensile tested at 500˚C. SEM fractograph of the center of the specimen of Figure 6.28 shows a mixture of fine and coarse deeply rounded dimples at higher magnification.

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FIGURE 6.30 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, tensile tested at 500˚C. High-magnification SEM fractograph of the area in the rectangle in Figure 6.29 shows serpentine glide in a coarse dimple.

FIGURE 6.31 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, roomtemperature tensile tested. Low-magnification SEM fractograph shows smooth fracture.

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FIGURE 6.32 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, roomtemperature tensile tested. SEM fractograph of the central region of Figure 6.31 shows fine equiaxed dimples.

FIGURE 6.33 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, roomtemperature tensile tested. SEM fractograph of the central region of Figure 6.31 shows fine equiaxed dimples at higher magnification.

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FIGURE 6.34 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature impact tested. Low-magnification SEM fractograph shows flat fracture.

FIGURE 6.35 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature impact tested. Higher-magnification SEM fractograph shows dimples and secondary cracks.

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FIGURE 6.36 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, room-temperature impact tested. Higher-magnification SEM fractograph shows dimples and secondary cracks. Tear ridges (arrow) are also seen.

FIGURE 6.37 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, impact tested at −70˚C. Low-magnification SEM fractograph shows smooth fracture.

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FIGURE 6.38 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, impact tested at −70˚C. Higher-magnification SEM fractograph shows dimples.

FIGURE 6.39 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, impact tested at −70˚C. Higher-magnification SEM fractograph shows dimples and secondary cracks. The fracture features of −70˚C-tested specimen are similar to those of room-temperature-tested specimen.

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FIGURE 6.40 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, roomtemperature impact tested. Low-magnification SEM fractograph shows fracture features.

FIGURE 6.41 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, roomtemperature impact tested. High-magnification SEM fractograph of the area in the rectangle in Figure 6.40 shows dimples.

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FIGURE 6.42 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, roomtemperature impact tested. SEM fractograph shows fine and coarse dimples a little away from the notch.

FIGURE 6.43 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, roomtemperature impact tested. SEM fractograph of the central region of the specimen shows fine equiaxed dimples. The fracture features are similar to the low-oxygen-content specimen, but the dimple size seems to be slightly finer.

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FIGURE 6.44 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–895 MPa. Low-magnification SEM fractograph shows faceted fracture.

FIGURE 6.45 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–895 MPa. SEM fractograph of fracture surface shows the origin (arrow).

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FIGURE 6.46 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–895 MPa. SEM fractograph shows cleavage-like features with fatigue striations at the center of the specimen.

FIGURE 6.47 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–895 MPa. SEM fractograph shows cleavage-like features with fatigue striations away from the origin.

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FIGURE 6.48 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–940 MPa. Low-magnification SEM fractograph shows smoother fracture with the absence of facets compared with lowstress range (Figure 6.44).

FIGURE 6.49 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–940 MPa. High-magnification SEM fractograph of the area in the rectangle (Figure 6.48) shows the origin.

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FIGURE 6.50 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–940 MPa. High-magnification SEM fractograph of the center of the specimen shows fatigue striations (arrow marked).

FIGURE 6.51 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, low-cycle fatigue tested at a stress range of 0–940 MPa. SEM fractograph shows fatigue striations and secondary cracks away from the origin.

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FIGURE 6.52 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, high-cycle fatigue tested at 685 MPa. Low-magnification SEM fractograph shows origin.

FIGURE 6.53 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, high-cycle fatigue tested at 685 MPa. High-magnification SEM fractograph shows a view of the origin.

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FIGURE 6.54 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, α+β heat-treated at 960˚C/1h/AC + 530˚C/6h/AC, high-cycle fatigue tested at 685 MPa. SEM fractograph at higher magnification shows fatigue striations and secondary cracks away from the origin.

FIGURE 6.55 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC. Optical micrograph of specimen shows transformed β microstructure with prior β boundaries. A thin continuous α film is present at the grain boundaries.

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FIGURE 6.56 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature tensile tested. SEM macrograph shows faceted fracture features.

FIGURE 6.57 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature tensile tested. Low-magnification SEM fractograph shows dimples on the facets.

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FIGURE 6.58 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature tensile tested. High-magnification SEM fractograph shows dimples on the facets. Alignment of dimples along the α/β interface is also seen.

FIGURE 6.59 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, roomtemperature tensile tested. Low-magnification SEM fractograph shows faceted fracture features.

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FIGURE 6.60 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, roomtemperature tensile tested. High-magnification SEM fractograph shows dimples on the facets. Alignment of dimples along the α/β interface is more prominent. The fracture features of both low- and high-oxygen-content alloys are similar.

FIGURE 6.61 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, tensile tested at 500˚C. Low-magnification SEM fractograph shows smoother fracture surface as compared with room-temperature-tested specimen.

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FIGURE 6.62 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, tensile tested at 500˚C. High-magnification SEM fractograph shows equiaxed dimples and tear ridges.

FIGURE 6.63 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, tensile tested at 500˚C. High-magnification SEM fractograph shows both equiaxed and elongated dimples.

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FIGURE 6.64 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature impact tested. Low-magnification SEM fractograph shows mixed inter- and transgranular fracture modes. The fracture is more intergranular in comparison with tensile fracture (Figure 6.56).

FIGURE 6.65 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature impact tested. High-magnification SEM fractograph shows dimples and secondary cracks just below the notch.

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FIGURE 6.66 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (low oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, room-temperature impact tested. High-magnification SEM fractograph shows dimples and secondary cracks away from the notch.

FIGURE 6.67 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, roomtemperature impact tested. Low-magnification SEM fractograph shows mixed inter- and transgranular fracture modes. The fracture is more intergranular compared with the low-oxygen-content specimen.

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FIGURE 6.68 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, roomtemperature impact tested. High-magnification SEM fractograph shows dimples on the facets and secondary cracks just below the notch.

FIGURE 6.69 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si (higher oxygen content), β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, roomtemperature impact tested. High-magnification SEM fractograph shows dimples on the facets and secondary cracks away from the notch.

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FIGURE 6.70 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 465 MPa. Low-magnification SEM fractograph shows fatigue fracture surface with flat origin crisscrossed by linear features.

FIGURE 6.71 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 465 MPa. SEM fractograph at higher magnification shows fatigue striations and secondary cracks away from the origin.

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FIGURE 6.72 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 588 MPa. SEM fractograph at low magnification shows origin (arrow).

FIGURE 6.73 Ti-6.5Al-3.2Mo-1.8Zr-0.25Si, β heat-treated at 1030˚C/0.25h/AC + 530˚C/8h/AC, high-cycle fatigue tested at 588 MPa. SEM fractograph at higher magnification shows fatigue striations and secondary cracks away from the origin.

7 Beta Alloys* In beta alloys, the β phase is stabilized at room temperature by the addition of approximately 30% of β stabilizers like V, Mo, Nb, Ta, etc. The important alloy of this class is β-C, which is used in burn-resistant applications. More often, the so-called β alloys are metastable β alloys in which the β phase can be retained on fast cooling from the β solution-treatment temperature. Titanium alloys, in the solution-treated condition, generally have lower strength. However, these alloys can be aged to obtain very high strength levels due to the precipitation of fine-α phase upon aging. These alloys have an excellent combination of fracture toughness and strength, which can be tailored by selecting appropriate aging temperature and time. Ti-10V-2Fe-3Al, Ti-13V-11Cr-3Al, and Ti-15Mo are some of the prominent alloys of this class. Low-cost β alloys such as TIMETAL LCB (Ti-4.5Fe-6.8Mo-1.5Al) are attracting interest for industrial and automotive applications. Ti-10V-4.5Fe-1.5Al is another low-cost β alloy

being studied in DMRL [8]. Metastable β alloys contain 10 to 15% of β stabilizers. Beta alloys are normally used in the α+β solution-treated and aged condition. This chapter presents microstructures and fractographs of the alloys Ti-10V-2Fe-3Al (Ti-10-2-3) and Ti10V-4.5Fe-1.5Al in the solution-treated (ST) and ST + aged (STA) conditions. Fracture features of the solutiontreated specimens showed only ductile dimples for both alloys. The fracture surface of the β solution-treated and aged specimens (450˚C) revealed predominantly intergranular fracture features with dimples on the grain facets, and the specimens aged at higher temperatures showed more ductile features, with dimples interspersed with intergranular fracture features in both the alloys. The α+β solution-treated and aged specimens in both alloys showed ductile fracture with dimples. These alloys have high strength and good hardenability, and they are used in aircraft structural applications.

FIGURE 7.1 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ. Optical micrograph of β alloy shows coarse equiaxed β grains.

* All micrographs in this chapter are from Bhattacharjee, A., Joshi, V.A., Deshpande, D.G., Hussain, S.M., Nandy, T.K., and Gogia, A.K., Development of Low Cost Beta Titanium Alloys, I, DMR TR 200270, Defence Metallurgical Research Laboratory, Hyderabad, India, 2000. With permission.

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FIGURE 7.2 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ and aged at 600˚C/8h/AC. Scanning electron microscope (SEM) backscattered electron image shows fine-α precipitates in β matrix.

FIGURE 7.3 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ, tensile tested at room temperature. SEM macrograph shows transgranular fracture and a few voids.

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FIGURE 7.4 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows ductile fracture with dimples.

FIGURE 7.5 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ and aged at 450˚C/1h/AC, tensile tested at room temperature. SEM macrograph shows rough fracture features.

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FIGURE 7.6 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ and aged at 450˚C/1h/AC, tensile tested at room temperature. Highmagnification SEM fractograph shows mixed-mode, brittle, and ductile fracture with shallow dimples on the facets. Secondary cracks are also seen.

FIGURE 7.7 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ and aged at 600˚C/1h/AC, tensile tested at room temperature. SEM macrograph shows rough fracture features with voids.

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FIGURE 7.8 Ti-10V-2Fe-3Al, β solution treated at 820˚C/8h/WQ and aged at 600˚C/1h/AC, tensile tested at room temperature. Highmagnification SEM fractograph shows mixed-mode, brittle, and ductile fracture with shallow dimples on the facets. Secondary cracks are also seen.

FIGURE 7.9 Ti-10V-2Fe-3Al, α+β solution treated at 700˚C/8h/WQ and aged at 600˚C/4h/AC. SEM backscattered electron image shows equiaxed primary α and acicular α.

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FIGURE 7.10 Ti-10V-2Fe-3Al, α+β solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temperature. SEM macrograph shows cup-and-cone fracture with a prominent shear lip.

FIGURE 7.11 Ti-10V-2Fe-3Al, α+β solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows ductile fracture with dimples.

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FIGURE 7.12 Ti-10V-4.5Fe-1.5Al, β solution treated at 800˚C/8h/WQ. Optical micrograph of β alloy shows equiaxed β grains.

