THE STUDY OF DISLOCATION GLIDE IN ICE BY SYNCHROTRON RADIATION X-RAY TOPOGRAPHY

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THE STUDY OF DISLOCATION GLIDE IN ICE BY SYNCHROTRON RADIATION X-RAY

TOPOGRAPHY

S. Ahmad, M. Ohtomo, R. Whitworth

To cite this version:

S. Ahmad, M. Ohtomo, R. Whitworth. THE STUDY OF DISLOCATION GLIDE IN ICE BY SYN-

CHROTRON RADIATION X-RAY TOPOGRAPHY. Journal de Physique Colloques, 1987, 48 (C1),

pp.C1-175-C1-181. �10.1051/jphyscol:1987125�. �jpa-00226270�

JOURNAL DE PHYSIQUE

C o l l o q u e C1, s u p p l 6 m e n t a u n o 3 , Tome 4 8 , m a r s 1 9 8 7

THE STUDY OF DISLOCATION GLIDE IN ICE BY SYNCHROTRON RADIATION X-RAY TOPOGRAPHY

S. AHMAD, M. OHTOMO and R.W. WHITWORTH

D e p a r t m e n t of P h y s i c s , U n i v e r s i t y of B i r m i n g h a m , G B - B i r m i n g h a m B 1 5 2TT, G r e a t - B r i t a i n

R6sum6

-

A b s t r a c t

Using t h e X-ray beam from a synchrotron r a d i a t i o n source, sequences of topographs have been o b t a i n e d showing d i s l o c a t i o n s i n s i n g l e c r y s t a l s o f i c e w i t h exposure times o f l e s s t h a n 1 min. D i s l o c a t i o n motion under s t r e s s h a s been s t u d i e d i n t h e very e a r l y s t a g e s o f p l a s t i c deformation. The topographs r e v e a l d e t a i l s about d i s l o c a t i o n v e l o c i t i e s , t h e p r o c e s s o f m u l t i p l i c a t i o n and t h e n a t u r e o f g l i d e on b o t h b a s a l and p r i s m a t i c p l a n e s .

INTRODUCTION

X-ray topography i s a w e l l e s t a b l i s h e d t e c h n i q u e f o r o b s e r v i n g d i s l o c a t i o n s i n c r y s t a l s , and f o l l o w i n g t h e p i o n e e r i n g experiments o f Webb and Hayes [l] a number o f groups have a p p l i e d it t o s t u d y d i s l o c a t i o n motion i n i c e [2-81. I c e i s a

p a r t i c u l a r l y s u i t a b l e m a t e r i a l f o r s t u d y by t h i s technique because it h a s a

r e l a t i v e l y low X-ray a b s o r p t i o n c o e f f i c i e n t and because c r y s t a l s can b e grown which have a s u f f i c i e n t l y low d i s l o c a t i o n d e n s i t y f o r t h e motion o f i n d i v i d u a l d i s l o c a t i o n s t o be followed even i n c r y s t a l s a few m i l l i m e t r e s t h i c k .

Experiments u s i n g a c o n v e n t i o n a l X-ray s o u r c e and a Lang camera o r a v a r i a n t o f t h i s method r e q u i r e exposure times of t h e o r d e r o f an hour. The s t u d y o f d i s l o c a t i o n motion i n t h i s way r e q u i r e s much p a t i e n c e , and, a s we s h a l l s e e , i s s u b j e c t t o e f f e c t s o f recovery d u r i n g t h e o b s e r v a t i o n s . The s i t u a t i o n can be improved by t h e use o f a h i g h i n t e n s i t y X-ray g e n e r a t o r [8,9], b u t t h e a v a i l a b i l i t y o f t h e h i g h l y c o l l i m a t e d i n t e n s e beam o f ' w h i t e ' X-rays from a synchrotron r a d i a t i o n s o u r c e changes t h e c h a r a c t e r o f t h e t e c h n i q u e and makes it r e l a t i v e l y simple t o o b t a i n l o n g sequences o f high q u a l i t y topographs w i t h exposure t i m e s l e s s t h a n a minute. This paper p r e s e n t s p r e l i m i n a r y o b s e r v a t i o n s o b t a i n e d u s i n g t h e Synchrotron Radiation F a c i l i t y a t t h e SERC Daresbury Laboratory, England.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987125

