Nickel-Titanium Materials: Biomedical Applications 9783110666113, 9783110666038

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Yoshiki Oshida, Toshihiko Tominaga NiTi Materials

Also of interest Advanced Materials van de Ven, Soldera (Eds.), 2019 ISBN 978-3-11-053765-9, e-ISBN 978-3-11-053773-4

Shape Memory Polymers: Theory and Application Kalita, 2018 ISBN 978-3-11-056932-2, e-ISBN 978-3-11-057017-5

Intelligent Materials and Structures Abramovich, 2016 ISBN 978-3-11-033801-0, e-ISBN 978-3-11-033802-7

Metals and Alloys: Industrial Applications Benvenuto, 2016 ISBN 978-3-11-040784-6, e-ISBN 978-3-11-044185-7

Yoshiki Oshida, Toshihiko Tominaga

NiTi Materials

Biomedical Applications

Author Prof. Yoshiki Oshida School of Dentistry University of California San Francisco 513 Parnassus Ave San Francisco CA 94153-0340 USA Dr. Toshihiko Tominaga Department of Peridontology and Endodontology Hokkaido University Chome Kita 5 060-8648 Sapporo Kita Ward Japan

ISBN 978-3-11-066603-8 e-ISBN (PDF) 978-3-11-066611-3 e-ISBN (EPUB) 978-3-11-066621-2 Library of Congress Control Number: 2020942826 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2020 Walter de Gruyter GmbH, Berlin/Boston Cover image: sujit kantakad / iStock / Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Prologue This book is based on a review of about 3,500 carefully selected articles and presents itself as a typical example of evidence-based learning (EBL). Evidence-based literature reviews can provide foundation skills in research-oriented bibliographic inquiry, with an emphasis on such review and synthesis of applicable literature. Information is gathered by surveying a broad array of multidisciplinary research publications written by scholars and researchers. In order for EBL to be used effectively, the content of every publication must be critically evaluated in terms of its degree of reliability. There is an established protocol for ranking the reliability of sources – this is an especially useful tool in the medical and dental fields. The ranking, from the most to least reliable evidence source, follows: (1) clinical reports using placebo and double blind studies; (2) clinical reports not using placebo, but conducted according to well-prepared statistical test plans; (3) study reports on time effect on one group of patients during predetermined period of time; (4) study/comparison reports, at one limited time, on many groups of patients; (5) case reports on a new technique and/or idea; and (6) retrospective reports on clinical evidence. Unfortunately, the number of published articles increases in this descending reliability order. The greatest advantage of the EBL review is that it helps identify common phenomena in a diversity of fields and literature. It is then possible to create synthetic, inclusive hypotheses that deepen and further our understanding. As important as the diversity of source material is to the EBL review, it can pose unique challenges. The sources used in this review are mainly journal articles published in the medical/dental and engineering fields. These two groups’ studies have very different analytical criteria and inherent issues. In medical/dental journals, authors typically present statistically analyzed results; this lends these studies a high degree of reliability. Though we may be confident in the conclusions drawn from statistical analysis, it can be difficult to develop generalized ideas from this literature. Some controversy exists in the medical and dental fields on the relationship between in vitro and in vivo test results. This situation is further complicated and confusing because among the various in vivo tests exists a wide variety of animal model species. The in vitro studies are also not without weakness – it is almost impossible to broadly extrapolate in vitro test results, since it is very rare to find articles where identical test methods are employed. In contrast to medical/dental studies, the data presented in the engineering journals are normally not subjected to statistical analysis. The characterization of data is considered more important to interpretation results because researchers try to explain phenomena, mechanisms, and kinetics. Discoveries in the engineering realm are vitally important to the advancement of medicine and dentistry: the materials and most of the technologies currently employed in medical and dental fields were originally developed in the engineering field. In this book, the authors draw upon https://doi.org/10.1515/9783110666113-202

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 Prologue

our unique experience in both fields to bridge the gap between medical/dental and engineering research. For those who might be interested in literature survey in similar scope of this book, here is a lengthy list of journals: Journal of Light Metals, Journal of Japan Institute of Light Metals, Journal of Advances Oral Research, Advances Dent Res, Journal of Less-Common Metals, Journal of Surgical Orthopaedic Advances, British Medical Journal, British Editorial Society of Bone and Joint Surgery, Cell, Clinical Implant Dentistry and Related Research, Clinical Oral Investigations, Critical Review in Oral Biology and Medicine, General Dentistry, Rare Metal Materials and Engineering, Werktoffe und Korrosion (Materials and Corrosion), International Dental Journal, International Journal of Oral & Maxillofacial Implants, International Journal of Oral Surgery, International Journal of Periodontics & Restorative Dentistry, International Journal of Prosthodontics, Implant Dentistry, International Journal of Molecular Sciences, Journal of Arthroplasty, Journal of Bacteriology, Journal of Bone and Joint Surgery, Journal of Bone and Mineral Research, Journal of Cardiovascular, Engineering & Technology, Journal of Chronic Diseases, Journal of Clinical Investigations, Journal of Clinical Pathology, Journal of Prosthetic Dentistry, Journal of Oral Implants, Journal of Oral Implantology, Journal of Oral Rehabilitation, Journal of Orthopaedic Trauma, Journal of Prosthodontics, Journal of Molecular Cell Biology, Journal of Orthopaedics, Oral Microbiology, Immunology, Proceedings of the Royal Society, Tissue Engineering, Toxicology, Dentistry and Practices, Journal of Dentistry and Oral Disorders, Journal of Dental and Oral Health, Orthodontic Waves, Progress in Organic Coatings, Quintessence International, Medical Devices & Technology, Journal of Medical Devices, Medical Devices: Evidence and Research, The Open Medical Devices Journal, Medical Science Monitor, The Journal of Adhesion, Journal of Japanese Society for Dental Materials and Devices, Journal of Alloys and Compounds, CrystEngComm, Journal of Electrochemical Society, The American Society for Metals, Wear, Journal of Vacuum Science and Technology, Journal of the Japan Institute of Metals, Dental Material Journal, Metallurgical and Materials Transaction A, Metallurgical and Materials Transaction B, Materials Letters, Material Science and Engineering: A, Material Science and Engineering: B, Material Performance, Material Science Forum, Transactional Society of Biomaterials, Thin Solid Films, Journal of Materials Chemistry, Journal of Material Science, Journal of Material Science Letters, Journal of Materials Processing Technology, Journal of Materials Science: Materials in Medicine, Journal of Nanoscience and Nanotechnology, Journal of Biochemical and Biophysical Methods, Journal of Metals, Journal of Biomechanics, Journal of Biomedical Materials Research A, Journal of Biomed Mater Res B, Journal of Bio-Medical Materials & Engineering, Journal of Applied Biomaterials, Journal of Association for Advancement of Medical Instrumentation, International Journal of Nanomedicine, Dental Materials Journal, Clinical Materials, Biomedical Materials, BioMedical Engineering OnLine, Biomedical Journal, CRC Critical Reviews in Biocompatibility, Annals of Biomedical Engineering, ACS Nano, ASM International, Applied Surface Science, Advanced Material Process, Journal of Materials and Manufacturing Processes, Journal of Colloid and