FIGURE 7.13 Ti-10V-4.5Fe-1.5Al, β solution treated at 800˚C /8h/WQ + 600/1h/AC. Secondary electron image of β alloy shows equiaxed β grains with fine-α precipitation.

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FIGURE 7.14 Ti-10V-4.5Fe-1.5Al, β solution treated at 800˚C/8h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows fully ductile fracture features.

FIGURE 7.15 Ti-10V-4.5Fe-1.5Al, β solution treated at 800˚C/8h/WQ, tensile tested at room temperature. SEM fractograph of the center of the specimen (Figure 7.14) at intermediate magnification shows fine dimples and tear ridges (arrow).

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FIGURE 7.16 Ti-10V-4.5Fe-1.5Al, β solution treated at 800˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph of a different area of the specimen (Figure 7.14) shows dimples and some inclusions (probably silicates [marked A]).

FIGURE 7.17 Ti-10V-4.5Fe-1.5Al, β solution treated at 800˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph of the surface of the specimen (Figure 7.14) shows fine and coarse dimples.

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FIGURE 7.18 Ti-10V-4.5Fe-1.5Al, β solution treated and aged at 800˚C/8h/WQ + 450˚C/1h/AC, tensile tested at room temperature. SEM macrograph shows crystalline fracture features.

FIGURE 7.19 Ti-10V-4.5Fe-1.5Al, β solution treated and aged at 800˚C/8h/WQ + 450˚C/1h/AC, tensile tested at room temperature. SEM fractograph shows mixed-mode fracture of intergranular and ductile fracture features at intermediate magnification.

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FIGURE 7.20 Ti-10V-4.5Fe-1.5Al, β solution treated and aged at 800˚C/8h/WQ + 450˚C/1h/AC, tensile tested at room temperature. Higher-magnification SEM fractograph shows fine dimples on the grain facets.

FIGURE 7.21 Ti-10V-4.5Fe-1.5Al, β solution treated and aged at 800˚C/8h/WQ + 500˚C/1h/AC, tensile tested at room temperature. Low-magnification SEM fractograph shows crystalline fracture features.

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FIGURE 7.22 Ti-10V-4.5Fe-1.5Al, β solution treated and aged at 800˚C/8h/WQ + 500˚C/1h/AC, tensile tested at room temperature. SEM fractograph shows mixed mode of intergranular- and ductile-fracture features at intermediate magnification. Ductile mode is more prominent, and intergranular fracture is less compared with the specimen aged at 450˚C (Figure 7.19).

FIGURE 7.23 Ti-10V-4.5Fe-1.5Al, β solution treated and aged at 800˚C/8h/WQ + 500˚C/1h/AC, tensile tested at room temperature. SEM fractograph shows fine dimples near the surface of the specimen (Figure 7.21) at higher magnification.

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FIGURE 7.24 Ti-10V-4.5Fe-1.5Al, α+β solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC. SEM backscattered electron image shows globular primary α and acicular α.

FIGURE 7.25 Ti-10V-4.5Fe-1.5Al, α+β solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temperature. SEM macrograph shows ductile fracture with shear lip.

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FIGURE 7.26 Ti-10V-4.5Fe-1.5Al, α+β solution treated at 700˚C/8h/WQ and aged at 500˚C/8h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows dimples and a big void.

8 Titanium Aluminides* 8.1 Ti3Al-BASED ALLOYS These are intermetallics possessing excellent high-temperature creep and stress-rupture properties. A Ti-Al system shows two intermetallics on the Ti-rich side: Ti3Al and TiAl. Both of these have excellent high-temperature properties but suffer from poor room-temperature ductility. Over the years, several alloys with ternary and quaternary additions have been investigated in an attempt to achieve adequate room-temperature ductility. The alloys based on Ti3Al contain large amounts of niobium and small additions of other β stabilizers like Ta, Mo, etc., and they may consist of a B2 phase in addition to α2 (Ti3Al) or a derivative of α2 known as O (Ti2AlNb), which has an orthorhombic lattice. The alloys show microstructural variations similar to those observed in conventional titanium alloys and have similar response to thermomechanical processing. However, microstructure in these alloys can be more complex due to the presence of the O phase and several other phase transformations in the Ti-Al-Nb system. Microstructures and fractographs of the Ti3Al-based alloys are presented in this chapter, including Ti-24Al11Nb [6, 9], Ti-24Al-20Nb [10], Ti-24Al-27Nb [10], and Ti-25Al-15Nb [11], with and without Ta and Mo additions [11]. All of the alloys were tensile tested after cooling from different temperatures using different cooling rates. In the alloy Ti-24Al-20Nb, α+β water-quenched (WQ) specimens, the fractographs show a mixture of cleavage facets and dimpled regions. The extent of the dimpled regions, which are more prominent in the high-

temperature solution-treated specimens, can be related to the fracture of the B2 phase, and the cleavage facets can be ascribed to the fracture of the α2 or O phase. In the Ti-24Al-27Nb alloy, cleavage features dominate the fracture surface at all solution-treated (ST) temperatures, although a few dimples are apparent in the specimens that were solution treated at 980˚C and 1020˚C. The size of the cleavage facets is very large to be associated with the particle size of the O phase in the microstructure. This could be due to the fracture of the large unrecrystallized B2 grains, while the dimpled regions are due to the fracture of the fine recrystallized B2 grains. Fractographs of alloy Ti-24Al-20Nb in the β-treated condition show transcrystalline features at all cooling rates. At high cooling rates, quasicleavage fracture features are seen. At low cooling rates, large cleavage facets with cleavage of similarly oriented laths are observed. In the β-treated Ti-24Al-27Nb alloy, a mixture of transcrystalline and intercrystalline fracture is seen at high cooling rates. Large cleavage facets with river markings are seen at this cooling rate.

8.2 TiAl-BASED ALLOYS Microstructures and fractographs of TiAl-based γ aluminides of two alloys with composition Ti-47Al-2Nb-2Cr and Ti-48Al-4Nb-1Mo are presented. Both alloys were tensile tested at room temperature and the fractographs show transcrystalline fracture with cleavage facets.

* The micrographs shown in Figure 8.55 through Figure 8.105 have all been taken from Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi, V.A., Sagar, P.K., and Banerjee, D., Development of Advanced High Temperature Ti-alloys, VI, Effect of Nb in Ti3 base alloys, DMR TR 94175, Defence Metallurgical Research Laboratory, Hyderabad, India, 1994. With permission. The micrographs shown in Figure 8.106 through 8.159 are from Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi, V.A., Kamat, S.V., and Banerjee, D., Development of an Advanced High Temperature Titanium Alloy, V, DMR TR 93166, Defence Metallurgical Research Laboratory, Hyderabad, India, 1993. With permission.

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FIGURE 8.1 Ti-24Al-11Nb, α2+β heat-treated, 1020˚C/8h/WQ. Optical micrograph shows equiaxed α2 and B2. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.2 Ti-24Al-11Nb, 1020˚C/8h/WQ, tensile tested at room temperature. Scanning electron microscope (SEM) macrograph shows smooth fracture features. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

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FIGURE 8.3 Ti-24Al-11Nb, 1020˚C/8h/WQ, tensile tested at room temperature. SEM fractograph at intermediate magnification shows cleavage fracture features with secondary cracks. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

FIGURE 8.4 Ti-24Al-11Nb, 1020˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows cleavage fracture features in the center of the specimen with secondary cracks. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

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FIGURE 8.5 Ti-24Al-11Nb, α2+β heat-treated at 1060˚C/4h/WQ. Optical micrograph shows α2 and B2. α2 has a higher aspect ratio that resulted due to processing at high temperature close to the β transus of the alloy. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.6 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows roughfaceted transcrystalline fracture features.

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FIGURE 8.7 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph of central region of the specimen (Figure 8.6) shows cleavage steps.

FIGURE 8.8 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph of central region of the specimen (Figure 8.6) shows shallow dimples on the cleavage facets.

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FIGURE 8.9 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows dimples on the facets and fluting (A) associated with α2 failure.

FIGURE 8.10 Ti-24Al-11Nb, α2+β heat-treated at 1060˚C/4h/WQ. Optical micrograph shows equiaxed α2 and B2. Equiaxed α2 has resulted due to processing at temperature lower than the β transus of the alloy (compare with Figure 8.5). (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

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FIGURE 8.11 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture features. The difference in fracture features as compared with Figure 8.6 is due to the difference in processing temperature. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

FIGURE 8.12 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph shows cleavage fracture features and a few dimples.

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FIGURE 8.13 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. SEM fractograph at intermediate magnification shows cleavage facets with secondary cracks. The facet size is of the order of primary α2 size.

FIGURE 8.14 Ti-24Al-11Nb, 1060˚C/4h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows feather markings on the cleavage facets (A). Dimples (B) around the facets are also seen at places.

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FIGURE 8.15 Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. SEM macrograph shows radial fracture with origin at the center.

FIGURE 8.16 Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. Low-magnification SEM fractograph shows quasicleavage fracture features with secondary cracks.

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FIGURE 8.17 Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.16 shows cleavage facets and secondary cracks.

FIGURE 8.18 Ti-24Al-11Nb, 1060˚C/4h/AC, tensile tested at room temperature. High-magnification SEM fractograph of a different area shows fan-shaped cleavage facets (A) and a few dimples (arrow).

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FIGURE 8.19 Ti-24Al-11Nb, α2+β heat-treated at 1100˚C/2h/WQ. Optical micrograph shows α2 and B2. α2 has a higher aspect ratio that resulted due to processing at higher temperature close to the β transus of the alloy. The volume fraction of α2 decreases with an increase in solution-treatment temperature. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.20 Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows coarse cleavage facets and secondary cracks.

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FIGURE 8.21 Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. SEM fractograph shows river pattern on cleavage facets. Cleaved steps (arrow) and secondary cracks are also seen.

FIGURE 8.22 Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows fine dimples on some cleavage facets.

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FIGURE 8.23 Ti-24Al-11Nb, α2+β heat-treated at 1100˚C/2h/WQ. Optical micrograph shows equiaxed α2 and B2. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.24 Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

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FIGURE 8.25 Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows transgranular fracture with cleavage of equiaxed α2 (arrow) and dimples. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.26 Ti-24Al-11Nb, 1100˚C/2h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows faceted failure with shallow dimples on facets. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

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FIGURE 8.27 Ti-24Al-11Nb, 1100˚C/4h/FC. Optical micrograph shows equiaxed α2 and retained B2. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

FIGURE 8.28 Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM macrograph shows rough fracture features. Origin is shown by the arrow.

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FIGURE 8.29 Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM fractograph of the center of the specimen (Figure 8.28) at intermediate magnification shows cleavage fracture features with secondary cracks.

FIGURE 8.30 Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM fractograph of a different region of the specimen (Figure 8.28) at high magnification shows cleavage and secondary cracks.