C1-176 JOURNAL DE PHYSIQUE

EXPERIMENTAL DETAILS

S i n g l e c r y s t a l s o f i c e were grown i n o u r l a b o r a t o r y by t h e technique

d e s c r i b e d i n 1101 o r an e a r l i e r v e r s i o n o f i t . Specimens measuring 20 x 1 0 x 1.5 mm were c u t from them i n v a r i o u s o r i e n t a t i o n s and mounted i n a compression s t r e s s i n g a p p a r a t u s c o n t a i n e d i n a small c r y o s t a t cooled by c o l d n i t r o g e n gas t o t h e d e s i r e d temperature (normally -20°C). The c r y o s t a t had m u l t i - l a y e r melinex windows, and was mounted on a goniometer w i t h t h e synchrotron r a d i a t i o n beam p a s s i n g t h r o u g h - t h e l a r g e f a c e s o f t h e specimen. Each Laue d i f f r a c t i o n s p o t s o formed i s a p r o j e c t e d image o f t h e specimen i n which d i s l o c a t i o n s a r e v i s i b l e . The c r y s t a l o r i e n t a t i o n was normally chosen s o t h a t a (1i00} topograph was formed u s i n g 0.8 8 r a d i a t i o n , and t h i s topograph was recorded on h i g h r e s o l u t i o n 35 mm f i l m with exposures o f 10 t o 40 s . Without moving t h e specimen known compressive s t r e s s e s could be a p p l i e d a l o n g i t s l e n g t h under remote c o n t r o l t y p i c a l l y f o r p e r i o d s o f 60 s , and sequences o f topographs were o b t a i n e d showing t h e motion o f d i s l o c a t i o n s produced by

s u c c e s s i v e a p p l i c a t i o n s o f t h e s t r e s s . The c r y s t a l was exposed t o t h e X-rays o n l y d u r i n g t h e t a k i n g of a topograph and not d u r i n g deformation. With t h e l o n g e r wavelengths i n t h e X-ray spectrum absorbed by an aluminium f i l t e r t h e r e was no v i s i b l e evidence o f r a d i a t i o n damage, and u s i n g an embedded thermocouple we were unable t o d e t e c t any s i g n i f i c a n t h e a t i n g by t h e beam.

Figure 1 Topographs o f d i s l o c a t i o n s p r o j e c t e d on (0001) p l a n e with d i f f r a c t i o n v e c t o r 1100

.

.

OBSERVATIONS

The o n s e t o f deformation i n b a s a l s l i p i s i l l u s t r a t e d i n f i g u r e 1, which shows a sequence o f topographs i n which t h e d i s l o c a t i o n s a r e imaged on t h e b a s a l p l a n e . D i s l o c a t i o n segments moving under s t r e s s t e n d t o t a k e up 60° o r screw o r i e n t a t i o n s s o a s t o form loops o f hexagonal shape, b u t i f t h e c r y s t a l i s allowed t o s t a n d f o r an hour o r s o w i t h no s t r e s s t h e s e l o o p s become more rounded. Extensive

d i s l o c a t i o n m u l t i p l i c a t i o n o c c u r s , w i t h most o f t h e d i s l o c a t i o n s s p r e a d i n g o u t from r e g i o n s where t h e r e i s a l r e a d y some handling o r mounting damage. A p a r t i c u l a r l y c l e a r example o f a d i s l o c a t i o n s o u r c e i s seen i n t h e lower r i g h t o f t h e s e topographs;

two segments s p i r a l i n o p p o s i t e d i r e c t i o n s around a common p o i n t , marked w i t h a n arrow i n ( a ) , a t which t h e r e must be a s t e p from one b a s a l p l a n e t o a n o t h e r [ll].

