Electron Microscopy and Structure of Materials [Reprint 2020 ed.] 9780520323230

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ELECTRON MICROSCOPY AND STRUCTURE OF MATERIALS

Electron Microscopy and S t r u c t u r e of M a t e r i a l s

GARETH THOMAS,

Editor

RICHARD M. FULRATH, Associate

Editor

Department of Materials Science and Engineering, and Lawrence Berkeley Laboratory University of California Berkeley, California ROBERT M. FISHER, Associate Editor U.S. Steel Corporation Research Center Monroeville, Pennsylvania

Proceedings of the Fifth International Materials Symposium "The Structure and Properties of Materials—Techniques and Applications of Electron Microscopy," held at the University of California, Berkeley, September 13-17, 1971

University of California Press, Berkeley, Los Angeles, London

University of California Press Berkeley and Los Angeles, California University of California Press, Ltd. London, England

Copyright © 1972, by The Regents of the University of California ISBN:

0-520-02114-2

Library of Congress Catalog Card Number: Printed in the United States of America

79-175119

iii

PREFACE This fifth in a series of International Materials Symposia had as its theme "Techniques and Applications of Electron Microscopy" and was held exactly ten years after the inaugural symposium "Electron Microscopy and Strength of Crystals" in 196l. In planning the conference it was felt that major new advances in electron microscopy, such as high voltage instruments, velocity analysis, high resolution scanning and special effects associated with many beam interactions, particularly the "disappearance voltage" phenomenon, channelling and improved resolution of lattice defects and the broadening areas of applications should be brought to the attention of a wide range of scientists and engineers. Although a spectrum of materials and fields of specialization have been covered here, the goals and techniques of materials characterization are, nevertheless, common, as is the challenge of utilizing the knowledge generated by research. It is hoped that this conference and the published proceedings will contribute to the exchange of ideas and improve the interdisciplinary approach and levels of sophistication necessary to solve even the most practical problems in engineering materials research. The conference was arranged by inviting speakers to give representative reviews of the current state of the art of characterization of microstructure, and in each session these reviews were followed by contributed papers describing current research programs. Some selection on the basis of relevance to the conference theme was made after abstracts were submitted, but the papers themselves have not been critically refereed because, in order to ensure rapid publication, camera-ready manuscripts were required at the time of the conference. Their contents and accuracy are, therefore, the responsibility of the authors, but the discussions included after each paper serve to indicate clarifications and points of differing opinions. The Committee: R. M. Fisher R. M. Fulrath G. Thomas-chairman

V

Contents I.

T e c h n i q u e s and New D e v e l o p m e n t s in E l e c t r o n and Scanning M i c r o s c o p y

Some Recent T r e n d s in T h e o r y and Application of T r a n s m i s s i o n a n d S c a n n i n g M i c r o s c o p y of Crystalline Materials P . B. H i r s c h

1

Applications and R e c e n t D e v e l o p m e n t s in Transmission Electron Microscopy W . L . B e l l a n d G. T h o m a s

23

F u n d a m e n t a l A s p e c t s a n d A p p l i c a t i o n s of High Voltage E l e c t r o n Metallography R. M. F i s h e r

60

S t u d i e s of F r a c t u r e i n t h e H i g h V o l t a g e Electron Microscope R . W. B a u e r , R . H. G e i s s , R . L,. L y l e s , a n d H. G . F . W i l s d o r f

85

D y n a m i c S t u d i e s of P l a s t i c D e f o r m a t i o n b y M e a n s of H i g h V o l t a g e E l e c t r o n M i c r o s c o p y T. Imura

104

n - B e a m L a t t i c e I m a g e s of C o m p l e x O x i d e s J . G. A l l p r e s s a n d J . V . S a n d e r s

134

S i m u l a t i o n of E l e c t r o n T r a n s m i s s i o n I m a g e s of C r y s t a l s Containing R a n d o m and P e r i o d i c A r r a y s of C o h e r e n c y S t r a i n C e n t e r s P . J . F i l l i n g h a m , H. J . L e a m y , and L. E. Tanner