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FIGURE 8.31 Ti-24Al-11Nb, 1100˚C/4h/FC, tensile tested at room temperature. SEM fractograph of a different region of the specimen (Figure 8.28) at high magnification shows cleavage facets with feathery fracture features and secondary cracks.

FIGURE 8.32 Ti-24Al-11Nb, 1130˚C/1h/OQ. Optical micrograph shows very fine α2 microstructure within B2 grains. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

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FIGURE 8.33 Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. Low-magnification SEM fractograph shows coarse transgranular fracture features. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

FIGURE 8.34 Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. SEM fractograph shows cleavage facets and secondary cracks.

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FIGURE 8.35 Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. SEM fractograph shows a grain with triple point and cleavage facets.

FIGURE 8.36 Ti-24Al-11Nb, 1130˚C/1h/OQ, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.35 shows faceted transgranular fractures with shallow dimples in region A and intergranular ductile fracture features with coarse and fine dimples in region B. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

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FIGURE 8.37 Ti-24Al-11Nb, 1150˚C/1h/AC. Optical micrograph shows acicular α2 in B2 grains. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

FIGURE 8.38 Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM macrograph shows mixed-mode fracture with coarse cleavage and intergranular features. Origin and intergranular facets are shown by white and black arrows, respectively. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

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FIGURE 8.39 Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. Low-magnification SEM fractograph of the origin shows grain-boundary-initiated failure. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.40 Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM fractograph of Figure 8.39, seen at high magnification, shows the origin and grain-boundary-initiated failure in detail.

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FIGURE 8.41 Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM fractograph at low magnification shows propagation of transcrystalline failure on the macroscopic facets extending on the B2 grains. Shallow dimples on the facets are also apparent.

FIGURE 8.42 Ti-24Al-11Nb, 1150˚C/1h/AC, tensile tested at room temperature. SEM fractograph of Figure 8.41 at high magnification shows propagation of transcrystalline failure on the macroscopic facets extending on the B2 grains.

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FIGURE 8.43 Ti-24Al-11Nb, 1150˚C/1h/FC. Optical micrograph shows coarse colony structure of α2 and B2. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

FIGURE 8.44 Ti-24Al-11Nb, 1150˚C/1h/FC, tensile tested at room temperature. SEM macrograph shows faceted transgranular fracture. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

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FIGURE 8.45 Ti-24Al-11Nb, 1150˚C/1h/FC, tensile tested at room temperature. SEM fractograph shows cleavage fracture with feathery features and secondary cracks. The orientation of cleavage facet changes across the colony boundaries.

FIGURE 8.46 Ti-24Al-11Nb, 1150˚C/1h/FC, tensile tested at room temperature. High-magnification SEM fractograph of individual facets shows feathery cleavage of α2 plates, which are separated by tear ridges. (Courtesy of Gogia, A.K., Banerjee, D., and Nandy, T.K., Metallurgical Transactions A, 21A, 1990. With permission.)

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FIGURE 8.47 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC. Optical micrograph shows α2 + transformed B2 microstructure.

FIGURE 8.48 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC, tensile tested at room temperature. Low-magnification SEM fractograph shows a fracture surface with secondary cracks.

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FIGURE 8.49 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC, tensile tested at room temperature. SEM fractograph of center of Figure 8.48 shows quasicleavage fracture features.

FIGURE 8.50 Ti-24Al-11Nb, 1060˚C/4h/WQ + 800˚C/24h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows terraced fracture surface (arrow) with smooth plateau and ridges.

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FIGURE 8.51 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC. Optical micrograph shows α2 + transformed B2 microstructure. (From Gogia, A.K., Microstructure, Tensile Deformation and Fracture in Ti3Al Base Alloys Containing Niobium, Ph.D. thesis, Dept. of Metallurgy, Institute of Technology, Banaras Hindu University, Varanasi, 1990. With permission.)

FIGURE 8.52 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC, tensile tested at room temperature. Low-magnification SEM fractograph shows radial transgranular fracture and origin (arrow).

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FIGURE 8.53 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC, tensile tested at room temperature. SEM fractograph shows quasicleavage fracture features with prior beta boundary.

FIGURE 8.54 Ti-24Al-11Nb, 1150˚C/1h/AC + 750˚C/24h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows quasicleavage fracture features.

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FIGURE 8.55 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ. SEM backscattered electron image shows α2 (dark) + O (gray) + B2 (bright) phases.

FIGURE 8.56 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ. SEM backscattered electron image shows α2 (dark) + O (dark gray needles) + B2 (light gray, matrix) phases. The volume fraction of the O phase is less compared with the 900˚C solution-treated sample.

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FIGURE 8.57 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ. SEM backscattered electron image shows α2 (dark) + B2 (gray, matrix) phases. The volume fraction of the B2 phase increases with increasing solution-treatment temperature.

FIGURE 8.58 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ, tensile tested at room temperature. SEM macrograph shows origin (arrow) and secondary cracks.

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FIGURE 8.59 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ, tensile tested at room temperature. SEM fractograph of Figure 8.58 at intermediate magnification shows cleavage facets and a few dimples.

FIGURE 8.60 Ti-24Al-20Nb, α2+β solution treated at 900˚C/1h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows clear cleavage facets and a few dimples. Cleavage facets are attributed to the fracture of the α2 or O phase, and dimples are due to fracture of the β phase.

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FIGURE 8.61 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ, tensile tested at room temperature. SEM macrograph shows fracture surface with secondary cracks.

FIGURE 8.62 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ, tensile tested at room temperature. SEM fractograph shows a mixture of dimples and cleavage facets. Secondary cracks are also apparent.

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FIGURE 8.63 Ti-24Al-20Nb, α2+β solution treated at 980˚C/1h/WQ, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.62 shows a mixture of dimples and cleavage facets. The dimpled region is related to the fracture of the β phase, and cleavage facets are related to the fracture of the α2 or O phase.

FIGURE 8.64 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture surface.

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FIGURE 8.65 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ, tensile tested at room temperature. SEM fractograph shows a mixture of dimples and cleavage facets.

FIGURE 8.66 Ti-24Al-20Nb, α2+β solution treated at 1060˚C/1h/WQ, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.65 shows a mixture of dimples and cleavage facets. The dimpled region is related to the fracture of the β phase, and the cleavage facets are related to the fracture of the α2 or O phase. A dimpled region surrounding the cleavage facets is also apparent.

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FIGURE 8.67 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.016˚C/sec. SEM backscattered electron image shows coarse colony structure comprising the α2, O, and B2 phases.

FIGURE 8.68 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.1˚C/sec. SEM backscattered electron image (Figure 8.67) shows comparatively fine colony structure comprising the α2, O, and B2 phases.

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FIGURE 8.69 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 2.5˚C/sec. SEM backscattered electron image shows fine basketweave structure. The structure is very fine due to the faster cooling rate.

FIGURE 8.70 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. Low-magnification SEM fractograph shows rough transcrystalline fracture features.

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FIGURE 8.71 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. High-magnification SEM fractograph shows transcrystalline fracture with large cleavage facets, with cleavage of similarly oriented laths in a colony and a few dimpled areas (arrow).

FIGURE 8.72 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. SEM macrograph shows fine features with smooth appearance.

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FIGURE 8.73 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. High-magnification SEM fractograph shows transcrystalline fracture with large cleavage facets with cleavage of similarly oriented laths in a colony and a few dimpled areas.

FIGURE 8.74 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. Low-magnification SEM fractograph shows coarse transgranular quasicleavage fracture features.

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FIGURE 8.75 Ti-24Al-20Nb, β treated at 1140˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. High-magnification SEM fractograph shows transgranular quasicleavage with cleavage steps.

FIGURE 8.76 Ti-24Al-20Nb, β treated at 1140˚C/1h/WQ, fracture-toughness tested (K1c). Low-magnification SEM fractograph shows classic cleavage fracture.

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FIGURE 8.77 Ti-24Al-20Nb, β treated at 1140˚C/1h/WQ, fracture-toughness tested (K1c). High-magnification SEM fractograph of Figure 8.76 shows river pattern on the cleaved facets and fine dimples in between the facets.

FIGURE 8.78 Ti-24Al-27Nb, α2+β solution treated at 900˚C/24h/WQ. SEM backscattered electron image shows two phases, B2 (bright), in the matrix of O phase.

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FIGURE 8.79 Ti-24Al-27Nb, α2+β solution treated at 940˚C/16h/WQ. SEM backscattered electron image shows two phases, B2 (bright), in the matrix of O phase.

FIGURE 8.80 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ. SEM backscattered electron image shows two phases, O (dark), in the matrix of B2.

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FIGURE 8.81 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ. SEM backscattered electron image shows two phases, O (dark), in the matrix of B2. The volume fraction of B2 seems to be increasing with increasing solution-treatment temperature.

FIGURE 8.82 Ti-24Al-27Nb, α2+β solution treated at 900˚C/24h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture surface.

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FIGURE 8.83 Ti-24Al-27Nb, α2+β solution treated at 900˚C/24h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph of Figure 8.82 shows predominantly cleavage fracture.

FIGURE 8.84 Ti-24Al-27Nb, α2+β solution treated at 900˚C/24h/WQ, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.83 shows fine cleavage facets.

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FIGURE 8.85 Ti-24Al-27Nb, α2+β solution treated at 940˚C/16h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture surface.

FIGURE 8.86 Ti-24Al-27Nb, α2+β solution treated at 940˚C/16h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows predominantly cleavage fracture.

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FIGURE 8.87 Ti-24Al-27Nb, α2+β solution treated at 940˚C/16h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows cleavage facets, which are slightly coarse as compared with samples treated at 900˚C (Figure 8.84).

FIGURE 8.88 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ, tensile tested at room temperature. SEM macrograph shows fracture surface and numerous secondary cracks.

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FIGURE 8.89 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows predominantly cleavage fracture features and some dimples. Dimpled region (A) indicates failure of fine recrystallized B2 grains, and the large cleavage facets (B) are due to unrecrystallized B2 grains.

FIGURE 8.90 Ti-24Al-27Nb, α2+β solution treated at 980˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.89 shows a large cleavage facet with river markings.

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FIGURE 8.91 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ, tensile tested at room temperature. SEM macrograph shows rough fracture surface.

FIGURE 8.92 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ, tensile tested at room temperature. SEM fractograph shows predominantly coarse cleavage fracture features with secondary cracks.

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FIGURE 8.93 Ti-24Al-27Nb, α2+β solution treated at 1000˚C/6h/WQ, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.92 shows coarse cleavage facets and a few dimples.

FIGURE 8.94 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec. SEM backscattered electron image shows colonies of O and B2 phases. The dark phase seen between the laths and at the prior β boundary is an Al-rich and Nb-lean phase.

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FIGURE 8.95 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec. SEM backscattered electron image shows colonies of O and B2 phases. The dark phase seen between the laths and at the prior β boundary is an Al-rich and Nb-lean phase.