Prologue 

 VII

Interface Science, Journal of the Less-Common Metals, Journal of Adhesion Science an Technology, Journal of the American Ceramic Society, Journal of Applied Physics, International Journal of Implant Dentistry, American Journal of Orthodontics and Dentofacial Orthopedic, The Angle Orthodontist, Journal of the Mechanical Behavior of Biomedical Materials, Materials and Manufacturing Processes, American Journal of Dentistry, Acta Biomaterialia, Acta Materialia, Biomaterials, Surface and Coatings Technology, Surface Technology, Tribology Industry, Biotribology, Digest Journal of Nanomaterials and Biostructures, Journal of Biomaterials Science-Polymer Edition, Journal of Material Design and Applications, Materials & Design, Journal of Engineering Tribology, Biomaterials Science: Processing, Properties and Applications, Scanning, Advanced Engineering Materials, Advanced Healthcare Materials, Advances in Materials Science and Engineering, Materials and Corrosion, Corrosion, Corrosion Science, Electrochimica Acta, Bioelectrochemistry, Biochemistry, Chemistry of Materials, International Archives of Allergy and Applied Immunology, Langmuir, Journal of Chemical Education, ACS Applied Materials and Interfaces, ACS Biomaterials Science and Engineering, Industrial and Engineering Chemistry Research, Faraday Discussions, Colloids Surface B: Biointerfaces, Journal of Electroceramics, Surface and Interface Analysis, Endodontic Topics, Contact Dermatitis, Journal of Endodontics, International Endodontic Journal, European Endodontic Journal, Journal of Investigative Dermatology, Tribology Letter, Transactions of the Electrochemical Society. We suspect that there should be more than the above-mentioned journals. This book discusses the distinct and unique properties and applications of the shape-memory alloys. The first part (Chapters 1–5) discusses history of material’s development, types of NiTi-based alloys, fundamental phenomena and mechanisms of shape-memory effect, superelasticity, phase transformation behavior, and heat treatment. The second part (Chapters 6 and 7) covers fabrication technologies to produce various forms of NiTi products and properties required when NiTi materials are used as biomaterials. The third part (Chapters 8 and 9) discusses basic properties in chemical and electrochemical reactions and mechanical properties (fatigue, torsion, and fracture). The fourth part (Chapter 10) touches high-temperature SMAs that are basically utilized in engineering and industrial fields; however, it should not be avoided from this book since the robots that had been developed in industry are employed in medical field. As discussed in the last chapter, the robots will be used mostly in AI-related medical field. The fifth part (Chapters 11–14) covers various properties and behaviors of NiTi materials when they are exposed to various biological environments and required surface modifications to improve surface characteristics. The sixth part (Chapters 15–17) is the main part of this book, covering medical and dental applications of NiTi biomaterials. This book ends with Chapter 18 on future perspective.

Table of Contents Prologue 

 V

Chapter 1 Introduction   1 References   6 Chapter 2 History of development and naming of NiTi materials  References   10

 7

Chapter 3 NiTi-based alloys and alloying element effects    12 3.1 Ni or Ti elemental variation in binary NiTi alloy systems  3.1.1 Ni-rich NiTi alloy   13 3.1.2 Equiatomic or near-equiatomic NiTi alloy   16 3.1.3 Ti-rich NiTi alloy   18 3.2 NiTi-X ternary alloys   20 3.2.1 First transition metal elements   21 3.2.1.1 V   21 3.2.1.2 Cr   22 3.2.1.3 Mn   23 3.2.1.4 Fe   23 3.2.1.5 Co   24 3.2.1.6 Cu   25 3.2.2 The second transition metal elements   31 3.2.2.1 Y   31 3.2.2.2 Zr   32 3.2.2.3 Nb   34 3.2.2.4 Mo   36 3.2.2.5 Pd   36 3.2.2.6 Ag   38 3.2.3 Third transition metal elements   39 3.2.3.1 Dy   39 3.2.3.2 Er   39 3.2.3.3 Hf   39 3.2.3.4 Ta   45 3.2.3.5 W   46 3.2.3.6 Re   46 3.2.3.7 Pt   47 3.2.3.8 Au   48

 12

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3.2.4 3.2.4.1 3.2.4.2 3.2.4.3 3.2.4.4 3.3 3.3.1 3.3.2 3.4 References 

Others   49 Al   49 Si   50 Mg   50 Na, P   51 NiTi–XY and NiTi–XYZ more complicated alloy systems  Quaternary alloy systems   51 Quinary alloy systems   54 Functionally graded NiTi materials    55  57

Chapter 4 Martensitic phase transformation and its related phenomena  4.1 Temperature-related transformation and stress-induced transformation   71 4.2 Stress-induced transformation    78 4.3 Multiple step transformation   83 References   86

 51

 71

Chapter 5 Heat treatments and their related phenomena   92 5.1 Stability   92 5.2 Precipitation and [T-T-T] diagram   97 5.3 Thermocycling   99 5.4 Heating rate and cooling rate   100 5.5 Cryogenic treatment   101 5.6 Aging   101 5.7 Annealing   106 5.8 Thermomechanical treatment   108 References   111 Chapter 6 Fabrication, synthesis, and product forms   119 6.1 Casting and other melting   121 6.2 Powder sintering technology   124 6.2.1 Pressureless powder sintering   125 6.2.2 Pressurized powder sintering    129 6.2.3 Metal injection molding    132 6.2.4 Space holder method    133 6.2.5 Powder treatment    135 6.3 Combustion syntheses    137 6.4 Mechanical alloying and rapid solidification processing 

 141

Table of Contents 

6.4.1 6.4.2 6.5 6.5.1 6.5.2 6.6 6.6.1 6.6.2 6.6.3 6.6.4 6.6.5 6.7 6.8 6.9 6.9.1 6.9.1.1 6.9.1.2 6.9.1.3 6.9.1.4 6.9.1.5 6.9.1.6 6.9.2 6.9.2 1 6.9.2.2 6.9.2.3 6.9.2.4 6.9.3 6.9.3.1 6.9.3.2 6.9.3.3 6.9.3.4 6.9.3.5 6.9.3.6 6.9.3.7 6.9.4 6.10 References 

Mechanical alloying    141 Rapid solidification processing (RSP)   144 Additive manufacturing    144 Selective laser melting   145 Electron beam melting   147 Joining technology   148 Adhesion bonding   149 Solid-state joining   150 Diffusion bonding   152 Soldering and brazing   153 Fusion welding   154 Controlled porosity    156 Amorphization and crystallization   159 Product forms   162 One-dimensional product forms   162 Wire   162 Fiber    165 Strands    165 Suture   165 Cable   166 Staples   167 2-D product forms   167 Film and foil   167 Strip   168 Sheet   169 Mesh   170 3-D product forms   170 Tube   170 Ribbon   173 Truss   174 Spring   175 Scaffold   176 Foam   178 Stent   180 Others   181 Other important technologies for NiTi forming   189

Chapter 7 Properties and their effects on biofunctionality   210 7.1 Basic mechanical and physical properties  7.1.1 In general   212

 210

 186

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7.1.2 7.1.3 7.1.4 7.1.5 7.1.6 7.1.7 7.1.8 7.1.9 7.1.10 7.1.11 7.1.12 7.1.13 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.3 7.3.1 7.3.2 7.3.3 7.4 7.5 References 

Various factors affecting mechanical properties    216 Temperature effects   218 Tensile properties    220 Compression and hardness   221 Tension/compression   222 Bending characteristics   223 Load-deflection behavior   227 Torsional properties   230 Creep behaviors   237 Joint strengths   238 Sterilization effects   239 Bleaching effect   243 Machinability, cutting, and shaping efficiency   245 Machining and machinability   245 Cutting efficiency   247 Shaping ability and canal preparation   249 Technical sensitivity of endodontic treatment   257 Surface-related phenomena and other properties   262 Surface characterization   262 Wettability and spreadability   268 Biofilm formation and cell adhesion   271 Damping behavior   277 Electric characteristics   279  280

Chapter 8 Corrosion and oxidation   297 8.1 Discoloration   297 8.2 Corrosion and electrochemical corrosion in various media  8.2.1 Fluoride-containing solution   299 8.2.2 Chlorine ion containing solution   303 8.2.3 Artificial saliva   305 8.2.4 Simulated body fluid   308 8.2.5 Sweat   311 8.2.6 Others   311 8.2.7 Influencing factors on NiTi corrosion behaviors   314 8.2.7.1 Surface condition   314 8.2.7.2 Coating   315 8.2.7.3 Polishing   316 8.2.7.4 Shot peeing   317 8.2.7.5 Surface oxidation   317 8.2.7.6 Heat treatment and welding   320

 298

Table of Contents 

8.2.7.7 8.2.8 8.2.9 8.2.10 8.2.10.1 8.2.10.2 8.2.10.3 8.2.10.4 8.2.10.5 8.2.10.6 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.3.5 8.3.6 8.3.7 8.3.8 References 