163

Kilovolt E l e c t r o n E n e r g y Dissipation vs. P e n e t r a t i o n Distance in High A t o m i c Number Materials M . S. C h u n g , W. J . D e v o r e , a n d T. E. Everhart

171

E n e r g y A n a l y s i s and E n e r g y Selection in E l e c t r o n M i c r o s c o p y and E l e c t r o n Diffraction J. Silcox and R. Vincent

188

vi

T h e U s e of a n A n a l y t i c a l E l e c t r o n M i c r o s c o p e (EMMA-4) to I n v e s t i g a t e Solute C o n c e n t r a t i o n s in Thin Metal Foils G. W. L o r i m e r , M . J . N a s i r , R . B. N i c h o l s o n , K . N u t t a l l , D . E . W a r d , and J. R. Webb

222

D e t e r m i n a t i o n of C o m p o s i t i o n a l P r o f i l e s N e a r G r a i n B o u n d a r i e s by E l e c t r o n D i f f r a c t i o n I . G. G r e e n f i e l d a n d I . T w e e r

236

Analytical Methods in P h o t o e m i s s i o n E l e c t r o n Microscopy L. Wegmann

246

A p p l i c a t i o n of T h e r m i o n i c E m i s s i o n E l e c t r o n M i c r o s c o p y t o t h e S t u d y of P h a s e Transformations K . R . K i n s m a n a n d H. I . A a r o n s o n

2 59

C r y s t a l l o g r a p h i c A s p e c t s of S c a n n i n g E l e c t r o n Microscopy: The E l e c t r o n Channelling P a t t e r n Technique E . M. Schulson

286

A p p l i c a t i o n of S e l e c t e d - A r e a E l e c t r o n Channeling P a t t e r n s Obtained in the S E M to Deformation Studies R . S t i c k l e r a n d G. R . B o o k e r .

301

M i c r o s t r u c t u r e with the SEM—A New A p p r o a c h Through SEM Fractography O. Johari

313

S o m e C o m m e n t s o n t h e U s e of t h e S c a n n i n g E l e c t r o n Microscope in F r a c t o g r a p h y J . D. E m b u r y a n d D . O s b o r n e •

333

S c a n n i n g E l e c t r o n M i c r o s c o p y S t u d y of a n I o n Etched U r a n i u m Alloy G. J . T h o m a s a n d C . J . M i g l i o n i c o

339

High T e m p e r a t u r e Scanning E l e c t r o n Microscopy R. F u l r a t h

347

vii

H. A.

Applications

Interfaces

Some S u r f a c e and I n t e r f a c e P r o b l e m s in Material Science D. A. E v e r e s t and A. Kelly

352

Dislocation Behavior and C o n t r a s t E f f e c t s A s s o c i a t e d with Grain Boundaries and Related Internal Boundaries M. J. Marcinkowski

382

A p p l i c a t i o n s of t h e S E M , T E M , a n d F I M i n t h e A n a l y s i s of S t r u c t u r e a n d E n e r g y of Metal Interfaces L . E . M u r r , O . T . I n a l , a n d G. I . Wong

.

.

Contact S t r e s s e s Between Metal Particles K . E a s t e r l i n g a n d A . Tholcin B.

.

.