FIGURE 8.96 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec. SEM backscattered electron image shows fine basketweave structure (not resolved) of O + B2 due to higher Nb content and a dark phase (Al rich and Nb lean) at the boundaries. Microstructure is similar to Figure 8.69 but is very fine.

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FIGURE 8.97 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. SEM macrograph shows transgranular fracture.

FIGURE 8.98 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. Low-magnification SEM fractograph shows predominantly fine cleavage fracture with secondary cracks.

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FIGURE 8.99 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.016˚C/sec, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.98 shows cleavage facets with secondary cracks.

FIGURE 8.100 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. SEM macrograph shows transgranular fracture features.

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FIGURE 8.101 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. Low-magnification SEM fractograph shows relatively coarse cleavage fracture with secondary cracks as compared with Figure 8.98.

FIGURE 8.102 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 0.1˚C/sec, tensile tested at room temperature. High-magnification SEM fractograph shows coarse cleavage facets with river markings and secondary cracks.

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FIGURE 8.103 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. SEM macrograph shows very coarse transgranular cleavage fracture.

FIGURE 8.104 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. SEM fractograph shows very coarse cleavage facets with river markings.

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FIGURE 8.105 Ti-24Al-27Nb, β treated at 1120˚C/1h and cooled at 2.5˚C/sec, tensile tested at room temperature. SEM fractograph shows cleavage facets with river markings. The fracture features coarsened with the increase in cooling rate from 0.016˚C/sec to 2.5˚C/sec.

FIGURE 8.106 Ti-25Al-15Nb, α2+β heat-treated at 900˚C/16h/WQ. Optical micrograph shows α2 and B2 phases. (Transmission electron microscope [TEM] observations of this sample revealed small amounts of O phase.)

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FIGURE 8.107 Ti-25Al-15Nb, α2+β heat-treated at 980˚C/8h/WQ. Optical micrograph shows α2 and B2 phases.

FIGURE 8.108 Ti-25Al-15Nb, α2+β heat-treated at 1020˚C/6h/WQ. Optical micrograph shows α2 and B2 phases.

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FIGURE 8.109 Ti-25Al-15Nb, α2+β heat-treated at 1060˚C/4h/WQ. Optical micrograph shows α2 and B2 phases. A change in the volume fraction of α2 is observed with the increase in solution-treatment temperature.

FIGURE 8.110 Ti-25Al-15Nb, α2+β heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture surface.

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FIGURE 8.111 Ti-25Al-15Nb, α2+β heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows cleavage features and secondary cracks.

FIGURE 8.112 Ti-25Al-15Nb, α2+β heat-treated at 980˚C/8h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture surface.

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FIGURE 8.113 Ti-25Al-15Nb, α2+β heat-treated at 980˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows cleavage features and secondary cracks.

FIGURE 8.114 Ti-25Al-15Nb, α2+β heat-treated at 1020˚C/6h/WQ, tensile tested at room temperature. SEM macrograph shows smooth fracture surface.

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FIGURE 8.115 Ti-25Al-15Nb, α2+β heat-treated at 1020˚C/6h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows coarse cleavage fracture features with river markings and secondary cracks.

FIGURE 8.116 Ti-25Al-15Nb, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. SEM macrograph shows rough fracture surface.

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FIGURE 8.117 Ti-25Al-15Nb, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows coarser cleavage fracture features with river markings and secondary cracks. Cleavage facet size increases with increasing the solution-treatment temperature.

FIGURE 8.118 Ti-25Al-15Nb, β heat-treated at 1170˚C/1h and cooled at 4˚C/sec. Optical micrograph shows extremely fine lath structure of α2 with small amounts of primary α2.

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FIGURE 8.119 Ti-25Al-15Nb, β heat-treated at 1170˚C/1h and cooled at 0.7˚C/sec. Optical micrograph shows basket-weave structure of α2 laths. Equiaxed primary α is present at the prior β grain boundaries and within the grains.

FIGURE 8.120 Ti-25Al-15Nb, β heat-treated at 1170˚C/1h and cooled at 4˚C/sec, tensile tested at room temperature. SEM macrograph shows rough cleavage fracture features.

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FIGURE 8.121 Ti-25Al-15Nb, β heat-treated at 1170˚C/1h and cooled at 4˚C/sec, tensile tested at room temperature. Highmagnification SEM fractograph of Figure 8.120 shows cleavage steps and river markings.

FIGURE 8.122 Ti-25Al-15Nb, β heat-treated at 1170˚C/1h and cooled at 0.7˚C/sec, tensile tested at room temperature. SEM macrograph shows rough transcrystalline fracture features.

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FIGURE 8.123 Ti-25Al-15Nb, β heat-treated at 1170˚C/1h and cooled at 0.7˚C/sec, tensile tested at room temperature. Highmagnification SEM fractograph of Figure 8.122 shows coarse leaf-like cleavage facets with river markings.

FIGURE 8.124 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 900˚C/16h/WQ. Optical micrograph shows α2 and B2 phases.

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FIGURE 8.125 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 980˚C/8h/WQ. Optical micrograph shows α2 and B2 phases.

FIGURE 8.126 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 1060˚C/4h/WQ. Optical micrograph shows α2 and B2 phases.

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FIGURE 8.127 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 1100˚C/2h/WQ. Optical micrograph shows α2 and B2 phases. The volume fraction of α2 decreases with increasing solution temperature.

FIGURE 8.128 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 900˚C/16h/WQ, tensile tested. SEM macrograph shows smooth fracture.

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FIGURE 8.129 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 900˚C/16h/WQ, tensile tested. High-magnification SEM fractograph shows predominantly cleavage fracture features.

FIGURE 8.130 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 980˚C/8h/WQ, tensile tested. SEM macrograph shows smooth fracture.

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FIGURE 8.131 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 980˚C/8h/WQ, tensile tested. High-magnification SEM fractograph shows predominantly cleavage fracture features with secondary cracks.

FIGURE 8.132 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested. Low-magnification SEM fractograph shows transcrystalline fracture features.

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FIGURE 8.133 SEM fractograph of Ti-25Al-11Nb-4Ta, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested. High-magnification fractograph showing predominantly cleavage fracture features (A) and a few dimpled areas (B).

FIGURE 8.134 SEM fractograph of Ti-24Al-11Nb-4Ta, α2+β heat-treated at 1100˚C/2h/WQ, tensile tested. Low-magnification fractograph showing coarser transcrystalline fracture features as compared with samples solution treated at temperatures 900˚C to 1060˚C.

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FIGURE 8.135 Ti-24Al-11Nb-4Ta, α2+β heat-treated at 1100˚C/2h/WQ, tensile tested. High-magnification SEM fractograph shows predominantly large cleavage fracture features and some shallow dimples.

FIGURE 8.136 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 10˚C/sec. Optical micrograph shows fine lath structure of α2.

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FIGURE 8.137 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 2.5˚C/sec. Optical micrograph shows coarser α2 laths as compared with Figure 8.136.

FIGURE 8.138 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 0.1˚C/sec. Optical micrograph showing coarse colony structure of α2 laths. Coarse α2 laths are observed at prior β grain boundaries also. As the cooling rate decreases coarsening of the laths occurs.

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FIGURE 8.139 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 10˚C/sec, tensile tested. SEM macrograph shows coarse cleavage fracture features. The fracture initiation is at the prior β boundary, and propagation in the cleavage mode is apparent across the entire prior β grain.

FIGURE 8.140 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 10˚C/sec, tensile tested. SEM fractograph shows rough cleavage facets with secondary cracks.

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FIGURE 8.141 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 10˚C/sec, tensile tested. High-magnification SEM fractograph of Figure 8.140 shows cleavage fracture initiating at the prior β boundary.

FIGURE 8.142 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 2.5˚C/sec, tensile tested. SEM macrograph shows transcrystalline fracture with rough cleavage facets.

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FIGURE 8.143 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 2.5˚C/sec, tensile tested. SEM fractograph shows rough cleavage facets.

FIGURE 8.144 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 2.5˚C/sec, tensile tested. High-magnification SEM fractograph of Figure 8.143 shows grain-boundary-initiated fracture (arrow).

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FIGURE 8.145 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 0.1˚C/sec, tensile tested. SEM macrograph shows transcrystalline fracture with cleavage facets.

FIGURE 8.146 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 0.1˚C/sec, tensile tested. Low-magnification SEM fractograph shows cleavage facets traversing the colony of similarly oriented laths.

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FIGURE 8.147 Ti-24Al-11Nb-4Ta, β solution treated at 1170˚C/1h/cooled at 0.1˚C/sec, tensile tested. High-magnification SEM fractograph shows feathery cleavage facets with secondary cracks.

FIGURE 8.148 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 900˚C/16h/WQ. Optical micrograph shows α2 in the matrix of B2.

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FIGURE 8.149 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 980˚C/8h/WQ. Optical micrograph shows α2 in the matrix of B2.

FIGURE 8.150 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1020˚C/6h/WQ. Optical micrograph shows α2 in the matrix of B2.

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FIGURE 8.151 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1060˚C/4h/WQ. Optical micrograph shows α2 in the matrix of B2. The volume fraction of α2 decreases with increasing solution-treatment temperature.

FIGURE 8.152 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. SEM macrograph shows rough fracture features.

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FIGURE 8.153 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 900˚C/16h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows fine cleavage fracture features with secondary cracks. Dimples are also seen at places.

FIGURE 8.154 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 980˚C/8h/WQ, tensile tested at room temperature. SEM macrograph shows rough fracture features. Secondary cracks are also seen.

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FIGURE 8.155 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 980˚C/8h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows slightly coarser cleavage fracture features compared with Figure 8.153.

FIGURE 8.156 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1020˚C/6h/WQ, tensile tested at room temperature. Low-magnification SEM fractograph shows transcrystalline fracture features with the origin near to the center (arrow).

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FIGURE 8.157 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1020˚C/6h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows large cleavage facets and secondary cracks.

FIGURE 8.158 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. SEM macrograph shows transcrystalline fracture features and the origin (arrow). Secondary cracks are also seen.

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FIGURE 8.159 Ti-25Al-14Nb-1Mo, α2+β heat-treated at 1060˚C/4h/WQ, tensile tested at room temperature. High-magnification SEM fractograph shows very large cleavage facets with river markings.

FIGURE 8.160 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. Optical micrograph shows fine recrystallized α grains surrounding large grains with twins.

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FIGURE 8.161 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. Optical micrograph shows fine recrystallized α grains with twins surrounding the large lamellae of α2+γ.

FIGURE 8.162 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. SEM backscattered electron image of Figure 8.161 shows lamellae of α2+γ within the recrystallized grains.

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FIGURE 8.163 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC. SEM backscattered electron image of the arrow-marked area in Figure 8.162 shows lamellae of α2+γ and massive γ phase at high magnification.