Surface modification   321 Galvanic corrosion   331 Microbiology-induced corrosion   335 Localized corrosion and stress-assisted corrosion  Pitting corrosion   340 Stress corrosion cracking   341 Reaction with hydrogen   342 Hydrogen embrittlement   344 Corrosion fatigue   345 Tribocorrosion   346 Oxidation and oxides   348 Oxidation   348 Air-formed oxide   350 Passivation   352 Oxidation at elevated temperatures   356 Crystal structures of Ti oxides   360 Characterization of oxides   363 Oxide growth, stability, and breakdown   368 Reaction with hydrogen peroxide   369  371

 338

Chapter 9 Fatigue, fracture, and creep   389 9.1 Fatigue of NiTi, in general   389 9.2 Fatigue on endodontic instruments   393 9.2.1 Thermomechanical fatigue   394 9.2.2 Cyclic fatigue   395 9.2.3 Torsional fatigue   401 9.2.4 Bending fatigue   403 9.3 Various factors influencing fatigue behaviors of NiTi alloys  9.3.1 Surface conditions   406 9.3.2 Heat treatment   408 9.3.3 Sterilization effect   411 9.3.4 Material’s parameters   415 9.4 Fatigue on orthodontic archwires   416 9.5 Fatigue on other applications   422 9.6 Fracture, fracture toughness (KIC), and fractography   426 9.6.1 Fracture mechanisms    426 9.6.2 Fracture toughness   428 9.6.3 Fractography   430 9.6.4 Fracture of endodontic instruments   432 9.6.4.1 Rotary file   434

 406

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9.6.4.2 9.6.5 9.6.6 9.7 9.8 References 

Reciprocating file   437 Removal of fractured or separated endodontic files  Fracture of orthodontic archwires   446 Creep   448 Cavitation erosion   450  451

Chapter 10 Nonmedical applications   466 10.1 High-temperature shape-memory alloys   10.2 Applications   469 10.2.1 Actuators    470 10.2.2 Heat engine   472 10.2.3 Seismic and vibration application   472 10.2.4 Battery and electrode   474 10.2.5 Others   475 References   477

 439

 466

Chapter 11 Properties in biological environment   480 11.1 Ion release and dissolution   480 11.1.1 Metallic ion release   481 11.1.2 Ni release, in general   484 11.1.3 Nickel ion release from orthodontic appliances   485 11.1.3.1 Appliances, in general   485 11.1.3.2 Archwire   490 11.1.4 NiTi dissolution from endodontic instruments   494 11.1.5 Prevention of nickel ion release   495 11.1.5.1 Oxidation   495 11.1.5.2 Chemical or electrochemical treatment   496 11.1.5.3 Surface modification   497 11.1.5.4 Coating   498 11.2 Allergic reaction, in general   500 11.2.1 Ni allergy   501 11.2.1.1 Orthodontic appliances   502 11.2.1.2 Ear pierce   505 11.2.1.3 Contact dermatitis   506 11.2.2 Ti allergy   507 11.2.2.1 Dental implant and orthopedic implant    507 11.3 Biocompatibility   514 11.3.1 In general   514 11.3.2 Corrosion resistance and biocompatibility   515

Table of Contents 

11.3.3 11.3.3.1 11.3.3.2 11.3.3.3 11.3.4 11.3.4.1 11.3.4.2 11.3.4.3 11.3.4.4 11.3.4.5 11.3.4.6 11.3.4.7 11.4 11.5 11.5.1 11.5.2 11.6 11.6.1 11.6.2 11.6.3 11.7 11.7.1 11.7.2 11.7.2.1 11.7.2.2 References 

Evaluation   517 In vitro evaluation   517 In vitro and in vivo evaluation   520 In vivo evaluation   521 Improvement   522 Oxidation treatment   522 HA coating   523 TiN coating   524 Carbon coating   525 Coating of composites   526 Plasma treatment   526 Porous structure   528 Hemocompatibility   529 Cytocompatibility and cytotoxicity   533 Cytocompatibility   533 Cytotoxicity   535 Magnetic resonance imaging compatibility  In general   539 Artifacts   542 Indications and contraindications   543 Osseointegration and its evaluation   545 Osseointegration   545 Stability and evaluation   553 In vitro evaluation of stability   554 In vivo animal evaluation of stability   554  556

 539

Chapter 12 Surface modification and engineering   573 12.1 Introduction   573 12.2 Nature of surface and interface   574 12.3 Surface modifications   575 12.3.1 Mechanical modification   576 12.3.2 Polishing   577 12.3.3 Chemical and electrochemical modification   579 12.3.4 Physical modification   584 12.3.5 Hydroxyapatite coating   588 12.3.6 Thermal modification   590 12.4 Surface modifications for dental and medical devices  References   594

 593

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Chapter 13 Sterilization   604 13.1 Process   605 13.2 Influences   607 13.2.1 Surface characteristics   607 13.2.2 Fatigue   610 13.2.3 Torsion   612 13.2.4 Cutting efficiency   614 13.2.5 Corrosion   615 13.2.6 Fracture   615 13.2.7 Cases in orthodontic archwire  References   617

 616

Chapter 14 Biotribology and wear debris toxicity   620 14.1 In general   620 14.2 Friction   622 14.2.1 General characterization   622 14.2.2 Frictional behavior between brackets and archwires   624 14.2.3 Effect of lubricant   629 14.2.3.1 Human saliva   630 14.2.3.2 Artificial saliva   631 14.2.3.3 Other lubricant media   632 14.2.3.4 Effects of metallic orthodontic devices on saliva   634 14.2.4 Influencing parameters   635 14.2.4.1 Surface roughness   636 14.2.4.2 Surface treatment   636 14.2.4.3 Coating   638 14.2.4.4 Type and setting of bracket   639 14.3 Wear and wear debris toxicity   639 14.3.1 Wear phenomenon   639 14.3.2 Influencing factors on wear   642 14.3.3 Improvement of wear resistance   643 14.3.3.1 Improvement by metal addition   644 14.3.3.2 Ion implantation modification   645 14.3.3.3 Composite reinforcement   646 14.3.3.4 Surface modification   646 14.3.4 Wear debris toxicity   647 14.4 Fretting   651 14.5 Tribocorrosion   653 References   654

Table of Contents 

Chapter 15 Dental/medical applications  References   667

 664

Chapter 16 Dental applications   670 16.1 In general   670 16.2 Endodontic applications   671 16.2.1 Development history of endodontic instruments   671 16.2.1.1 Incubation period   671 16.2.1.2 The first generation (early 1990s)   672 16.2.1.3 The second generation (late 1990s)   678 16.2.1.4 The third generation (2000–2010)   681 16.2.1.5 The fourth generation (2010)   685 16.2.1.6 The fifth generation (since 2013)   689 16.2.2 Bioperformance and efficacy   694 16.2.2.1 Shaping   694 16.2.2.2 Centering   697 16.2.2.3 Cutting   698 16.2.2.4 Glide path   699 16.2.2.5 Handpiece versus engine-drive   701 16.2.2.6 Nickel titanium versus stainless steel   703 16.2.2.7 Among Nickel titanium files   707 16.2.2.8 Affecting factors on efficacy   709 16.2.2.9 Irrigation   710 16.2.3 Fracture and separation   713 16.2.3.1 Fracture mechanism   714 16.2.3.2 Surface characteristics   716 16.2.3.3 Fractography   716 16.2.3.4 Factors influencing fracture phenomena   718 16.2.3.5 Fracture rate   720 16.2.3.6 Resistance and preventions   723 16.2.4 Fatigue and its related phenomena   724 16.2.4.1 Basic mechanisms   724 16.2.4.2 Factors influencing fatigue behaviors   726 16.2.4.3 New approach and analysis   728 16.2.5 Torsion   729 16.2.5.1 Torsional behaviors   729 16.2.5.2 Comparisons in torsional behaviors   730 16.2.5.3 Influencing factors on torsional properties   733 16.2.6 Separation   735 16.2.7 Removal of separated instruments   737

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16.2.8 16.2.8.1 16.2.8.2 16.2.8.3 16.2.8.4 16.2.9 16.2.9.1 16.2.9.2 16.2.9.3 16.2.9.4 16.2.10 16.2.10.1 16.2.10.2 16.3 16.3.1 16.3.2 16.3.2.1 16.3.2.2 16.3.2.3 16.3.2.4 16.3.2.5 16.3.2.6 16.3.2.7 16.3.2.8 16.3.2.9 16.3.2.10 16.3.2.11 16.3.2.12 16.3.2.13 16.3.2.14 16.3.2.15 16.3.2.16 16.3.2.17 16.3.2.18 16.3.2.19 16.3.3 16.3.4 16.3.4.1 16.3.4.2 16.4 16.4.1