417 427

Plastic Deformation

D i s l o c a t i o n S t r u c t u r e a n d t h e S t r e n g t h of Titanium H . C o n r a d , K. O k a s a k i , V . G a d g i l , and M. Jon

438

T h e N a t u r e a n d S t a b i l i t y of D i s l o c a t i o n D i s t r i b u t i o n s i n b. c . c . M e t a l s J . D. Boyd and J . D. E m b u r y

470

S y n e r g y of T r a n s m i s s i o n E l e c t r o n M i c r o s c o p y , Selected A r e a X - R a y T o p o g r a p h y and X - R a y Line P r o f i l e A n a l y s i s in D e f o r m a t i o n Studies of B e r y l l i u m S . W e i s s m a n n a n d V. C . K a n n a n

484

A v o i d a n c e of S o f t e n i n g i n D i l u t e C u - Z r A l l o y s C. E . Sohl, A. Kidron, and R . J . De A n g e l i s

494

T r a n s f o r m a t i o n s Involving C o h e r e n t P h a s e s and E f f e c t s o n M e c h a n i c a l P r o p e r t i e s of A l l o y s H. W a r l i m o n t

505

T h e I n f l u e n c e of C o h e r e n c y S t r a i n F i e l d s o n t h e Dislocation A r r a n g e m e n t in T w o - P h a s e Alloys K . H a r t m a n n a n d H. H a b e r k o r n

538

viii

The Deformation Behavior of T w o - P h a s e Aluminium C r y s t a l s A. T. S t e w a r t and J . W. M a r t i n

549

Strengthening by Dislocation S u b s t r u c t u r e s in Multiphase Aluminum A l l o y s H. A. Lipsitt, C. M. Sargent, and G. C. W e a t h e r l y

559

A n a l y s i s of M i c r o s t r u c t u r e s in N i c k e l - B a s e A l l o y s : Implications for Strength and Alloy De sign J . M. Oblak and B. H. Kear

566

Some A s p e c t s of Structure P r o p e r t y Relationships in M a t e r i a l s J . Nutting

617

The Effect of Carbide P r e c i p i t a t i o n on the M e c h a n i c a l P r o p e r t i e s in C e r t a i n Cobalt Based A l l o y s V. R a m a s w a m y , P . R. Swann, and D. R. F . West

637

I n t e r g r a n u l a r F r a c t u r e of a Spinodally Decomposed C u - N i - F e Alloy R. J . Livak and W. W. G e r b e r i c h

647

M i c r o s t r u c t u r e - Me c h a n i c a l Pr ope r t y - F r a c t u r e Relationships in the A l l - B e t a Titanium Alloy Ti- 13V- H C r - 3A1 G. H. Narayanan and T. F. Archbold

657

T r a n s m i s s i o n and Scanning Electron Microscope Observations of Niobium-Hafnium A l l o y s R. W. C a r p e n t e r

667

Slip and M e c h a n i c a l Twinning in Ni-Ni^Nb D i r e c t i o n a l l y Solidified Eutectic Alloy C. G r o s s i o r d , G. Lesoult, and M. Turpin

678

The Role of M e t a l l o g r a p h i c Techniques in the Understanding of and Use of S u p e r p l a s t i c i t y R. B. Nicholson

689

C r e e p in O r d e r e d O r d e r e d b. c. c. P . R. Strutt, and Y. H.

722

B i n a r y and T e r n a r y Alloys G. M. Rowe, J . C. I n g r a m , Choo

ix

Application of a Stabilized Recovery-Induced Substructure for Improving C r e e p R e s i s t i v i t y Y. K. Lindroos and K. O. Yilpponen

732

F e a t u r e s of Fatigue M e c h a n i s m s C l a r i f i e d by Scanning Electron M i c r o s c o p y D. E. MacDonald and W. A. Wood

742

L o w - C y c l e Fatigue in Udimet 500 G. P . Sabol, T. F. Hengstenberg, and D. M. Moon

753

Fatigue Deformation in Ni-Cr L a m e l l a r Composite s R . Kossowsky, K. Sadananda, and M. Doner

764

C.