FIGURE 8.164 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC. Optical micrograph shows fine lamellar structure of α2+γ.

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FIGURE 8.165 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC. Optical micrograph shows fine lamellar structure of α2+γ and a thin layer of very fine recrystallized α grains at the boundaries.

FIGURE 8.166 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 950˚C/6h/AC. Optical micrograph shows coarse colonies of fine lamellae of α2+γ.

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FIGURE 8.167 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 950˚C/6h/AC. Optical micrograph shows lamellar structure of α2+γ at higher magnification.

FIGURE 8.168 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC, tensile tested at room temperature. SEM macrograph shows rough fracture surface with transgranular crack.

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FIGURE 8.169 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC, tensile tested at room temperature. SEM fractograph shows cleavage fracture features with secondary cracks.

FIGURE 8.170 Ti-47Al-2Nb-2Cr, forged at 1200˚C/24h/AC + 950˚C/6h/AC, tensile tested at room temperature. High-magnification SEM fractograph shows cleavage facets with river markings.

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FIGURE 8.171 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM macrograph shows rough fracture surface with facets.

FIGURE 8.172 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractograph shows faceted fracture with secondary cracks. The effect of the microstructure is also seen on the fracture surface.

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FIGURE 8.173 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. High-magnification SEM fractograph of the arrow-marked region in Figure 8.172 shows fine dimples on a cleavage facet.

FIGURE 8.174 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractograph of Figure 8.172 at higher magnification shows cleavage facets and delamination (arrow).

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FIGURE 8.175 Ti-47Al-2Nb-2Cr, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractograph of a different area at high magnification shows feathery cleavage facets.

FIGURE 8.176 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM macrograph shows rough fracture surface.

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FIGURE 8.177 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. High-magnification SEM fractograph of Figure 8.176 shows deep secondary cracks and cleavage facets.

FIGURE 8.178 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. High-magnification SEM fractograph of a different area shows fan-like cleavage facets with river markings.

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FIGURE 8.179 Ti-48Al-4Nb-1Mo, forged at 1400˚C/0.5h/AC + 960˚C/6h/AC, tensile tested at room temperature. SEM fractograph shows feathery pattern on the cleavage facet.

Study: Failure Investigation Report 9 Case of IMI 550 High-Pressure Compressor (HPC-I) Aero Engine Blade* 9.1 INTRODUCTION A failed high-pressure compressor (HPC) stage-1 aero engine rotor blade of titanium alloy IMI 550 was received to assess the cause of premature failure [12]. The failure of the blade occurred during the trial run of the engine. The blade failed at the aerofoil hub and caused secondary damage to the blades in the remaining stages.

9.2 INVESTIGATION A schematic top view of the failed blade sample is shown in Figure 9.1. The fracture surface showed a smooth region (marked A), a comparatively rough region (marked B), and a shear-lip region (marked C). Low- and high-magnification photographs of the smooth region and shear lip are shown in Figure 9.2. The origin of the fracture was seen in the smooth region (marked by an arrow). A detailed fractography study was conducted using the scanning electron microscope (SEM). A small sample cut from the blade (as shown by the arrow in Figure 9.1) was utilized for microstructural observation. Electron probe microanalysis (EPMA) was used for chemical analysis. No coloration was observed on the fracture surface or on the rest of the blade, indicating that there was no thermal damage.

9.2.1 CHEMICAL ANALYSIS The specified chemistry and the chemistry of the sample evaluated by EPMA are presented in Table 9.1. The alloy chemistry is within the specification limits of IMI 550.

9.2.2 MICROSTRUCTURE The alloy IMI 550 is generally used in the α+β heattreated condition (900˚C/1h/AC + 500˚C/24h/AC). The microstructure of the sample showed equiaxed α and transformed β of approximately equal volumes (Figure 9.3). The microstructure appeared to be consistent with the heat treatment. Large α stringers (>100 µm) aligned along the blade axis were observed at many locations. Because the blades were machined from bar stock, no

oxygen enrichment or alpha casing was expected, and none was observed.

9.2.3 FRACTOGRAPHY The failed sample was seen in the SEM for a detailed observation of the fracture surface. The apparent origin of the fracture observed at low magnification is shown in Figure 9.2. The same is shown at higher magnification in Figure 9.4. The origin appeared to be associated with machining grooves, as seen Figure 9.4b. A prominent beach mark was observed on the fracture surface (Figure 9.5). Observation of the fracture surface at higher magnifications (Figure 9.6, Figure 9.7, and Figure 9.8) showed voids in the thumbnail-shaped origin. Flat smooth features (marked A in Figure 9.7) and tear ridges were observed throughout the fracture surface. Secondary cracking was also observed, and the extent of the secondary cracking was observed to increase at locations farther from the origin. These secondary cracks appeared to be aligned. The thumbnail shape of the origin, presence of prominent beach mark, alignment of secondary cracking parallel to the origin and beach mark, and smooth appearance of the fracture surface in region A are indicative of fatigue failure. Extensive tearing also suggests that the blade sample might have undergone fatigue deformation closer to lowcycle fatigue conditions. However, the fracture features did not provide conclusive and unambiguous evidence for the fatigue conditions, and the observed features should only be used in conjunction with other data such as stress analysis. Samples that fail under fatigue conditions usually exhibit fatigue striations on the fracture surface. However, the sample under investigation exhibited no such features. The appearance of striations in fatigue-tested samples, however, depends upon material and microstructural conditions. Alpha + beta heat-treated titanium alloys have not been found to exhibit these striations (Figure 9.9), while in β-treated conditions, titanium alloys usually show prominent striations (Figure 9.10). The fracture features of an α+β heat-treated laboratory-tested specimen shown

* All the figures in this chapter are from Gogia, A.K., Muraleedharan, K., Joshi, V.A., and Banerjee, D., Failure Investigation of HPC Blade, unpublished report, 2000. With permission.

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section through the origin and the presence of several deep machining notches (≈10 to 20 µm). The fracture appears to have initiated at one such notch. Upon tilting the sample, it was seen clearly that the fracture originated at the machining notch (Figure 9.14). Large numbers of small voids were observed near the fracture surface (Figure 9.13b). The density of these voids was observed to decrease with increasing distance from the fracture surface, as indicated by the computer-processed image shown in Figure 9.13c. These voids may be due to stress concentration ahead of the fatigue crack. The absence of these voids away from the fracture surface suggests that they were not present beforehand and are not due to material defect. The presence of machining notches in the fillet radius region (Figure 9.15) suggests the use of an improper machining and polishing process on the blades. This is also supported by the fact that these notches were seen even in an unused blade (Figure 9.16).

TABLE 9.1 Specified and Analyzed Composition of Alloy IMI 550 Element Al Sn Mo Si

Specifications (wt.%)

EPMA (wt.%)

3.0–5.0 1.5–2.5 3.0–5.0 0.3–0.7

4.2 2.5 4.3 0.6

Source: From Gogia, A.K., Muraleedharan, K., Joshi, V.A., and Banerjee, D., Failure Investigation of HPC Blade, unpublished report, 2000. With permission.

in Figure 9.9 are otherwise similar to those observed in the sample under investigation and are very distinct from those resulting from the other modes of failure, such as tensile overload. Region B revealed dimpled failure due to tensile overload, as shown in Figure 9.11. The tensile failure in region B can occur once the crack in region A has grown sufficiently large so that the stress level exceeds the tensile strength of the material in the remaining load-bearing section of the blade. Prima facie, the failure appeared to have occurred under fatigue conditions. The origin of the fatigue crack appeared to be associated with machine marks. To investigate further, the failed blade sample was sectioned through the origin (Figure 9.12) to observe underlying defects that might have initiated the crack. The sectioning was done by marking the origin precisely and cutting with a low-speed diamond saw. The section was polished and etched for observation under SEM. Figure 9.13 shows the

9.2.4 STRESS-CONCENTRATION EFFECTS OF A NOTCH A geometrical discontinuity in the blade, such as a machining notch, can result in stress concentration at the discontinuity. For a small elliptical notch, the maximum stress at the ends of a notch is given by the equation σmax = σ(1+2)a/b where a and b are axes of the elliptical notch. In the present case, the notch can be approximated as a semicircle and thus a ≈ b, in which case σmax = 3σ. The stress concentration due to machining can result in stresses close to yield, even at low applied stress, that can cause fatigue failure.

A B C

Fracture surface

FIGURE 9.1 Schematic sketch of the failed blade.

Polished for microstructure

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Origin 2 mm

A

C

a

Origin

A

C b

1 mm

Shear lip

FIGURE 9.2 Macrograph of the origin and the shear-lip regions A and C of Figure 9.1 shown at (a) low magnification and (b) high magnification.

9.2.5 ANALYSIS

OF THE

DEPOSITS

A small sample was cut from the blade area showing a thicker deposit and analyzed by electron probe microanalysis (EPMA). The morphology of the deposit is shown in Figure 9.17. The EPMA results showed that the deposit contains Al, Si, and O (Figure 9.18). The deposit could be oxides of aluminum and silicon.

9.3 CONCLUSION • •

• •

The microstructure and chemistry of the material were within specifications. Machining notches on the blade, observed mainly at the fillet radius region, were the result of an improper machining and polishing process. A deposit on the remaining blades — a result of the blade failure — showed oxides of Al and Si. Blade failure appeared to be due to fatigue. The fatigue crack was initiated at the site of a machining notch on the blade surface.

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FIGURE 9.3 Optical micrographs from the failed blade: (a) low-magnification image showing primary α + transformed β microstructure with elongated α stringers and (b) high-magnification micrograph showing primary α + transformed β microstructure with large α stringers.

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Origin

a

Origin

b

FIGURE 9.4 SEM micrographs of the fracture surface near the origin: (a) low magnification and (b) high magnification. The machine marks on the surface are clearly seen in (b).

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Origin

Beach mark

Shear lip

FIGURE 9.5 SEM micrograph shows distinct features of the fracture. A prominent beach mark, as shown, may be related to a stress jump in the life cycle of the failed blade. The final failure region, marked with a shear lip, is shown at the bottom of the micrograph.

FIGURE 9.6 Fracture features at the origin at a higher magnification. Voids are seen at the origin.

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FIGURE 9.7 Fracture features away from the origin, in between the origin and the beach mark, at a higher magnification. Flat smooth regions, marked A, and tear ridges are seen.

FIGURE 9.8 Fracture features outside of the beach mark showing secondary cracks at a higher magnification.

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FIGURE 9.9 Fatigue features on the fracture surface of α+β heat-treated Ti-6.5Al-3.2Mo-1.8Zr-0.25Si alloy tested under HCF condition.

FIGURE 9.10 Fatigue striations on the fracture surface of β heat-treated Ti-6.5Al-3.2Mo-1.8Zr-0.25Si alloy tested under HCF condition.