 Table of Contents

Surface modification and mechanothermal modification   744 Surface modification   744 Thermal treatment   745 Thermomechanical treatment   750 Electropolishing   752 Sterilization   754 Cleaning efficiency and evaluation   754 Comparative studies among various sterilization methods   756 Sterilization effects on cutting efficiency   758 Effects on mechanical properties   759 Technical-sensitive endodontic treatment   762 Undergraduate students   762 Experienced students and practitioners   766 Orthodontic applications   770 Past, present, and future of orthodontic mechanotherapy   771 Orthodontic archwires   771 In general   771 Cross-sectional shapes and material types   773 Mechanical properties in general   775 Load-deflection characteristics   779 Bending behaviors   784 Biotribology   787 Chemical and electrochemical corrosion   795 Corrosion in artificial saliva   799 Galvanic corrosion   801 Metallic ion release   803 Nickel ion release   805 Nickel allergy issue   807 Cytotoxicity   809 Fracture   811 Fatigue   812 Torsional action   813 Joining   815 Heat treatment   817 Coating   817 Orthodontic brackets   820 Mini-orthodontic implants   823 Temporary anchorage pins   823 Mini-orthodontic implants   824 Dental implants   827 Blade type dental implant   828

Table of Contents 

 832 16.4.2 Fixators  16.4.3 Prosthesis   833 References   833 Chapter 17 Medical applications   865 17.1 In general   865 17.2 Cardiovascular applications   866 17.2.1 Cardiovascular catheter   866 17.2.2 Inferior vena cava filter   867 17.2.3 Atrial septal occluder   869 17.3 Various bioapplications of stents   871 17.3.1 In general   871 17.3.2 For coronary artery disease   873 17.3.3 Stents for artery   875 17.3.4 Stents for colorectal and bowel obstruction, and gastrointestinal strictures   877 17.3.5 Stents for ureteropelvic junction   879 17.3.6 Stents for biliary obstruction   881 17.3.7 Intratracheal stents   882 17.3.8 Stents for esophageal strictures   882 17.3.9 Stents for otosclerosis   883 17.4 Orthopedic applications   884 17.4.1 Spinal vertebra spacer   884 17.4.2 Orthopedic bone plate   886 17.4.3 Compression staples   887 17.4.4 Intramedullary nail   889 17.5 Scaffold structure   893 17.5.1 In general   893 17.5.2 Scaffold structure   894 17.5.2.1 Mesh scaffold structure   894 17.5.2.2 Fibrous scaffold structure   894 17.5.2.3 Foam scaffold structure   894 17.5.3 Porous scaffold structures   895 17.6 Bone implant   898 17.7 Other applications   905 17.7.1 Nitinol suture   905 17.7.2 Muscle   908 17.7.3 Glove   910 17.7.4 Drug delivery system   911 17.8 New FDA testing guidelines for nitinol materials   912 References   913

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Chapter 18 Future perspective   922 18.1 What was foreseen, 10 years ago   922 18.2 Current status and future perspective   923 References   929 Epilogue  Index 

 931  933

Chapter 1 Introduction There are various types of materials. About 100 pure elements, 783 out of possible 3,403 combinations of binary alloys, and 334 out of 91,881 possible tertiary alloys are considered as practically usable metallic materials. There are, furthermore, about 2,000 to 5,000 types of plastics, and about 10,000 kinds of ceramics (including oxides, nitrides, and carbides). Moreover, there are three major types of composites: metal-matrix compounds, plastic-matrix compounds, and ceramic-matric compounds. Among the above list of variety of materials, if you limit yourself to search for material(s) that is equally utilized in both industry/engineering field and medical/ dental area, there are only two metallic materials selected: stainless steel and titanium materials [1, 2]. Of interest, with further narrow-down applications, these two metallic materials are also used in orthodontic appliances (such as archwires) and endodontic instruments (such as files and reamers). In materials science, it is common to classify materials into two categories: structural materials and functional materials. Structural materials are nonactivatable materials that bear load. The key properties of such structural materials in relation to bearing load are elastic modulus, yield strength, ultimate tensile strength, hardness, ductility, fracture toughness, fatigue, and creep resistance. In addition, if materials corrode or wear, their ability to carry load will be degraded. Classical materials such as metals and alloys have played a significant role as structural materials for many centuries. Engineers have designed components and selected alloys by employing the classical engineering approach of understanding the macroscopic properties of the material and selecting the appropriate one to match the desired performances based on the application. With advancements in materials science and with increasing space and logistical limitations, scientists have been constantly developing high-performing materials for various applications [3]. On the other hand, functional materials  possess properties that allow an energy conversion inside the material on the basis of physical effects. They can be used directly as material-based energy converter; in other words, it is possible to modify their material properties deliberately and reversibly. Functional materials are not assigned to one single material group. For listing just a few, examples of functional materials are as follows: piezoelectric materials (ZnO, BaTiO3, etc.), optomechanical materials ( LiNbO3, KTN, ZnO, etc.), optical materials (SiO2, GaAs, glasses, Al2O3, YAG, etc.), magnetic materials (Fe, Fe-Si, NiZn and MnZN ferrites, γ-Fe2O3, Co-Pt-Ta-Cr, etc.), shape-memory materials (NiTi, ZrO2, AuCd, CuZnAl, polymer gels, magnetorheological fluids, etc.), electroactive polymers (carbon nanotubes), and energy technology and environment (UO2, Ni–Cd, ZrO2, LiCoO2, etc.) [4]. This historical classification of materials appears to be unclear and some types of materials have been employed as a structural material while it exhibits https://doi.org/10.1515/9783110666113-001

2 

 Chapter 1 Introduction

its unique functionality. As going on with this book, NiTi materials are the typical materials falling onto dual-characteristic materials. By applying the surface technology (such as cladding, chemical/electrochemical treatment, and physiochemical/physical deposition), the surface zone can be modified to exhibit certain type(s) of function, the so-called functionalization. As we will discuss later, unique properties of NiTi [shape memory effect (SME) and/or superelasticity (SE)] can be added onto the surface layer of structural materials. This functionality can be manipulated on the surface zone to make a whole material as the functionally graded material (FGMs) which may be characterized by the variation in composition and structure gradually over volume, resulting in corresponding changes in the properties of the material. The materials can be designed for specific function and applications. Various approaches based on the bulk (particulate processing), preform processing, layer processing, and melt processing are used to fabricate the FGMs. Biomaterials are those materials that are used in the human body. Biomaterials should have two important properties: biofunctionality and biocompatibility [1–5]. Good biofunctionality means that the biomaterial can perform the required function when it is used as a biomaterial. Biocompatibility means that the material should not be toxic within the body. Because of these two rigorous properties required for the material to be used as a biomaterial, not all materials are suitable for biomedical applications. The use of biomaterials in the medical field is an area of great interest as average life has increased due to advances in the use of surgical instruments and biomaterials [6, 7]. The  NiTi alloys  have been investigated extensively for 30 years after the establishment of basic understanding on the relationship among the  microstructure, transformation behavior, and SME/SE phenomena. Many applications have been successfully developed in both engineering and medical fields. In particular, SE has been used for medical applications such as stents, guide wires and orthodontic arch wires, and endodontic files and reamers, as mentioned earlier. The popularity of titanium biomaterials in both medical and dental fields can be recognized by counting numbers of peer-reviewed manuscripts published in variety of journals. In Figure 1.1, the total accumulated number of published articles for every 5 years is plotted. There are two straight lines in semilog scale. Top line is data for titanium biomaterials, which contain commercially pure titanium, Ti-based alloys such as Ti-6Al-4V alloy and NiTi-based alloys. The bottom line presents accumulated publication for each 5-year span on NiTi biomaterials, which contain NiTi, nitinol, and TiNi for medical/dental applications. Since the line for titanium biomaterials includes publications on NiTi biomaterials, it can be roughly said that about 20–30% of total publications on Ti materials are about NiTi materials. As indicated clearly in the figure, the exponentially increasing trend of published papers on medical and dental NiTi biomaterials may be attributed to different reasons that might include (1) needs from medical and dental sectors, (2) increased researchers and scientists involved in the NiTi biomaterials, and (3) expanded industrial scale being associated with the above two.