Ferrous Alloys

S t r u c t u r e and Strength of F e r r o u s M a r t e n s i t e R. G. Davies and C. L . Magee

7 75

The Influence of T r a n s f o r m a t i o n Behavior on the S t r u c t u r e and M e c h a n i c a l P r o p e r t i e s of Low-Carbon S t e e l s P. C. Becker and A. T. Davenport

786

Effect of Lath Boundary P r e c i p i t a t i o n on F r a c t u r e Toughness of M a r t e n s i t e R. D. Goolsby, W. E. Wood, E. R. P a r k e r , and V. F . Zackay

798

The Influence of Antimony on the High T e m p e r a t u r e T e m p e r i n g of Low Carbon M a r t e n s i t e s Containing Mangane se D. Senicourt and P. R. Krahe

808

E l e c t r o n M i c r o s c o p y Study of Coherent P r e c i p i t a t i o n in F e - N i - C o - M o Mar aging Alloys J . Bourgeot, Ph. M a i t r e p i e r r e , J . Manenc, and B. Thomas

818

Use of an Allotropic P h a s e Change to Enhance Ductility in F e - T a A l l o y s R . H. Jones, E. R. P a r k e r , and V. F. Zackay

829

X

S t r u c t u r a l A s p e c t s of D e n s i f i c a t i o n in N i c k e l and I r o n - N i c k e l C o m p a c t s H. L . G a i g h e r , M. J . K o c z a k , and A. Lawley D.

839

Composite s

M i c r o m e c h a n i s m s of F r a c t u r e i n a W - 5 % F e - 5 % Ni P o w d e r C o m p o s i t e D. G. B r a n d o n , E . A r i e l , and J. Barta

849

Growth and C h a r a c t e r i z a t i o n of R e f r a c t o r y Oxide-Metal Composites R . J . G e r d e s and A . T . Chapman

859

Multiple F r a c t u r e s in F i l a m e n t R e i n f o r c e d Composite s G. R . K e r h u e l and R . H. B r a g g

870

E.

Environmental Effects

High Voltage M i c r o s c o p e Studies of Environmental Reactions P . R . Swann

878

T h e I n f l u e n c e of M i c r o s t r u c t u r e on the S t r e s s C o r r o s i o n C r a c k i n g of Light A l l o y s M. J . B l a c k b u r n and M. O. S p e i d e l

905

D i s l o c a t i o n Bands P r o d u c e d in a f e e N i - C o B a s e A l l o y by E l e c t r o l y t i c Hydrogen Charging J . M. R i g s b e e and R . B . B e n s o n , J r

920

F.

Radiation Damage

Study of P o i n t D e f e c t I n t e r a c t i o n s with D i s l o c a t i o n s by M e a n s of H i g h - V o l t a g e Microscopy K . U r b a n and M. Wilkens

929

E l e c t r o n I r r a d i a t i o n E f f e c t s on the S t r u c t u r e of Copper and A l u m i n u m M. M e s h i i and K . S h i r a i s h i

952

xi

E l e c t r o n D i s p l a c e m e n t D a m a g e i n Graphite and Aluminum S. M. O h r , A . W o l f e n d e n , and T . S. N o g g l e

964

Low T e m p e r a t u r e E l e c t r o n M i c r o s c o p y Applied to N e u t r o n R a d i a t i o n D a m a g e S t u d i e s of Materials A. Bourret

974

The R o l e of E l e c t r o n M i c r o s c o p y in the U n d e r s t a n d i n g of N e u t r o n I r r a d i a t i o n Produced Swelling A . W o l f e n d e n , K. F a r r e l l , and J. O. S t i e g l e r

984

R a d i a t i o n I n d u c e d D i s t o r t i o n and S w e l l i n g of Magne s i u m E . F . S t u r c k e n and C. W. K r a p p

996

C h a r a c t e r i z a t i o n of I r r a d i a t e d C e r a m i c Oxide Nuclear Fuels H. S. R o s e n b a u m , V. E . W o l f f , and T. E. Lannin G.