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FIGURE 9.11 Region B of Figure 9.1 shows ductile overload fracture features with dimples at higher magnification.

FIGURE 9.12 Micrograph of the blade sample showing the section through the origin that was sampled to investigate the microstructure of the blade underneath the fracture origin.

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Fracture surface

Origin

a

Fracture surface

b

50 µ m

FIGURE 9.13 Micrographs of the blade sample at the section through the origin: (a) secondary electron micrograph showing machining marks as indicated (the origin and the fracture surface are also marked); (b) backscattered electron micrograph showing the microstructure underneath the fracture surface; and (c) distribution of voids in the material below the fracture surface in the same area as in (b). The image in (c) is a binary image processed by an image-analysis routine to reveal only the darkest (void) areas in the micrograph in (b).

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50 µm c

FIGURE 9.13 (continued)

Fracture surface

Origin

FIGURE 9.14 Same sample as in Figure 9.13, in tilted condition, showing the origin and machine marks on the fillet radius as indicated.

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FIGURE 9.15 Micrographs showing machine marks (arrows) at the fillet radius sectioned through the origin.

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a

b

FIGURE 9.16 Micrographs of an unused blade: (a) aerofoil section and (b) root radius section. Arrows show machine marks.

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FIGURE 9.17 SEM micrograph shows morphology of the deposit on the blades.

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BSE Cu Strip

Deposit

Blade 50 µm

Ti

Al

Si

O

FIGURE 9.18 EPMA images show backscattered electron micrograph and x-ray elemental images from the same region taken across the blade with a deposit. The thickness of the deposit is about 100 µm, and the elemental images indicate the presence of Si, Al, and O. A copper adhesive tape strip (seen on the top part of the BSE micrograph) was attached to the outer surface of the blade to preserve the deposit during metallographic preparation of the cross section.

References 1. Titanium, Technology Information, Forecasting and Assessment Council, Department of Science and Technology, New Delhi, July 1991. 2. Gogia, A.K., Titanium (in-house publication), Vol. 2, Nos. 3 and 4, Defence Metallurgical Research Laboratory, Hyderabad, India, 1997. 3. Jaffee, R.I., Titanium Science and Technology, Vol. 3, Jaffee, R.I. and Burte, H.M., Eds., Plenum Press, New York, 1973, p. 1665. 4. Kornilov, I.I., The Science, Technology and Applications of Titanium, Jaffee, R.I. and Promisel, N.E., Eds., Pergamon Press, Oxford, 1970, p. 407. 5. Rosenberg, H.W., The Science, Technology and Applications of Titanium, Jaffee, R.I. and Promisel, N.E., Eds., Pergamon Press, Oxford, 1970, p. 851. 6. Gogia, A.K., doctoral thesis, Banaras Hindu University, Varanasi, India, 1990. 7. Banerjee, D., Saha, R.L., Mukherjee, D., and Muraleedharan, K., DMRL technical report, DMRL TR 8983, Defence Metallurgical Research Laboratory, Hyderabad, India, 1989. 8. Bhattacharjee, A., Joshi, V.A., Deshpande, D., Hussain, S.M., Nandy, T.K., and Gogia, A.K., DMRL technical report, DMRL TR 2000270, Defence Metallurgical Research Laboratory, Hyderabad, India, 2000. 9. Gogia, A.K., Banerjee, D., and Nandy, T.K., Metal. Trans. A., 21A, 609, 1990.

10. Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi, V.A., Sagar, P.K., and Banerjee, D., DMRL technical report, DMRL TR 94175, Defence Metallurgical Research Laboratory, Hyderabad, India, 1994. 11. Gogia, A.K., Nandy, T.K., Muraleedharan, K., Joshi, V.A., Kamath, S.V., and Banerjee, D., DMRL technical report, DMRL TR 93166, Defence Metallurgical Research Laboratory, Hyderabad, India, 1993. 12. Gogia, A.K., Muraleedharan, K., Joshi, V.A., and Banerjee, D., Failure Investigation of HPC Blade, unpublished report, 2000.

FURTHER READING 1. ASM, Metals Handbook: Fractography and Atlas of Fractographs, Vol. 9, American Society for Metals, Metals Park, OH, 1974. 2. Leyens, C. and Peters, M., Eds., Titanium and Titanium Alloys, Wiley-VCH GmbH & Co. KgaA, Germany, 2003. 3. Polmear, I.J., Light Alloys: Metallurgy of the Light Metals, Edward Arnold, London, 1989. 4. Donachie, M.J., Jr., Titanium, A Technical Guide, ASM International, Metals Park, OH, 1989.

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Index A Acicular α β alloys Ti-10V-2Fe-3Al (Ti-10-2-3), 101 Ti-10V-4.5Fe-1.5Al, 109 near-α alloys, 23, 32 Ti-24Al-11Nb, 130 Acicular β, VT9, 70 Aero engine rotor blade failure, see Failure investigation report, HPC-I aero engine blade Aerospace applications, 7–8, 10 Aging conditions, β alloys, see β alloys, microstructure and fractography Allotropic modifications, 9, 11 Alloy C, chemical composition, 17 Alloying element effects, 9 α, primary aero engine blade failure analysis, 206 α + β alloys Ti-6Al-4V, 59 VT9, 70 near-α alloys, IMI 834, 38, 49–50 Ti-25Al-15Nb, 171 α alloys, 19–23; see also Near-α alloys chemical composition, 17 crystal structure, 9 metallurgy, 9–10 phases observed in, 11 α phase aging and, 97 physical metallurgy, 11 α + β alloys chemical composition, 17 metallurgy, 10 microstructure, evolution of, 11, 13–14 microstructure and fractography, 59–95 Ti-6Al-4V, 59, 60–70 VT9, 59, 70–95 α + β solution/heat treatment aero engine blade failure analysis, 203–204 α + β alloys, VT9, 71–86 β alloys, 97 Ti-10V-2Fe-3Al (Ti-10-2-3), 102 Ti-10V-4.5Fe-1.5Al, 109–110 titanium aluminides, 111 Ti-24Al-11Nb, 114, 116, 121, 123 Ti-24Al-11Nb-4Ta, 173–179 Ti-24Al-20Nb, 139–144 Ti-24Al-27Nb, 150–158, 164 Ti-25Al-14Nb-1Mo, 185–191 Ti-25Al-15Nb, 165–170 α precipitates, β alloys Ti-10V-2Fe-3Al (Ti-10-2-3), 98 Ti-10V-4.5Fe-1.5Al, 103 α stringers, aero engine blade failure analysis, 206 α2 (Ti3Al), 10 chemical composition, 17 microstructure, evolution of, 13, 15 phases observed in, 11

α2 failure Ti-24Al-11Nb, 116 Ti-24Al-20Nb, 141, 143 α2 phase Ti-24Al-11Nb, 114, 121, 125, 127, 133–135, 137 Ti-24Al-11Nb-4Ta, 173–175, 179–180 Ti-24Al-20Nb, 141, 145 Ti-25Al-14Nb-1Mo, 185–187 Ti-25Al-15Nb, 164–166, 170 α2 + γ Ti-47Al-2Nb-2Cr, 192 Ti-48Al-4Nb-1Mo, 194 α volume fraction, near-α alloys, 23 Aluminides, see Titanium aluminides Aluminum α alloys, 19 chemical composition, 17–18 effects of alloying elements, 9–10 Aluminum equivalent (Rosenberg criterion), 9–10 Aluminum-rich phase, Ti-24Al-27Nb, 158–159 Analysis of deposits, HPC-I aero engine blade failure investigation, 205, 216–217 Applications of titanium alloys, 7–9 Architecture, applications of titanium alloys, 7 Automotive applications, 7–8

B B1/β ′, 1 1 B2/β 2 microstructure, evolution of, 15 phases observed in alloys, 11 B2 grains, Ti-24Al-27Nb, 156 B2 phase titanium aluminides, 111 Ti-24Al-11Nb, 114, 116, 121, 125, 127, 130, 132–133 Ti-24Al-11Nb-4Ta, 173–175 Ti-24Al-20Nb, 145 Ti-24Al-27Nb, 150–152, 158 Ti-25Al-14Nb-1Mo, 185–187 Ti-25Al-15Nb, 164–166 Basket-weave structure, 15 Ti-24Al-20Nb, 146 Ti-24Al-27Nb, 159 Ti-25Al-15Nb, 171 bcc phases, 11 Beach mark, aero engine blade failure analysis, 203, 208–209 β alloys chemical composition, 17 crystal structure, 9 metallurgy, 9–10 metastable, 10 microstructure, evolution of, 11–12 microstructure and fractography, 97–110 Ti-10V-2Fe-3Al (Ti-10-2-3), 97–102 Ti-10V-4.5Fe-1.5Al, 102–110 phases observed in, 11 β boundaries near-α alloys, IMI 834, 39

221

222

titanium aluminides Ti-24Al-11Nb-4Ta, 181–182 Ti-24Al-27Nb, 158 β−C, 97 β microstructure, aero engine blade failure analysis, 206 β phase fracture near-α alloys, 37 Ti-24Al-20Nb, 141, 143–144 β solution/heat treatment α + β alloys, VT9, 91–92, 95 β alloys, 97 Ti-10V-2Fe-3Al (Ti-10-2-3), 97–101 Ti-10V-4.5Fe-1.5Al, 103–108 and striation, 203 titanium aluminides Ti-24Al-11Nb-4Ta, 179–185 Ti-24Al-20Nb, 145–150 Ti-24Al-27Nb, 159–164 Ti-25Al-15Nb, 170–173 β stabilizers, see α + β alloys β structure/microstructure α + β alloys, VT9, 86 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 26, 28 IMI 834, 51 β-Ti alloy, chemical composition, 18 Bilbao, Guggenheim Museum, 7 Biomedical applications, 7–8 Body-centred cubic structure, β alloys, 9 Boron, effects of alloying elements, 9 Boundary initiated failure titanium aluminides Ti-24Al-11Nb, 131 Ti-24Al-11Nb-4Ta, 183 Boundary regions grain, 12 intergranular fracture, 3 near-α alloys, OT4-1, 24 Ti-25Al-15Nb, 171 IMI 685, 38 titanium aluminides Ti-24Al-11Nb, 131 Ti-24Al-11Nb-4Ta, 180, 183 Ti-24Al-27Nb, 158–159 Brittle fracture, Ti-10V-2Fe-3Al (Ti-10-2-3), 100–101 Bromine, effects of alloying elements, 9

C Carbon, effects of alloying elements, 9 Chemical analysis, HPC-I aero engine blade failure investigation, 203–204 Chemical applications, 7–8 Chemical composition of alloys, 17–18 Chlorine, effects of alloying elements, 9 Chromium chemical composition of alloys, 17–18 effects of alloying elements, 9–10 Cleavage, fractography, principles of, 1 Cleavage facets near-α alloys, 23 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 36, 37 IMI 834, 41, 45–46, 57 titanium aluminides, 111 Ti-24Al-11Nb, 118, 120–121, 128–129 Ti-24Al-11Nb-4Ta, 181, 183–185