The v e l o c i t i e s of s t r a i g h t 60° and screw d i s l o c a t i o n segments i n t h i s specimen a t a r e s o l v e d s t r e s s o f 0.16 MPa and - 2 0 O ~ a r e t y p i c a l l y 3 x lo-' m s - ' . This l i e s i n t h e range quoted by p r e v i o u s workers [5,7], although t h e y do n o t r e p o r t s e e i n g s t r a i g h t segments and w i t h t h e i r l o n g e r exposure t i m e s t h e i r c r y s t a l s would have been s u b j e c t t o some e f f e c t s o f recovery.

Figure 2 shows t h e development o f a p a i r o f l o o p s from a d i s l o c a t i o n of known Burgers v e c t o r l y i n g i n t h e c e n t r e o f ( a ) and g l i d i n g t o t h e r i g h t under s t r e s s . There a r e presumed t o be two s t e p s from one b a s a l p l a n e t o a n o t h e r a t P and Q.

60° segments form and g l i d e from P and from t h e lower s i d e o f Q i n a p e r f e c t l y normal way, b u t t h e segment g l i d i n g upwards from Q becomes jagged and moves a n o r d e r o f magnitude f a s t e r i n a d i r e c t i o n p a r a l l e l t o t h e Burgers v e c t o r . Such behaviour h a s been observed q u i t e f r e q u e n t l y and i s o f t e n a s s o c i a t e d w i t h a 'tongue' such a s t h a t s e e n a t R i n ( c ) . S e v e r a l examples can b e s e e n on c l o s e i n s p e c t i o n o f f i g u r e 1. Somewhat s i m i l a r f e a t u r e s have been r e p o r t e d by Higashi e t a l . [7,8]

and a t t r i b u t e d t o g l i d e on (1100) p r i s m a t i c p l a n e s , b u t i+e p r o j e c t i o n o f f i g u r e 2 i s such t h a t t h e segment a t R cannot i t s e l f l i e on t h e (1100) p l a n e . F a s t motion o f approximately edge o r 30° segments i s o c c u r r i n g , which may o r may n o t b e a s s o c i a t e d w i t h g l i d e o f t h e s t e p from Q a l o n g t h e ( l h ) p l a n e .

Figure 2 Development o f d i s l o c a t i o n loops on (0001) p l a n e a t -loOc. Resolved s t r e s s 0.06 MPa a p p l i e d f o r 1 min between ea& topograph. D i f f r a c t i o n v e c t o r g and Burgers v e c t o r b a s i n d i c a t e d .

C1-178 J Q U R N A L DE PHYSIQUE

To i n v e s t i g a t e f u r t h e r t h e p o s s i b i l i t y o f p r i s m a t i c g l i d e a specimen was o r i e n t e d s o t h a t compression would produce s l i p on t h e b a s a l p l a n e s b u t t h e topographic image i s p r o j e c t e d on a p l a n e p e r p e n d i c u l a r t o t h e s l i p p l a n e . The b a s a l p l a n e s a r e seen edge on i n f i g u r e s 3 ( a ) t o (dl and d i s l o c a t i o n s on them appear a s s t r a i g h t l i n e s . I n f i g u r e 3 ( e ) t h e specimen i s a s i n ( d ) b u t r o t a t e d by 15O s o t h a t t h e b a s a l p l a n e s a r e seen o b l i q u e l y with d i s l o c a t i o n s l y i n g randomly on them. Under s t r e s s t h e t r a c e s o f d i s l o c a t i o n s i n topographs ( a ) t o ( d ) change i n l e n g t h due t o g l i d e on t h e b a s a l p l a n e s . S h o r t segments connecting two b a s a l p l a n e s a r e edge d i s l o c a t i o n s with BurgErs v e c t o r s [1210] o r [ T l l ~ ] which can g l i d e on t h e p r i s m a t i c p l a n e s (1010) and (0110) r e s p e c t i v e l y . Such segments a r e s e e n t o

Figure 3 Topographs ( a ) t o ( d ) o f specimen w i t h b a s a l p l a n e s seen edge on and w i t h compressive s t r e s s v e r t i c a l i n diagram. I n ( e ) t h e specimen a t