1017

Non-Metals —Ceramics

The A p p l i c a t i o n s of S c a n n i n g and T r a n s m i s s i o n E l e c t r o n M i c r o s c o p y i n the S e m i c o n d u c t o r Industry E . S. M e i e r a n and T . R. C a s s

1027

D a r k - F i e l d E l e c t r o n M i c r o s c o p y of A m o r p h o u s Semiconductors M. L. R u d e e

10 64

The S t r u c t u r e of V a p o r D e p o s i t e d 4 D T r a n s i t i o n Metals R. L o o p , M. C o l l v e r , and R. H a m m o n d •

•

•

- 1 0 74

N o n - s t o i c h i o m e t r y in C e r a m i c Compounds M. H. L e w i s , J. B i l l i n g h a m , and P . S. B e l l

1084

P r e c i p i t a t i o n in Boron-Doped Vanadium Carbide G. E . H o l l o x , D. J. R o w c l i f f e , and J. W. E d i n g t o n

1116

xii

P l a s t i c D e f o r m a t i o n of T a n t a l u m C a r b i d e U p t o 2200® C J. L. M a r t i n , P . L a c o u r - Gayet, and P. Costa

1131

T r a n s m i s s i o n E l e c t r o n M i c r o s c o p y of Silicon Nitride A . G. E v a n s a n d J . V. S h a r p

1141

Domain Structure and Dislocation Configurations in Niobiumditelluride J. Van Landuyt and S. A m e l i n c k x

1155

M i c r o s t r u c t u r e s a n d M e c h a n i c a l P r o p e r t i e s of Mica G l a s s - C e r a m i c s C . K . C h y u n g , G. H . B e a l l , a n d D . G. G r o s s m a n

1167

A n a l y s i s of M i c r o s t r u c t u r e s i n A l u m i n a P. F. Becher

Ceramics 1195

Water Damage in Glass Fiber/Polyester Composite s K . H . G. A s h b e e

Resin 1205

E f f e c t s of M i c r o s t r u c t u r e o n D e f o r m a t i o n a n d F r a c t u r e of P o r t l a n d C e m e n t P a s t e R . B. W i l l i a m s o n a n d R . P . T e w a r i

1223

D e f o r m a t i o n of L u n a r a n d T e r r e s t r i a l M i n e r a l s J . M. C h r i s t i e , D. T. G r i g g s , R . M. F i s h e r , J . S. L a l l y , A . H . H e u e r , a n d S . V. Radcliffe

1234

E l e c t r o n M i c r o s c o p i c S t u d i e s of S o m e L u n a r a n d Terrestrial Pyroxenes P . E . C h a m p n e s s a n d G. W. L o r i m e r

1245

T h e r m a l and E l e c t r o n B e a m Induced of T o p a z M . S. H a m p a r

1256

Breakdown

Author Index

12 67

Subject Index

1269

I. TECHNIQUES AND NEW DEVELOPMENTS

1

SOME RECENT TRENDS IN THEORY AND APPLICATION OF TRANSMISSION AND SCANNING MICROSCOPY OF CRYSTALLINE MATERIALS P. B. Hirsch Department of Metallurgy, University of Oxford, Oxford, England SUMMARY Recent developments in transmission and scanning Electron Microscopy, aimed at improving the applications of these techniques to the study of materials, are reviewed. Results are described on the determination of partial dislocation separations, using the "weak beam technique" of transmission electron microscopy (TEM). An application of TEM at voltages up to 500 KV to the study of the dislocation structures in dispersion hardened alloys is described. The prospects of using the scanning microscope for detecting defects at the surface of solid specimens, and for transmission microscopy are discussed.