Titanium Alloys: An Atlas of Structures and Fracture Features

Ti-24Al-20Nb, 141–144, 147 Ti-24Al-27Nb, 153, 155–156, 158, 161, 164 Ti-25Al-14Nb-1Mo, 190–191 Ti-25Al-15Nb, 170, 173 Ti-47Al-2Nb-2Cr, 196, 198 Ti-48Al-4Nb-1Mo, 200–201 Cleavage fractures near-α alloys, IMI 834, 40–41, 43 titanium aluminides Ti-24Al-11Nb, 113, 115, 117, 126 Ti-24Al-11Nb-4Ta, 176, 178–179, 181 Ti-24Al-20Nb, 149 Ti-24Al-27Nb, 153–154, 156–157, 160, 163 Ti-25Al-14Nb-1Mo, 188–189 Ti-25Al-15Nb, 167–168 Ti-47Al-2Nb-2Cr, 196 Cleavage steps, titanium aluminides Ti-24Al-20Nb, 149 Ti-25Al-15Nb, 172 Closely packed hexagonal structure, α alloys, 9 Coalescence, intergranular fracture, 3 Coarse cleavage, titanium aluminides Ti-24Al-11Nb, 130 Ti-24Al-11Nb-4Ta, 181 Ti-24Al-27Nb, 162 Ti-25Al-14Nb-1Mo, 189 Ti-25Al-15Nb, 169–170 Coarse colony structure, Ti-24Al-20Nb, 145 Coarse granular fracture, IMI 834, 56 Cobalt, effects of alloying elements, 9 Colony boundaries IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 38 Ti-24Al-11Nb, 134 Colony structure, titanium aluminides Ti-24Al-11Nb, 133 Ti-24Al-11Nb-4Ta, 180 Ti-24Al-20Nb, 145 Ti-24Al-27Nb, 158 Ti-48Al-4Nb-1Mo, 194 Commercially pure (CP) titanium applications, 7 chemical composition, 18 microstructure and fractography, 19–22 Conical dimple, α alloys, 22 Continuous solid solution formation, 9 Conventional alloys, microstructure evolution, 11–15 Cooling rate, see also Specific heat treatments microstructure evolution, 11 titanium aluminides Ti-24Al-20Nb, 146–148 Ti-24Al-27Nb, 163–164 Copper, effects of alloying elements, 9 Corrosion resistance, 7–8 Covalent bonding, 9 CP titanium, see Commercially pure (CP) titanium Creep strength, 8 Creep testing, near-α alloys, 23 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 35–37, 38 IMI 834, 52–57 Creep-to-rupture fractures, intergranular, 3 Crystal structure, 3 cleavage fracture, 1 microstructure, 11 types of titanium alloys, 9 Crystalline fracture, Ti-10V-4.5Fe-1.5Al, 106–107

223

Index

Cup-and-cone fracture α + β alloys, Ti-6Al-4V, 59–60, 62, 64 β alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 102 Cyclic relaxation fracture, IMI 834, 46

D Delamination, Ti-47Al-2Nb-2Cr, 198 Deposits, aero engine blade failure analysis, 205, 217 Dimple rupture aero engine blade failure analysis, 204 fractography, principles of, 1–2 Dimples aero engine blade failure analysis, 210 α alloys, 20–22 α + β alloys, 59 Ti-6Al-4V, 60–65, 67 VT9, 72–73, 75, 77–80, 90–91, 93 β alloys, 97 Ti-10V-2Fe-3Al (Ti-10-2-3), 100–102 Ti-10V-4.5Fe-1.5Al, 104–105, 107–108, 110 near-α alloys, 23 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 30–31, 34, 37 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27–28 IMI 834, 40–41, 46–48, 52, 54–57 OT4-1, 25 titanium aluminides, 111 Ti-24Al-11Nb, 115–116, 118, 120, 122, 124, 129, 132 Ti-24Al-11Nb-4Ta, 178–179 Ti-24Al-20Nb, 141–144, 147, 150 Ti-24Al-27Nb, 156, 158 Ti-25Al-14Nb-1Mo, 188 Ti-47Al-2Nb-2Cr, 198 Ductile fracture β alloys, 97 Ti-10V-2Fe-3Al (Ti-10-2-3), 99–102 Ti-10V-4.5Fe-1.5Al, 104, 106, 108 dimple rupture, 1–2 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 29 IMI 834, 52–53, 55 titanium aluminides, Ti-24Al-11Nb, 129 Ductile overload, aero engine blade failure analysis, 210

E Electron probe microanalysis (EMPA), aero engine blade failure analysis, 203, 205, 217 Engine rotor blade failure, see Failure investigation report, HPC-I aero engine blade Engine valves, applications of titanium alloys, 7 Equiaxed α, primary β alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 101 Ti-25Al-15Nb, 171 Equiaxed α2, Ti-24Al-11Nb, 124 Equiaxed α2 and B2, Ti-24Al-11Nb, 116, 123 Equiaxed dimples, α + β alloys Ti-6Al-4V, 59, 61, 63 VT9, 75, 80, 90 Equiaxed grains, β alloys Ti-10V-2Fe-3Al (Ti-10-2-3), 97 Ti-10V-4.5Fe-1.5Al, 103 Eutectoid transformation, 9–10

F Faceted failure Ti-24Al-11Nb, 124, 133 Ti-47Al-2Nb-2Cr, 197 Facets, VT9, 93 Failure investigation report, HPC-I aero engine blade, 203–217 analysis of deposits, 205, 216–217 chemical analysis, 203–204 conclusion, 205 fractography, 203–204, 205–215 microstructure, 203, 206 stress-concentration effects of notch, 204 Fan-like cleavage facet, Ti-48Al-4Nb-1Mo, 200 Fatigue aero engine blade failure analysis, 203, 210 fractography, principles of, 2–3, 4 Fatigue resistance properties, 7–8 Fatigue testing α + β alloys Ti-6Al-4V, 66–69 VT9, 81–86, 94 near-α alloys, IMI 834, 42–46 Feathery features, 1 Ti-24Al-11Nb, 118, 127, 134 Ti-24Al-11Nb-4Ta, 185 Ti-48Al-4Nb-1Mo, 199, 201 Fine cleavage fracture Ti-24Al-27Nb, 160 Ti-25Al-14Nb-1Mo, 188 Fissures α + β alloys, Ti-6Al-4V, 67 near-α alloys, IMI 834, 43 Flat fracture, VT9, 76 Fluorine, effects of alloying elements, 9 Fluting, Ti-24Al-11Nb, 116 Forge temperature Ti-47Al-2Nb-2Cr, 191–198 Ti-48Al-4Nb-1Mo, 194–195, 199–201 Fractography, 1–5 α alloys, 19–22 cleavage, 1 dimple rupture, 1–2 fatigue, 2–3, 4 HPC-I aero engine blade failure investigation, 203–204, 205–215 intergranular, 3–5 Fracture behavior microstructure and, 15 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 26–27 OT4-1, 24 Fracture toughness testing, Ti-24Al-20Nb, 149

G Gallium, effects of alloying elements, 9 Gamma (TiAl), 10, 173–185 chemical composition, 17 microstructure, evolution of, 15 phases observed in, 11 Ti-24Al-11Nb-4Ta, 111, 173–185 Ti-25Al-14Nb-1Mo, 185–191 Ti-47Al-2Nb-2Cr, 111, 191–194, 195–199 Ti-48Al-4Nb-1Mo, 111, 194–195, 199–201

224

Gamma phase Ti-47Al-2Nb-2Cr, 192–193 Ti-48Al-4Nb-1Mo, 194 Globular α, Ti-10V-4.5Fe-1.5Al, 109 Grain boundaries, Ti-24Al-11Nb-4Ta, 180 Grain boundary failure, titanium aluminides Ti-24Al-11Nb, 131 Ti-24Al-11Nb-4Ta, 183 Grain boundary phases, 12 intergranular fracture, 3 near-α alloys, OT4-1, 24 Ti-25Al-15Nb, 171 Grain size, evolution of microstructure, 13 Granular fracture, see Intergranular fracture Guggenheim Museum, Bilbao, 7

H Hafnium, effects of alloying elements, 9 Herringbone structure, 1 High-cycle fatigue, 2 High-pressure compressor (HPC-I) aero engine blade failure, see Failure investigation report, HPC-I aero engine blade Hydrogen, effects of alloying elements, 9 Hydrogen damage, intergranular fracture, 3

I Image processing, aero engine blade failure analysis, 212 IMI 550 chemical composition, 17 engine rotor blade failure, see Failure investigation report, HPC-I aero engine blade IMI 685 chemical composition, 18 microstructure and fractography Ti-6Al-5-Zr-0.5Mo-0.25Si, 29–38 Ti-6Al-5-Zr-0.5Mo-0.30Si, 23, 26–28 IMI 834 chemical composition, 18 microstructure and fractography, 23, 38–57 Impact testing, VT9, 76–79, 91–92 Inclusions, Ti-10V-4.5Fe-1.5Al, 105 Indium, effects of alloying elements, 9 Intergranular facets, Ti-24Al-11Nb, 130 Intergranular fracture α + β alloys, VT9, 91–92 β alloys, Ti-10V-4.5Fe-1.5Al, 97, 106, 108 fractography, principles of, 3–5 near-α alloys, IMI 834, 56 titanium aluminides, Ti-24Al-11Nb, 129 Intermetallics (Ti3Al, Ti2AlNb, TiAl), see also Titanium aluminides high-temperature properties, 10 microstructure, 10 phases, precipitates, 11 Interstitial solid solution formation, 9 Iodine, effects of alloying elements, 9 Ionic bonding, effects of alloying elements, 9 Iron chemical composition, 17–18 effects of alloying elements, 9–10

L Lamellae Ti-47Al-2Nb-2Cr, 192–193 Ti-48Al-4Nb-1Mo, 194–195

Titanium Alloys: An Atlas of Structures and Fracture Features

Laths near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 36, 38 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27 titanium aluminides, 111 Ti-24Al-11Nb-4Ta, 179–180, 184 Ti-24Al-20Nb, 147–148 Ti-24Al-27Nb, 158–159 Ti-25Al-15Nb, 170 Leaf-like cleavage facets, Ti-25Al-15Nb, 173 Limited solid solution formation, 9