Chapter 1 Introduction 

 3

2

10,000 9 8 7 6 5 4 3

The number of published articles

2

Titanium biomaterials

1,000 9

8 7 6 5 4 3 2

100 9

NiTi biomaterials (NiTi, nitinol, TiNi)

8 7 6 5 4 3 2

10 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 Year Figure 1.1: Accumulated number of published articles on titanium biomaterials and NiTi biomaterials. Titanium line includes publications on NiTi materials.

Although there are various manuscripts as book chapters on NiTi materials, we value the books [8, 9] covering basic mechanisms of SME* and SE** and applications using these interesting phenomena. – * SME stands for shape-memory effect, and materials exhibiting this uniqueness are called as SMA (shape-memory alloys). Although SME does not, strictly speaking, describe the manner that such SMA exhibits an interesting phenomenon, rather it refers to the recovery of shape (or strain) after apparent “permanent” deformation which is previously induced at relatively cold temperatures by heating above a characteristic transformation temperature, so that it should be called as SRE (shape recovery effect). However, as majority of researchers and industry engineers utilize SME, in this book, we will follow the majority’s judgment.

4 

–

 Chapter 1 Introduction

SE refers to superelasticity. Again, here is a controversial issue for naming. Although the term superelasticity implies that the extent of elasticity is great (super) from a viewpoint of phenomenology, it is not the fact that material’s elasticity is super, rather it should refer to the isothermal recovery of relatively large (apparent elastic) strains during a mechanical load–unload cycle that occurs at temperatures above a characteristic transformation temperature. Hence, it should be called as pseudoelasticity. Due to a similar reason as SME, the term “super” can attract more people than the term “pseudo”; hence, we will follow the majority’s naming. **

Materials science and technology involved in titanium materials is typically interdisciplinary. When such titanium materials are treated as biomaterials, there should some limitations including biocompatibility and biofunctionality, so that certain types of titanium materials are accepted in medical and dental areas. Furthermore, when titanium biomaterials are employed as main materials for implant systems, additional requirements should be met before in vivo application. Such requirements should include osseointegration, biomechanical compatibility, and macro- and micromorphological compatibilities [1, 2, 10]. Figure 1.2 illustrates the complicated, yet nicely correlated interrelationship among different disciplines to establish a promising titanium materials science and engineering for assisting not only industry but also health providers, as well as the receiver of such services. In Figure 1.2, in order for engineered titanium materials to serve as titanium biomaterials, we will be discussing and reviewing numerous articles to prove that appropriate surface modifications and characterizations should be properly preformed and reflected to fabrication technologies and methods. Then, such titanium biomaterials are ready to be used in different dental and medical applications. We will review ever-growing Ti materials research and development for meeting specific aims, including V-free alloys, β-Ti alloys, Ti materials having better properties of fatigue, as well as wear, amorphatizable materials, materials exhibiting better superplastic formability, macro-, micro-, and nanoscale structure-controlled materials, SME and SE. These new Ti materials, along with conventional Ti materials, are characterized to evaluate whether they meet specific required characteristics. All these activities, as seen in the figure, are nicely correlated to establish titanium biomaterials, from which various dental and medical applications can be realized. Furthermore, currently and continuously in the future, with tremendous valuable and supportive technologies (including newly developed surface modification, near-net shape forming, better understating of bone healing mechanisms, and advanced tissue engineering materials and technologies) implant systems can be further developed to bring benefits to both patients by enhancing their quality of life level and clinicians’ professional satisfaction.

Supportive technologies V-free new α/β alloy development Near-net shape (NNS) forming advanced SPF on composites laser forming injection forming Bone-healing mechanisms Tissue-engineering protein coating scaffold structure scaffold material

Implant application

Applications Denfure Bridge/crown/coping Orthodontic archwires/brackets Endodontic files/reamers Post and cores Splint Orthopedic appliances (clamps/staples/stent) Total joint replacements (knee, hip)

Figure 1.2: Titanium materials flow chart involving various materials science and engineering disciplines [1, 11].

Characterization/compatibilities Corrosion resistance Passivation (auto healing effect) Oxidatioin Reaction with hydrogen Dissolution/metal ion release Wear resistance Biotribology/wear debris toxicity Toxicity Attachment cell attachment MIC (microbiologically-induced corrosion) product Compatibility biological mechanical morphological

Titanium biomaterials

Surface modifications Sand-blasting (alumina, titania) Shot-peening Coating (hydroxyapatite, Ca-P, Ti beads, Ti) Gold-color TiN coating Silver-color Ti2N coating Ti2O (titania) film coating Porosity controlled surface Foamed titanium

Titanium materials

Forming/machning Casting Machining EDM (electrical discharge machining) laser machining/forming Isothermal forming SPF (superplastic forming) ultrafine grain SPF transformation SPF DB (diffusion bond) SPF D/B P/M (powder metallurgy) MIM (metal injection molding) technique Laser welding Fusion welding Soldering Cementation

Shape/conditions Annealed Wrought Heat treated Sheet Rod Wire Plate Powder Beads

Type/crytallography α type (HCP) Near α type (α+β) type β type (BCC) TiNi (β or martensite) TiAl

Chapter 1 Introduction   5

6 

 Chapter 1 Introduction

References [1]

Oshida Y. Bioscience and Bioengineering of Titanium Materials. Elsevier, Amsterdam, 1st edition, 2007. [2] Oshida Y. Bioscience and Bioengineering of Titanium Materials. Elsevier, Amsterdam, 2nd edition, 2013. [3] Rao A, Srinivasa AR, Reddy JN. Introduction to Shape Memory Alloys. In: Design of Shape Memory Alloy (SMA) Actuators. 2015, 1–31; doi: 10.1007/978-3-319-03188-0. [4] Askeland DR, Phulé PP. The Science and Engineering of Materials. Pacific Grove, CA, USA, Thomson Brooks/Cole, 2003. [5] Shabalovskaya SA. On the nature of the biocompatibility and on medical applications of NiTi shape memory and superelastic alloys. Biomed. Mater. Eng. 1996, 6, 267–89. [6] Tathe A, Ghodke M, Nikaljie AP. A brief review: biomaterials and their application. Int. J. Pharm. Pharm. Sci. 2010, 2, 19–23. [7] Rice C. Shape Memory Alloys, Applications. Encyclopedia of Smart Materials. 2002; https://doi.org/10.1002/0471216275.esm071. [8] Duerig TW, Melton KN, Stöckel D, Wayman CM. Engineering Aspects of Shape Memory Alloys. Butterworth-Heinemann, London, UK, 1990. [9] Yahia L. Shape Memory Implants. Springer-Verlag, Heidelberg, Germany, 2000. [10] Oshida Y, Miyazaki T, Tominaga T. Some biomechanistic concerns on newly developed implantable materials. J. Dent. Oral Health 2018, 4, 5 pages; https://scientonline.org/ open-access/some-biomechanistic-concerns-on-newly-developed-implantable-materials.pdf. [11] Oshida Y, Tuna EB. Science and Technology Integrated Titanium Dental Systems. In: Basu et al. ed., Advanced Biomaterials – Fundamentals, Processing, and Applications. John Wiley & Sons, Hoboken, NJ, USA, 143–77.