2 INTRODUCTION The first International Materials Symposium was held ten years ago at Berkeley lender the title "Electron Microscopy and Strength of Crystals". A t that time the transmission electron microscopy (TEM) technique had become well established, including the development of specimen preparation techniques and of special specimen stages, and the theory of image contrast had been formulated. There was intense activity in applying the technique to a w i d e range of problems in Metallurgy and Materials Science, some of w h i c h were discussed at that conference. It had however already become clear that TEM suffered from (1) the technique is limited to a number of disadvantages:thin specimens, typically a few 1000X thick at 100 KV; in such specimens certain defects, e.g. dislocations, are liable to r e arrange or to b e lost under some conditions, and in situ treatments e.g. deformation, ageing, or annealing lead to results w h i c h are typical of thin foils and not necessarily of the bulk. For certain materials the preparation of sufficiently thin specimens is also very difficult. (2) the resolution is limited by a number of factors including chromatic aberration from the inelastically scattered electrons reaching the image, and for defects, the considerable w i d t h of the dislocation image observed under normal conditions, w i t h the crystal set near a Bragg position. (3) while TEM is a powerful method for determining the nature of lattice defects, it does not give readily useable information on the local chemical composition of the specimen. In this paper some of the advances will b e discussed briefly w h i c h have b e e n made, some of them rather recently, aimed at removing at least partially these limitations.

RESOLUTION W e a k Beam Technique Considerable advances have b e e n made in the design of electron microscopes, so that lattice spacings of2l - 2 £ have now been observed. Very recently Hashimoto et al have b e e n able to produce images for the first time in the TEM of single heavy atoms of e.g. thorium in specimens of thorium-benzene tetra carboxylic acid, supported on a graphite crystal. Dark field techniques were used to reduce the image noise from the supporting film. The question arises whether such instrumental resolution can be utilised in studies aimed at characterising materials, and defects in particular. One useful technique w h i c h has b e e n developed recently is the so called "weak b e a m

3

technique"^(WBT), first developed effectively by Cockayne, Ray and Whelan , in which the width of dislocation images is reduced by an order of magnitude compared with that of images obtained under ordinary conditions. In the standard method of TEM the dislocation is observed with the crystal set near to a Bragg reflecting position; the image wijlth is typically V§ £ , where £ is the extinction distance . For Cu in the lll®reflectio§, this gives an image width at 100 KV ^80 X; for Si the value would be ^200 X. In the WBT a high resolution dark field micrograph is taken in a reflection g with respect to which the perfect crystal has been set far removed from the Bragg condition. Contrast will be obtained effectively only from those parts of the crystal near the dislocation core, at which the reflecting planes are bent locally into the reflecting position . This occurs when sg

+

(fe

(JL-!)

=

0

(1)

where s is the distance of the Ewald sphere from the reciprocal lattice^point measured in the direction of the incident beam, R is the displacement at a point z in the column along the diffracted beam direction. The peak of the weak beam image will occur approximately at the point corresponding to a column within which (jg.R) satisfying (1) is at a turning poj.nt. The width of the images is typically ^15 X for s ^10 X , and the displacement from the core is of the saml order of magnitude. Many features of the images can be understood in terms of the kinematical theory. Fig 1 shows dislocation images in the same area of deformed silicon, ta^eg in strongly and weakly excited 220 reflections respectively ' . The weak beam picture reveals that the dislocations are dissociated, and new features, having the appearance of dissociated dipoles, can be observed. The WBT provides a powerful geans for determining accurately the position of columns in which — ( g . R ^ ) is sufficiently large e.g. close to dislocation cores, and the positions of the dislocation centres can be determined to within say 5 - 10 X. The facts that the width of the images is very small, and that the images are close to the cores, make the method suitable for accurate determinations of the widths of dissociated dislocations . Using the standard TEM methods direct measurements of ribbon widths have been limited to layer structures, e.g. graphite, molybdenum disulphide jtc., some of which were described

in the 1961 c o n f e r e n c e ^ .

F o r m e t a l s and

alloySgt^e

node method originated by Whelan , and improved by others ' is the best,^glthough a number of less direct, methods have also been used . For the pure f.c.c. metals Cu, Ag, Au the nodes are small and tjie^ode method is difficult to apply. Cockayne, Jenkins and Ray ' have recently applied the WBT to Ag and Cu,

Fig 1.