M Machining grooves, aero engine blade failure analysis, 203–204, 207, 212–215 Manganese chemical composition, 17–18 effects of alloying elements, 9 Marine applications, 7–8 Martensitic transformation, 11–13 Mechanical properties, microstructure and, 15 Metallurgy, physical, see Physical metallurgy Metastable β alloys, 10–11 α + β solution treatment and aging, 97 chemical composition, 17 Microstructure, see also Specific alloys and alloy classes aero engine blade failure analysis, 212 α alloys, 19–22 applications of titanium alloys, 8 HPC-I aero engine blade failure investigation, 203, 206 physical metallurgy, 10–16 conventional alloys, 11–15 titanium aluminides, 15 Microvoids, see Voids/microvoids Mixed mode fracture β alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 100–101 near-α alloys, IMI 834, 57 Ti-24Al-11Nb, 130 Molybdenum β alloys, 97 chemical composition, 17–18 effects of alloying elements, 9–10 titanium aluminides, 111

N Near-α alloys chemical composition, 17 metallurgy, 10 microstructure and fractography, 23–57 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 29–38 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 23, 26–28 IMI 834, 23, 38–57 OT4-1, 23–25 Near-β alloys, 10 Necking, α + β alloys, 59 Nickel, effects of alloying elements, 9 Niobium β alloys, 97 chemical composition, 17–18 effects of alloying elements, 9–10 Niobium-lean phase, Ti-24Al-27Nb, 158–159 Nitrogen, effects of alloying elements, 9 Notch, stress-concentration effects of, 204

225

Index

Notch tensile testing, IMI 834, 40–41 Nucleation, 1, 11

O O phase microstructure, evolution of, 15 titanium aluminides, 111 Ti-24Al-20Nb, 139, 141, 145 Ti-24Al-27Nb, 150–151, 158 Ti-25Al-15Nb, 164 O phase fracture, Ti-24Al-20Nb, 141, 143–144 Offshore applications, 7 Omega, 11, 13 Orthorhombic structure chemical composition, 17 microstructure, 11 titanium aluminides, 111 OT4-1 chemical composition, 18 microstructure and fractography, 23–25 Overload fracture α alloys, 20–21 α + β alloys, Ti-6Al-4V, 67 near-α alloys, IMI 685, 34 Oxidation resistance, 8 Oxide deposit, aero engine blade failure analysis, 205, 216–217 Oxygen α alloys, 19 effects of alloying elements, 9 Oxygen content, VT9, 70–80, 87–93

P Peritectoid transformation, 9 Phase diagrams, alloys, 11 Phosphorus, effects of alloying elements, 9 Physical metallurgy, 7–15 alloying element effects, 9 applications of titanium alloys, 7–9 microstructure, evolution of, 10–16 conventional alloys, 11–15 titanium aluminides, 15 phases observed in alloys, 11 physical properties of unalloyed titanium, 9 types of titanium alloys, 9–10 Physical properties, unalloyed titanium, 9 Plastic deformation, 1 Precipitates aging and, 97 β alloys Ti-10V-2Fe-3Al (Ti-10-2-3), 98 Ti-10V-4.5Fe-1.5Al, 103 microstructure, 11 Primary α, see α + β solution/heat treatment; α, primary

Q Quasicleavage, 1, 111 Ti-24Al-11Nb, 119, 136, 138 Ti-24Al-20Nb, 148–149 Quaternary additions, titanium aluminides, 111

R Radial fracture, Ti-24Al-11Nb, 119, 137 Recrystallization, 10, 111 Ti-24Al-27Nb, 156 Ti-47Al-2Nb-2Cr, 191–192, 194 Ripples, dimple rupture, 1 River markings, 1, 111 Ti-24Al-20Nb, 150 Ti-24Al-27Nb, 156, 162–164 Ti-25Al-14Nb-1Mo, 191 Ti-25Al-15Nb, 169–170, 172–173 Ti-47Al-2Nb-2Cr, 196 Ti-48Al-4Nb-1Mo, 200 Rock candy appearance, intergranular fracture, 3 Rosenberg criterion, 9–10 Rotor blade failure, see Failure investigation report, HPC-I aero engine blade Rough fracture near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 35 IMI 834, 39 titanium aluminides Ti-24Al-11Nb, 125 Ti-24Al-11Nb-4Ta, 183 Ti-24Al-27Nb, 157 Ti-25Al-14Nb-1Mo, 187–188 Ti-25Al-15Nb, 169, 171 Ti-47Al-2Nb-2Cr, 195 Ti-48Al-4Nb-1Mo, 199

S Secondary α, 13 Secondary cracks aero engine blade failure analysis, 203, 209 α + β alloys Ti-6Al-4V, 69 VT9, 76–78, 84, 86, 91, 93–95 β alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 100–101 near-α alloys, IMI 834, 41, 43–45, 48, 53, 55 titanium aluminides Ti-24Al-11Nb, 113, 118–122, 126–128, 134–135 Ti-24Al-11Nb-4Ta, 177, 181, 185 Ti-24Al-20Nb, 142 Ti-24Al-27Nb, 155, 157, 160–162 Ti-25Al-14Nb-1Mo, 188, 190 Ti-25Al-15Nb, 167–170 Ti-47Al-2Nb-2Cr, 196–197 Ti-48Al-4Nb-1Mo, 200 Selenium, effects of alloying elements, 9 Serpentine glide α alloys, 21 dimple rupture, 1 near-α alloys, OT4-1, 25 Silicides, microstructure evolution, 13 Silicon, chemical composition, 17–18 Slip-plane fracture, 2 Smooth fracture α + β alloys, VT9, 77 titanium aluminides Ti-24Al-11Nb, 123 Ti-24Al-11Nb-4Ta, 175–176 Ti-24Al-20Nb, 143 Ti-24Al-27Nb, 154 Ti-25Al-15Nb, 166–168

226

Solid solution formation, 9 Spheroidization, 10 Stabilizers, effects of alloying elements, 9 Strain markings, dimple rupture, 1 Stress-concentration effects of notch, 204 Stress corrosion, intergranular fracture, 3 Stress rupture, IMI 834, 47–49 Stretching α alloys, 21 dimple rupture, 1 Striations aero engine blade failure analysis, 203, 210 α + β alloys Ti-6Al-4V, 69 VT9, 82, 84, 86, 94–95 fracture mechanics, 2–3 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 33 IMI 834, 43–44, 47 Sulfur, effects of alloying elements, 9

T Tantalum β alloys, 97 chemical composition, 17–18 effects of alloying elements, 9–10 titanium aluminides, 111 Tear ridges α + β alloys, VT9, 77 β alloys, Ti-10V-4.5Fe-1.5Al, 104 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 31, 34, 36 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27, 28 IMI 834, 40, 52, 54 Ti-24Al-11Nb, 72, 134 Tellurium, effects of alloying elements, 9 Temperature, see also Specific alloys and testing regimes creep-to-rupture fractures, 3 high-temperature properties of intermetallics, 10 Ti-24Al-27Nb solution treatment, 152, 155 Tensile failure, aero engine blade failure analysis, 204 Tensile testing α + β alloys Ti-6Al-4V, 59, 60 VT9, 73–75, 87–89 β alloys Ti-10V-2Fe-3Al (Ti-10-2-3), 98–102 Ti-10V-4.5Fe-1.5Al, 104–110 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 26, 28–29, 35–36, 38 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 25 IMI 834, 39–41 titanium aluminides, 111 Ti-24Al-11Nb, 112–138 Ti-24Al-11Nb-4Ta, 175–178, 181–185 Ti-24Al-20Nb, 140, 142–144, 146–149 Ti-24Al-27Nb, 152–157, 160–164 Ti-25Al-14Nb-1Mo, 187–190 Ti-25Al-15Nb, 166–169, 171–173 Ti-47Al-2Nb-2Cr, 195–199 Ti-48Al-4Nb-1Mo, 199–201 Ternary additions, titanium aluminides, 111 Terraced fracture surface, Ti-24Al-11Nb, 136 Ti2AlNb

Titanium Alloys: An Atlas of Structures and Fracture Features

chemical composition, 17 microstructure, evolution of, 15 phases observed in, 11 Ti2Cu, 11, 13 Ti3Al, see α2 (Ti3Al) Time, temperature, and transformation (TTT) diagram, 11 TIMETAL LCB, 17, 97 Tin α alloys, 19 chemical composition, 17–18 Tire cracks, 3 Titanium aluminides, 111–201 chemical composition, 17–18 metallurgy, 10, 11, 15 microstructure, evolution of, 15 microstructure, TiAl-based (γ), 111, 173–201; see also γ phase Ti-24Al-11Nb-4Ta, 111, 173–185 Ti-25Al-14Nb-1Mo, 185–191 Ti-47Al-2Nb-2Cr, 111, 191–194, 195–199 Ti-48Al-4Nb-1Mo, 111, 194–195, 199–201 microstructure, Ti3Al-based Ti-24Al-11Nb, 111–112, Ti-24Al-20Nb, 111, 139–150 Ti-24Al-27Nb, 111, 150–164 Ti-25Al-15Nb, 111, 164–173 Ti-5Al-2.5Sn, 19 Ti-6Al-4V, 10, 12–14 Titanium-vanadium alloys chemical composition, 17–18 microstructure and fractography, 97–102 Ti-10V-4.5Fe-1.5Al, 102–110 Tongue formation, 1 Toughness, 8 Transcrystalline fractures, 111 Ti-24Al-11Nb, 114, 132 Ti-24Al-11Nb-4Ta, 177–178, 182, 184 Ti-24Al-20Nb, 146, 148 Ti-24Al-27Nb, 111 Ti-25Al-14Nb-1Mo, 189–190 Ti-25Al-15Nb, 172 Transformed B2, Ti-24Al-11Nb, 135, 137 Transformed β, 13 α + β alloys Ti-6Al-4V, 59 VT9, 70 near-α alloys, IMI 834, 49–50 Transgranular crack, Ti-47Al-2Nb-2Cr, 195 Transgranular fracture β alloys, Ti-10V-2Fe-3Al (Ti-10-2-3), 98 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 36 IMI 834, 39 titanium aluminides Ti-24Al-11Nb, 124, 128–129, 133, 137 Ti-24Al-20Nb, 149 Ti-24Al-27Nb, 160–161, 163 Twinning, Ti-47Al-2Nb-2Cr, 1, 191–192

V Vanadium β alloys, 97 chemical composition, 17–18 effects of alloying elements, 9–10 Voids/microvoids

227

Index

aero engine blade failure analysis, 204, 208 β alloys Ti-10V-2Fe-3Al (Ti-10-2-3), 98, 100 Ti-10V-4.5Fe-1.5Al, 110 dimple rupture, 1 intergranular fracture, 3 near-α alloys IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.25Si), 31, 38 IMI 685 (Ti-6Al-5-Zr-0.5Mo-0.30Si), 27 IMI 834, 52–54 VT9 chemical composition, 18 microstructure and fractography, 59, 70–95

W Wallner lines, 1 Water quenching, titanium aluminides, 111 Widmanstätten α structure, 11–12, 35, 37

Z Zirconium α alloys, 19 chemical composition, 17–18 effects of alloying elements, 9