Chapter 2 History of development and naming of NiTi materials Since the discovery of shape-memory effect (SME) phenomena with NiTi alloy (in 1960s) [1], which was originally developed for the needs of high refractory metallic material having high damping capacity [2], there were no further remarkable development on NiTi materials till another unique property of superelasticity (in 1980s) was observed. Physical metallurgy investigation was carried out in detail on the dislocation mechanism, Ti3Ni4 precipitation, and R-phase transformation [3]. Once these two unique characteristics associated with NiTi were discovered and understood fully, further R&D has been advanced remarkably. First, in order to rise the effective temperature for SME phenomena high, 80°C, various NiTi-based alloy systems have been developed by adding Zr, Hf, or Nb. Then, the applications (including idea only as paper patents) were proposed in aerospace field, automotive engine, power generation station, nuclear power plant, and so on, exhibiting the typical seed-oriented development. This activity in R&D is shown in 1980s through early 1990s, as shown in Figure 2.1. Then, the second remarkable activity was found in advanced fabrication technology for, particularly, manufacturing various shapes of products such as foam metal with controlled porosity, thin film, hybrid, composites, as shown representatively with NiTi thin film in Figure 2.1. During the 1990s, new powerful actuators were developed to expand their applications in various sectors of engineering, robotics, and sensing engineering. Since 2000, after the biocompatibility and safety of NiTi were evaluated, a variety of applications in both medical and dental fields were explored [2, 3]. For future perspective, we discuss this at the last portion of this book. The difference in denotation of NiTi and TiNi is more than individual’s preference, rather there is a pure scientific reason. Scientists, engineers, and people involved in the research and development take more considerations on material properties on an atomic level when dealing with elemental materials, alloys, and composites. Therefore, they generally consider atomic interactions from one element to another, how a certain stoichiometric property alters as the composition is changed or an additive (as an alloying element) is introduced. So, in general, they deal materials on an atomic percentage (at%) or molecular percentage (mol%) level when considering such properties. Unless otherwise stated, liquids and solids are generally expressed in weight percentages (wt%), and gasses are expressed in volume percentage (vol%) or mol%. Atomic percentages are all well and good when the concern is how individual atoms from one species or another provides a solution to a problem according to how they interact on an atomic level, that is, when an optical, chemical, electrical, and physical evaluation is taken into consideration. However, for actual manufacturing, individual atoms cannot be measured very easily. Therefore, it is effective and useful to utilize the wt% when manufacturing an alloy or composite material. https://doi.org/10.1515/9783110666113-002

8 

 Chapter 2 History of development and naming of NiTi materials 2

10,000 9

Ti biomaterial

8 7 6 5 4 3

The number of published articles

2

NiTi thin film

1,000 9

8 7 6 5 4 3 2

NiTi alloys 100 9

8 7 6 5 4 3 2

10

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 Year

Figure 2.1: Three major noticeable activity eras in research and development history of NiTi materials.

Based on the above discussion, when we consider NiTi alloys, as we will see various physical, chemical, electrochemical, optical, and other properties to which if we know at% it would be much easier to understand the underlying reasons and mechanisms thereof. For example, there is a term “equiatomic” or “near- equiatomic” NiTi (as known as nitinol: an acronym of NiTi and Naval Ordnance Laboratory) [4–6], simply implying that it is a 50%Ni and 50%Ti in at% level. Referring to Figure 2.2, the equilibrium phase diagram is normally described with two compositional scales (wt% and at%), so that it is clearly understood that “equiatomic” alloy does not indicate an alloy with “equiweight” composition. The conversion between wt% and at% can be easily done by knowing atomic weight of each constituent elements. Let WA and WB be weight percentage of elements A and B, MA and MB be atomic percentage of elements A and B, and AA and AB be respective atomic weights, we have

 9

Chapter 2 History of development and naming of NiTi materials 

(1) for converting from wt% to at% at%A = WA/[WA + (MA/MB)WB] × 100 at%B = WB/[WB + (MB/MA)WA] × 100 = 100 – at%A (2) for converting from at% to wt% wt%A = (AA/MA)/(AAMA + ABMB) × 100 wt%B = (AB/MB)/(AAMA + ABMB) × 100 = 100 – wt%A Weight Percent Nickel 1800

0

10

20

30

40

50

60

70

80

90

100

1670°C

L

1600

Temperature °C

1400

1455°C

1380°C 1310°C 1304°C

1200

1118°C

TiNi 1000

984°C

942°C

(βTi)

TiNi3

882°C 800

Ti2Ni

765°C (αTi)

600

(Ni)

0 Ti

10

20

30

630°C 40

50

60

Atomic Percent Nickel

70

80

90

100 Ni

Figure 2.2: Equilibrium phase diagram of [Ti–Ni] alloy system with both wt% and at% scales.

Hence, 50–50 equiatomic NiTi alloy can be converted to 55.08wt%Ni and 44.92wt%Ti alloy, using atomic weight of 58.69 for Ti and 47.87 for Ni, respectively. To describe an alloy system, say A–B alloy, we normally put a base element (with majority of composition) in A position; therefore, the description of NiTi is normally practiced in wt% level; on the other hand, in at% level, since it is equiatomic, both NiTi or TiNi can be accepted. However, historically, there is another term “nitinol” for NiTi, because of this fact NiTi is the majority description of an alloy composed of Ni and Ti elements. Now, move to the history of R&D for SME alloys (Figure 2.3) and NiTi-based alloys (including the binary alloy, ternary alloy, quaternary alloy, and quinary alloy, which are discussed in Chapter 3). Research and commercial applications include automobile, aerospace, robotic, and biomedical domains [5]. The demand for SMAs for

10 

 Chapter 2 History of development and naming of NiTi materials

NiTiFe AuCdZn NiAl FePt TiNb AuCd

NiTi*

CuZn InTl

CuAINi*

1950

1960

UNb CuZnAI*

AuCuZn CuZnGa CuSn AgCd CuZnSn CuZnSi

NiTiCu* NiTiTa

FeCrNi TiPdNi FeMnSi TiPd

1970 1980 *Commercially used

NiMnGa ZrCo

FeMn

ZrCu

CuAlAg TiVPd CuAlMn NiTiCoV

FeNiC ZrRh

1990

2000

Figure 2.3: History of the discovery of shape-memory alloys [1].

engineering and technical applications has been increasing in numerous commercial fields; such as in consumer products and industrial applications [7–9], structures and composites [10], automotives [11], aerospace [12–15], mini actuators and microelectromechanical systems [14, 16], robotics [17], biomedical [18–23], and even in fashion [24]. Since the discovery of nitinol in 1963, the history of R&D of NiTi is that of development of devices and appliances employed in medical and dental fields, which will be discussed in later chapters.

References [1]

[2] [3] [4] [5] [6] [7]

[8] [9] [10] [11]

Shaw J, Churchill C, Iadicola M. Tips and tricks for characterizing shape memory alloy wire: part 1 differential scanning calorimetry and basic phenomena. Exp. Techniques 2008, 32, 55–62. Buehler WJ, Gilfrich JV, Wiley RC. Effect of low-temperature phase changes on the mechanical properties of alloys near composition TiNi. Appl. Phys. 1963, 34, 1475–7. Miyazaki S, Otsuka K, Suzuki Y. Transformation pseudoelasticity and deformation behavior in a Ti-50.6 at% Ni alloy. Scripta. Met. 1981, 15, 287–92. Liu Y, Galvin SP. Criteria for pseudoelasticity in near-equiatomic NiTi shape memory alloys. Acta Mater. 1997, 45, 4431–9. Jani JM, Leary M, Subic A, Gibson MA. A review of shape memory alloy research, applications and opportunities. Mater. Design 2014, 56, 1078–113. Kaufmann GB, Mayo I. The story of nitinol: the serendipitous discovery of the memory metal and its applications. Chem. Educat. 1997, 2, 1–21. Wu, MH, Schetky IM. Industrial Applications for Shape Memory Alloys. In: International Conference on Shape Memory and Superelastic Technologies. Pacific Grove, CA, USA, 2000, 171–82. Zider RB, Krumme JF. Eyeglass frame including shape-memory elements. US Patents 4772112: 1988. Hautcoeur A, Eberthardt A. Eyeglass frame with very high recoverable deformability. US Patent 564027: 1997. Furuya Y. Design and material evaluation of shape memory composites. Intell. Mater. Syst. Struct. 1996, 7, 321–30. Stöckel D. Shape memory actuators for automotive applications. Mater. Des. 1990, 11, 302–7.