220 strong beam arid weak beam images of the same area in deformed silicon

Fig 2

Weak beam 220 image of dissociated dislocations in silver

5 12 13 and the latter has also been studied by Stobbs and Sworn ' Fig 2 shows dissociated dislocations in Ag; figs 3 and A show the variation of the separation of partial dislocations with orientation for Ag and Cu, and computed curves based on anisotropic elasticity for the range of stacking fault energies bounding the experimental points. It is clear that the separations are less than or of the same order as the widths of the images taken under normal conditions^ The mean values of y are founc^to be y^ = 16.3 ± 1.7 erg cm , and y 41 ± 9 erg cm . With ®Such small separations core effects may be important; the figures show the valy^s of y calculated assuming a Peierls model for the core. It is'5not known whether this model is a good description of the core, but this serves to indicate that the values of y derived from such small partial separations are liable to significant systematic error due to core effects. There is now the possibility that the electron microscope may give significant information about dislocation cores. It might be noted that the value of y^ is iijL^g(j>gd agreement with the latest extended node measurements; ' for after 20 during wlji^ch y was estimated to be as low as 24 erg cm and as high as 163 erg cm , the new measurement gives a^jjalue very close to the original estimate of^gullman in 1951 . (For review of values of y see Gallagher ) In the case of Si dislocations introduced by deformation have now been found to be dissociated , and both intrinsic and extrinsic nodes are observed. (Occasionally unextended nodes have been observed - Booker and Cullis private communication). The val^e of for intrinsic faults is found to be 51 ± 5 erg cm , and for extrinsic faults y must be of the same order . These results are in agreement with the original observations of Aerts, Delavignette, Siems and Amelinckx , although, because of the strong and many beam cond^igjjsgused in these experiments the interpretation was in doubt. ' ' Fig 5 shows a nearedge dislocation in Si deformed at 1200 C and annealed at 1350 C. In this case parts of the dislocation (pure edge) are constricted, and evidence from other sections jiuggests that these parts have climbed out of the slip plane . The WBT has also been applied to superdislocations in ordered alloys^. Marcincowski described the structure to be expected for a a [ill] superdislocation in the D0^ type ordered alloy, but direct observation was hindered by the small separation of the "partial" dislocations in practical cases. Recently Crawford, Ray and Cockayne resolved the fourfold dissociation of superlattice dislocations in a DO^ordered Fe-26at% A1 alloy. Figs 6 and 7 show the dissociation and the separations as a function of orientation from which the values of 77 ± 12 and 85 ± 16 erg cm have been obtained for the first and second nearest neighbour antiphase boundary energies

6

Fig 4

Fig 6

Separation of partial dislocations as a function of orientation for Cu Fig 5

Near-edge dislocation in Si deformed at 1200°C and annealed at 1350 C, showing constricted dislocations.

Four-fold dissociation Fig 7 in DO^-ordered Fe-26at%Al

Separation of "partial" dislocations as a function of orientation in ordered Fe-26at%Al

7

respectively. The WBT22JISO has important applications to the study of small loops and other defects, and these must be explored further. Energy Filtering In principle the energy selecting microscopes which have been developed recent^,^^or example those employing the Castaing-Henry System ' , should enable images of higher resolution or from thicker specimens for a given resolution, to be obtained, since chromatic aberration effects due to energy losses in the specimen can be effectively eliminated. Only a few instruments of this type have so far been built, and they have not really been much utilised for improving the resolution. The reason for this may be partly that such instruments do not yet attain the resolving power now available with the best commercial microscopes. The usefulness of these instruments for improving resolution should be further assessed, although it is likely that they will in due course be superceded by the higl^ resolution scanning electron microscopes now being developed , in which the absence of an objective eliminates the chromatic error due to energy losses in the specimen, and energy analysis can in any case be carried out very easily.