(dl h a s been r o t a t e d 15O about t h e d i r e c t i o n o f t h e 1100 d i f f r a c t i o n v e c t o r . Temperature -20°c. Resolved s t r e s s on b a s a l p l a n e 0.14 m a , and on a c t i v e p r i s m a t i c p l a n e s approx. 0.17 MPa.

move under t h e a p p l i e d s t r e s s a s a t L and M; t h e y have v e l o c i t i e s s i m i l a r t o t h o s e o f t h e f a s t segments d i s c u s s e d i n connection w i t h f i g u r e 2. There i s evidence of more e x t e n s i v e p r i s m a t i c s l i p moving i n from t h e t o p edge o f t h e topographs i n f i g u r e 3. I n o u r l i m i t e d experiments we have n o t s e e n screw d i s l o c a t i o n s c r o s s g l i d e from one b a s a l p l a n e t o a n o t h e r , and we do n o t know whether t h e s t e p s on d i s l o c a t i o n s from one b a s a l p l a n e t o a n o t h e r have been produced by c r o s s g l i d e o r by climb. I t seems t h a t edge d i s l o c a t i o n s a r e very mobile on t h e p r i s m a t i c p l a n e s b u t screw d i s l o c a t i o n s a r e n o t .

The p r i s m a t i c q l i d e i n f i g u r e 3 o c c u r s simultaneously w i t h b a s a l s l i p .

F i g u r e 4 shows a specimen viewed edge on $ t h e b a s a l p l a n e s a s i n f i g u r e 3 b u t w i t h t h e a x i s o f compression p e r p e n d i c u l a r t o

LOOO~]

d i s l o c a t i o n s would have moved and c o u l d have l e d t o e x t e n s i v e d i s l o c a t i o n m u l t i p l i c a t i o n on t h o s e p l a n e s .

F i g u r e 4 ( a ) t o ( c ) Specimen compressed along [li00] and observed i n p l a n e p e r p e n d i c u l a r t o (0001). ( d ) same a s ( c ) b u t r o t a t e d 15O a b o u t [1i00].

Temperature -20°c. Resolved s t r e s s on p r i s m a t i c p l a n e s approx. 0.5 MPa.

CONCLUSIONS

Synchrotron r a d i a t i o n topography i s a powerful t o o l f o r t h e s t u d y o f t h e p r o c e s s o f d i s l o c a t i o n movement i n i c e . Our p r e l i m i n a r y c o n c l u s i o n s a r e :

1. Under s t r e s s on t h e b a s a l p l a n e s d i s l o c a t i o n s tend t o t a k e up and g l i d e i n screw and 60° o r i e n t a t i o n s , s u g g e s t i n g t h a t some form of P e i e r l s s t r e s s i s l i m i t i n g t h e i r motion.

2 . S h o r t segments of edge d i s l o c a t i o n can g l i d e on p r i s m a t i c p l a n e s an o r d e r o f magnitude more r a p i d l y t h a n screw o r 60° d i s l o c a t i o n s g l i d e on t h e b a s a l p l a n e . 3. The f a c t t h a t p l a s t i c deformation o f i c e i s e a s i e r on t h e b a s a l p l a n e s t h a n on

t h e p r i s m a t i c p l a n e s a r i s e s from t h e immobility o f screw d i s l o c a t i o n s on t h e p r i s m a t i c p l a n e s and t h e consequent l a c k o f an e f f e c t i v e m u l t i p l i c a t i o n mechanism.

4. Edge-like segments on t h e b a s a l p l a n e sometimes move much f a s t e r t h a n s c r e w o r 60° segments. These f a s t segments c o u l d be a s s o c i a t e d with s t e p s on t h e p r i s m a t i c p l a n e , b u t t h i s i s n o t c l e a r .

These o b s e r v a t i o n s have major i m p l i c a t i o n s f o r models o f d i s l o c a t i o n m o b i l i t y [12], b u t it i s eremature t o s p e c u l a t e on t h e s e i s s u e s h e r e .