References 

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

 11

Bill C, Massey K, Abdullah EJ. Wing morphing control with shape memory alloy actuators. J. Intell. Mater. System Struct. 2013, 24, 879–98. Hardt DJ, Lagoudas DC. Aerospace applications of shape memory alloys. Proc. Inst. Mech. Eng., Part G: J. Aerospace Eng. 2007, 221, 535–52. Humbeek JV. Non-medical applications of shape memory alloys. Mater. Sci. Eng. A, 1999, 134–48. McDonald SI. Shape memory alloy applications in space systems. Mater. Des. 1991, 12, 29–32. Sun I, Huang WM, Ding Z, Zhao Y, Wang CC, Purnawali H, Tang C, Huang WM, Wang CC. Stimulus-responsive shape memory materials: a review. Mater. Des. 2012, 33, 577–640. Furuya Y, Shimada H. Shape memory actuators for robotic applications. Mater. Des. 1991, 12–21–8. Petrini I, Magliavacca F. Biomedical applications of shape memory alloys. J. Metall. 2011, 2–11. Song C. History and current situation of shape memory alloys devices for minimally invasive surgery. Open. Med. Dev. J. 2010, 2, 24–31. Morgan NB. Medical shape memory alloy applications – the market and its products. Mater. Sci. Eng. A, 2004, 378, 16–23. Machado LG, Savi MA. Medical applications of shape memory alloys. Braz. J. Med. Biol. Res. 2003, 36, 693–91. Mantovani D. Shape memory alloys: properties and biomedical applications. JOM 2000, 52, 36–44. Duerig T, Pelton A, Stöckel D. An overview of nitinol medical applications. Mater. Sci. Eng. A, 1999, 273/275, 149–60. Langenhove LV, Hertleer C. Smart clothing: a new life. Int. J. Clothing Sci. Technol. 2004, 16, 63–72.

Chapter 3 NiTi-based alloys and alloying element effects 3.1 Ni or Ti elemental variation in binary NiTi alloy systems NiTi is made of approximately equal amounts of nickel and titanium (at atomic % level), and small variations in these proportions (in other words, Ni/Ti ratio) have a radical effect on the properties of the alloy and, in particular, its transformation temperature, metallurgical characteristics, physical and mechanical properties, and chemical and electrochemical behaviors as well. For example, Sanjabi et al. [1], studying mechanical and metallurgical properties in NiTi thin films, concluded that the transformation from low-temperature martensitic phase (M-phase) to the hightemperature parent phase took place below room temperature in Ni-rich NiTi while it occurred above room temperature in Ti-rich and near-equiatomic NiTi. Nanoindentation tests demonstrated superelasticity (SE) in Ni-rich NiTi and martensitic deformation in Ti-rich and near-equiatomic NiTi compositions. Belyaev et al. [2] studied the influence of the chemical composition of a NiTi alloy on the martensite stabilization effect. The Ni50-Ti, Ni49.5-Ti, and Ni49.0-Ti (at%) alloys were water-quenched from 900 °C to exhibit the B2 ↔ B19′ transformation on cooling and heating without the intermediate R-phase formation. It was reported that the martensite stabilization effect was observed in NiTi alloys, regardless of the chemical composition and value of the preliminary strain. When the residual strain was less than 2.5%, the martensite stabilization effect values were close to each other, whereas if the residual strain exceeded 2.5%, the martensite stabilization effect values in the Ni50-Ti and Ni49.5-Ti were larger than in the Ni49.0-Ti (at%) alloys. TiNi films with different Ni/Ti ratios were prepared by cosputtering technique [3] to investigate compositional effects on residual stress evolution. It was found that for the film of Ti with 51.3 at%, a two-step transformation was observed among martensite, R-phase, and austenite; and the residual stress was quite low at room temperature. For the films with Ti contents of 47.3 at% and 53 at%, residual stress was quite high due to the high intrinsic stress and partial relaxation of stress caused by the R-phase transformation. When the films with Ti contents of 47.3 at% and 53 at% were annealed at 650 °C, residual stress in films decreased significantly, because postannealing could probably modify the film structure, reduce the intrinsic stress, increase the transformation temperatures, and cause martensite transformation above room temperature. Furthermore, Yoneyama et al. [4] investigated the ingots with 51.0 and 50.5 at% Ni by conducting tensile testing and differential scanning calorimetry (DSC). It was shown that Ni51-Ti (at%) showed a brittle property while Ni50.5-Ti (at%) exhibited a low value of the apparent proof strength with relatively large elongation, and residual strain increased with increasing titanium content. Yan et al. [5], investigating corrosion resistance laser spot-welded joint NiTi wires in Hank’s solution, found that the corrosion resistance improved due to https://doi.org/10.1515/9783110666113-003

3.1 Ni or Ti elemental variation in binary NiTi alloy systems 

 13

decrease of the surface defects and the increase of the Ti/Ni ratio, which may be attributed to more stable passive TiO2 film formation. The unique properties of shape-memory alloys (SMAs) are controlled by and are dependent on four external parameters: temperature (T), stress (σ), strain (ε), and time (t) [6]. These parameters cannot be changed independently. The complete mechanical behavior of SMAs has to be determined from a (T, σ, ε) diagram, in which the temperature axis covers the general temperature range from approximately 50 °C below and approximately 100 °C above MS (upon cooling, the starting temperature for phase transformation from parent phase to M-phase). The value of a point along the (T, σ, ε) surface is not always constant and can move in any direction in that space. This time dependency can result from creep, stress relaxation, and changes due to variations in the chemical free energy of martensite and/or parent phases (which are stable at relatively high temperature and called as B2 crystal structure). During the phase transformation between B2 phase (A: austenite) and B19′ (M: martensite) phase, unique properties of shape-memory effect (SME) and SE take place. For enhancing these phenomena, normally postdeformation annealing, thermal/mechanical cycling, aging, and others are applied on NiTi materials [7–11]. By these additional thermal or thermomechanical treatment, the intermediate rhombohedral R-phase is frequently produced, from which an intermetallic compound, Ni4Ti3, is precipitated coherently and is normally not shown in the equilibrium phase diagram since it is in unstable phase, thereby resulting in generating an internal strain field. According to the appearance of R-phase, the transformation could be a multistep process; B2↔R↔M, and during the B2↔R transformation, SME and SE are also recognized [12]. The phase transformation temperature is very sensitive to alloy component, thermal/thermomechanical treatment conditions, and alloying elements [13]. With increasing temperature, the behavior changes from one-way effect (thermal memory effect) over SE (mechanical memory effect) to the stress–strain characteristic of conventional metal alloys. The position of the human body temperature on T-axis can be adjusted sensitively by the chemical composition and the thermomechanical treatment of the material [6]. In the following section, we divide the section into three major portions: Ni-rich NiTi, near- or equiatomic NiTi, and T-rich NiTi alloys.

3.1.1 Ni-rich NiTi alloy The phase changing processes are executed by the forward and reverse transformation such as cubic austenitic phase (A-phase) B2 to monoclinic M-phase B19′ and vice versa. Phase transformation temperatures (AS: on heating, transformation starting temperature from M-phase to parent A-phase, AF: on heating, transformation finishing temperature from M-phase to parent A-phase, MS: on cooling, transformation starting temperature from parent A-phase to M-phase, and MF: on cooling,