OBSERVATIONS IN THICKER CRYSTALS High Voltage Electron Microscopy (HVEM) The advantages of HVEM for microstructural studies of materials are now well knowijgand have been described in detail by others (for reviews see ). Recipes for maximising penetratio^gin HVEM have been given by Humphreys, Thomas, Lally and Fisher , based on calculations using the many beam dynamical theory, and supporting experiments. Thus, for example, while for Cu the optimum orientation for transmission in bright field at 100 KV is positive deviation from the first order Bragg position, at 1000 KV the symmetry position is best. These results can be explained in terms of the channelling behaviour of the various Bloch waves as a function of voltage. Improvements in penetration at 1000 KV of 3 - 5 times relaj^ve to 100 KV have been observed in a number of materials. Humphreys has recently considered the problem of possible advantages with respect to penetration in crystalline specimens at voltages 1 MeV to 10 MeV. Fig 8 shows curves of thickness as a function of voltage for Fe for different crystal

100

300 500 1000 3000 5000 Accelerating VoltQg« in kv B.F. PENETRATION, IRON. l(t)=0 0011(0)

10000

Fig 8

0.2 SHEAR

9

STRAIN

Critical particle diameter as a function of shear strain for a to g and y to 6 transitions in deformed Cu-20% Zn alloys with silica particles

wW lion

10

Rows of [lio] prismatic Fig 11 loops (A) and primary [lOl] prismatic_loops (B); 6 structure; (111) crossslip plane section

y structure w i t h a row of [Oil] loops at B. Primary loops at C; (111) (primary plane) section

9 orientations, given that the intensity of the transmitted beam is 10 of the incident beam. From these curves it would seem that there would be little gain in penetration in increasing the voltage above about 1 MtíV. It has been suggested, however, that if the maximum aperture size, which is usually limited at 1 MeV by the Bragg angle, could be increased by setting the crystal at the first order reflection of a high index systematic row, more inelastically scattered intensity could be accepted, and since contrast is better preserved at high voltages, increased penetration might be achieved. Hashimoto et al have used a "multi-beam imaging technique", in which the aperture ^s increased to allow several diffracted beams to form the image . The brightness and resolution of the image are found to be better than in conventional bright field images. These novel techniques must be explored further before their usefulness can be assessed in relation to electron microscopy at and above 1 MeV. The application of HVEM to electron irradiation studies is well known and will not be discussed here. A "bonus" of HVEM is the so called critical voltage effect 3 1 ' 3 2 » 3 3 . The atomic scatteriig factor, f, increases with electron mass and therefore voltage. Consider a reflection 2g; it receives contributions from 0, g, 3g etc; some of these1 contributions are of opposite sign, and consequently can give rise to zero or minimum intensity at a critical voltage. The critical voltage is very sensitive to small variations in f, and can therefore in principle be used to determine f accurately. Values of f accurate to within about 1%, comparable with the best X-ray methods seem attainable, and the method may also be used to determine Debye temperatures, ordering effects, segregation in local areas, etc . This appears to be a potentially powerful technique for a number of applications. Several applications of HVEM will be discussed in this conference. I should like to describe briefly one particualr study of the dislocation structures in deformed two phase Cu - 20% Zn alloys containing s i l ^ a particles, carried out recently by Humphreys and Stewart . One of the fir^íj studies on dispersion hardened alloys was described by Ashby at the 1961 Conference. This is a case where the HVEM (up to 500 KV) was used to complement observations at 100 KV in order to study the structures at the larger particles and to avoid dislocation loss or rearrangement. The mean sizes of the silica particles varied between about O.lu and 1 - 2 p. The dislocation structure is found to be a function of strain and particle size, an increase in either parameter producing a more complex structure. Fig 9 shows the transition curves for the different structures observed, which are summarised in table I.

10

STRUCTURE

BURGERS VECTOR

VOIDS

ROTATIONS

YES

_

101 110 101 Oil 110 Oil

CRITICAL PARTICLE DIAMETER (pm)

a

P

e

P

P

-

Y

P

P

-

p

p

YES

-

0.11