14 

 Chapter 3 NiTi-based alloys and alloying element effects

transformation finishing temperature from parent A-phase to M-phase) are decided by the range of phase limit. The state of phase presented in  NiTi such as austenite or martensite decided the application. Therefore, controlling the microstructure of NiTi alloy through amendment of transformation temperatures by changing the chemical composition of  NiTi  alloy, and heat treatment process is a challenging mission in today’s materials advancement. Generally, aged Ni-rich NiTi alloys undergo martensitic transformations on cooling from high temperatures in two steps: B2 to R and then R to B19′ (normal behavior). However, under certain aging conditions, the transformation can also occur in three or more steps (unusual multiple step behavior) [14–18]. Aging of Ni-rich NiTi alloys was studied [19], by DSC and showed two transformation peaks on cooling after short aging times, three after intermediate aging times, and finally again two peaks after long aging times (2–3–2 transformation behavior). The three-step transformation was explained by two basic elements: (1) the composition inhomogeneity evolved during aging as Ni4Ti3 precipitates grow and (2) the difference between nucleation barriers for R-phase (small) and B19′ (large) [20]. The effect of 450 °C aging on the microstructure and on the martensitic transformations in a Ni-rich (50.8 at% Ni) NiTi SMA was studied [20] using transmission electron microscopy (TEM), X-ray diffraction (XRD), neutron diffraction, and DSC. It was found that on cooling from the high-temperature phase, two distinct peaks were observed after short-aging times, three peaks after intermediate-aging times, and two peaks again after long-aging times (2–3–2 transformation behavior). The first peak on cooling represents the formation of R-phase and the second peak is associated with the formation of M (B19′), suggesting that the burst-like transformation events during the growth of thermoelastic martensite and on the effect of oxidation are related to NiTi microstructures [20]. The effects of Ni concentration in Ni–Ti binary alloys on the multistep transformation were studied [21,22]. Using Ti–50.6, 50.8, and 51.0  at% Ni alloys, the effects of Ni concentration and aging conditions on the multistage martensitic transformation in aged Ni–Ti alloys were investigated by DSC and in situ scanning electron microscopy [22] by heat treating at 950 °C for 1 h and then aged at 500 °C for 1 h. It was found that although the triple-stage transformation appeared in the Ti–50.6 and 51 at% Ni alloys during cooling, the transformation sequence of the two alloys was completely different, and quadruple-stage transformation was observed in the Ti–50.8  at% Ni alloy [22]. The origin of the abnormal multistage martensitic transformations was investigated [23,24]. Followed by aging Ni-rich NiTi alloys, finely dispersed Ni4Ti3 particles embedded in B2 matrix are normally observed. With this situation, the B2 matrix (parent A-phase) normally undergoes two-stage martensitic transformation B2-R-B19′. However, as described earlier, there is also evidence of three-stage transformation. The origin of such abnormal three-stage transformation remains controversial. Fan et al. [23] conducted a comparative study between single crystals and corresponding polycrystals to find that all single crystals exhibit normal two-stage transformation,

3.1 Ni or Ti elemental variation in binary NiTi alloy systems 

 15

being independent of Ni content. It was further mentioned that, by comparison, polycrystals with low Ni content (50.6at%Ni) show three-stage transformation, but those with high Ni content (51.5at%Ni) again exhibit normal two-stage transformation. These new findings are consistent with a simple scenario that different transformation behaviors are a result of competition between preferential grain-boundary precipitation of Ni4Ti3 particles and a tendency for homogeneous precipitation when supersaturation of Ni is large [23]. Similar results were obtained by Zhou et al. [24]. According to the study, after aging at intermediate temperatures (400–500 °C), it was found that Ni-rich NiTi alloys undergo an abnormal three-stage martensitic transformation behavior (one-stage R and two-stage B19′), which stems from a preferential Ni4Ti3 precipitation around grain boundary. On the other hand, if aged at low temperatures (250–300 °C), they undergo two-stage R-phase transformation. Studying on low-Ni (50.6at%Ni, 51at%Ni) and high-Ni (52at%Ni) polycrystals, it was found that the former exhibited two-stage R-phase transformation, whereas the latter showed onestage R-phase transformation. It was further concluded that the different transformation behavior of low-Ni and high-Ni polycrystals stems from a competition between two opposing tendencies: (1) for preferential precipitation in the grain boundary and (2) for homogeneous precipitation across the whole grain with high Ni content [24]. There are numerous articles supporting the fact that the precipitation of Ni4Ti3 phase is responsible for the multistage transformation in Ni-rich NiTi alloys, leading to SME and/or SE characteristics [25–31]. Chu et al. [32] investigated the effect of aging temperature on the reverse martensitic transformation in the Ni-rich NiTi alloy after the treatment of the solution at 1,050  °C for 4  h, using differential scanning calorimeter. It was reported that the type of reverse martensitic transformation changed from one step M-phase→Aphase (after solution treatment) through two steps M-phase→R→A-phase (after aged at 400, 450, and 475  °C) back to one step M-phase →A-phase (after aged at 500 °C). Both the austenite finish temperature (AF) and the peak temperature corresponding to the transformation of R→A-phase decreased with the rise of the aging temperature. Zhang et al. [33] reported that during repeatedly imposed thermally induced martensitic transformations in NiTi SMAs, the MS decreases. The temperature dependency of the phase transformation was investigated by Olbricht et al. [34]. Using binary ultra-fine-grained pseudoelastic NiTi wires, the phase transformations were studied in a wide temperature range by mechanical loading/unloading experiments, resistance measurements, DSC, thermal infrared imaging, and TEM. It was mentioned that the R-phase always forms prior to M-phase (B19′) when good pseudoelastic properties are observed; and the stress-induced A-phase (B2) to R-phase transformation occurred in a homogeneous manner, contrary to the localized character of the B2/R to B19′ transformations [34]. The electrical resistance variations of Ni50.9Ti49.1 shape-memory wires were studied during aging treatment at different temperatures via in situ electrical resistance measurement [35]. It was shown that during aging treatment, a cyclic behavior was observed in the electrical

16 

 Chapter 3 NiTi-based alloys and alloying element effects

resistance variations, which could be related to the precipitation process. The evaluation of postaging transition temperatures was conducted using DSC analysis, and the precipitation process is found to occur in four different stages, suggesting that two-, three-, or four-step martensitic transformation could be observed, depending on the stress level around precipitates [35].

3.1.2 Equiatomic or near-equiatomic NiTi alloy A small amount variation of Ni content from the equiatomic Ni/Ti ratio affects the changes in characteristic temperatures during the phase transformation; in particular, MS and AF (shape recovery temperature), as shown in Figure 3.1 [36–40]. It is clearly noticed that within the composition range at which the NiTi phase exists at ambient temperature, both MS and AF depend quite strongly on composition, particularly on the Ni-rich side, whereas Ti-rich alloys show less sensitivity. The composition dependency of MS and AF has important practical consequence, because the precise composition control is required when melting the alloys. It is normally believed that the NiTi alloy containing about 50.0  at%Ni shows typical SME, whereas NiTi alloy with more than 50.5  at%Ni exhibits SE phenomenon; hence, the SE appearance is more sensitive to the temperature control. Nickel/titanium ratio 0.92 0.96 1.0 1.04 1.08

150

50

Af temperature (°C)

Ms temperature (°C)

100

0 –50

–100 –150

47

48 49 50 51 52 Nickel content (Atomic %)

53

Figure 3.1: Effect of Ni contents on MS and AF temperatures in near-equiatomic NiTi alloy, where closed circles refer to MS temperatures and open circles represent AF temperatures.

3.1 Ni or Ti elemental variation in binary NiTi alloy systems 

 17

Lin et al. [41] examined the effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy by internal friction and shear modulus measurements, hardness test, and TEM observation. It was found that the martensite stabilization can be induced by cold rolling at room temperature. Both deformed martensite structures and deform-induced dislocations/vacancies are considered to be related to the martensite stabilization. After the occurrence of the first reverse martensitic transformation of B19′ → B2, the martensite stabilization dies out and the transformation temperatures are depressed by retained dislocation on subsequent thermal cycles. The experimental results indicate that the martensite stabilization can depress the rate of martensitic transformation in the equiatomic TiNi alloy [41]. Martensite stabilization was also mentioned [42]. Structural changes with cooling and heating of Ni–Ti alloys involve three phases: austenite (B2), R-phase, and martensite (B19′). Jordan et al. [43], studying on an equiatomic Ni50Ti50, employed techniques such as DSC and internal friction to investigate structural transformation between any two phases, which is identified in these measurements by a peak in the studied property versus temperature plot. It was noted that, although, upon cooling, there is the evidence of two transformations, austenite → R-phase and R-phase → martensite, have been established in the literature during the reverse transformation from martensite on heating, the transformation can be associated with one or two peaks. It was observed that the reverse transformation of stress-induced martensite occurred at a temperature of approximately 20 K higher than that of thermal martensite. The increase in temperature for the reverse transformation was indicative of a stabilization effect, which is attributed to the change in the accommodation morphology of martensite variants from a self-accommodating state for the thermal martensite to an orientated state for the stress-induced martensite [44]. Lin et al. [45] studied the effect of cold rolling during the reverse transformation on the tensile behavior. It was reported that if the cold-rolled equiatomic TiNi alloy is subjected to a reversed martensitic transformation at temperature