C1-180 JOURNAL D E PHYSIQUE

ACKNOWLEDGEMENTS

This work was supported by research grants from the Science and Engineering Research Council. We are grateful for the use of the facilities at the Daresbury Laboratory and for advice in particular from Dr D. K. Bowen and Dr G. F. Clarke.

REFERENCES

WEBB, W. W. and HAYES, C. E., Phil. Maq.

16

I]

[3] JONES, S. J. and GILRA, N. K., Physics and Chemistry of Ice, Eds E. Whalley, S. J. Jones and L. W. Gold (Ottawa, Royal Society of Canada, 1973) 344-349.

FFUDA, A. and HIGASHI, A., Crystal Lattice Defects

4

MAI, C., C. R. Acad. Sci. Paris, B282 (1976) 515-518.

MA?, C., PEREZ, J., TATIBOU~T, J. and VASSOILLE, R., J. Physique Lett.

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(1978) L-307 -310.

[7] HIGASHI, A., FUKUDA, A., HONDOH, T., GOTO, K. and AMAKAI, S. Yamada

Conference IX on Dislocations in Solids, Eds H. Suzuki, T. Ninomiya, K. Sumino and S. Takeuchi (University of Tokyo Press, 1985) 511-515.

(81 HIGASHI, A., FUKUDA, A., HONDOH, T., GOTO, K. and AMAKAI, S . Bull. Fac.

Enqng. Hokkaido Univ. No. 125 (1985) 111-121.

GOTO, K., HONDOH, T. and HIGASHI, A. Jap. J. appl. Phys.

25

OHTOMO, M., AHMAD, S. and WHITWORTH, R. W. This conference.

AHMAD, S., OHTOMO, M. and WHITWORTH, R. W. Nature, Lond.

319

COMMENTS

T. HONDOH

I am impressed by your beautiful topographs showing Frank-Read type dislocation sources in ice. Did you observe dislocation generation at free surfaces also ? Do you think which mechanism, F-R source or surface/grain boundary, is dominant in plastic deformation of ice ?

I am also impressed by your clear observation of edge segments gliding fast on prismatic planes which was not observed clearly in our study. In my opinion, it is not surprising that the velocity for prismatic glide is much faster than that for basal glide. The velocity observed for prismatic glide should be compared with the kink velocity on basal glide which cannot be observed by X-pay topography but should be much faster than the dislocation velocity observed for basal glide. Then, both must be the same order of magnitude.

Answer :

In our crystals dislocation generation comes from dislocations already in the crystal, mainly from mounting damage at the ends. We have seen multiplication occur where a dislocation cuts the surface, but have not observed the nucleation of a fresh loop at the ice surface. We have not examined a crystal containing a sub-boundary.The prismatic segments which were observe to move rapidly correspond to super-jogs on basal dislocations not to kinks.There is no reason to expect a jog to move particularly rapidly,indeed jogs are usually thought of obstacles to glide.

A. HIGASHI

I am pleased to hear that edge segments of dislocations on the prism plane move faster than screws on the basal plane as we have predicted from our observations of

X-ray topography (see Fukuda, Hondoh, Higashi, this symposium). My question is, whether any stress was exerted on the immobile part (step in F-R source) of dislocation between two adjacent basal planes or not, because the steps seems to be the same as the jog which we observed to move fast under non-basal shear stress.

Answer :

I agree that a jog in a basal dislocation is an edge segment on a prismatic plane, but in our experiments not all such segments glide on the prismatic plane. Immobile ones may be too short, otherwise pinned or in unfavourable orientation. We do not have enough evidence to offer any fuller explanation.

J. PEREZ

DO you know the force applied on each type of dislocation, as given by the PEACH and XOEHLER formula ; is this any difference between the force applied on fast moving dislocation segments and that on basal dislocations ?

Answer :

This can in principle be calculated in every cade from the geometry of the system where the Burgers vector has been identified by using several diffraction vector. It cannot explain the differences between screw and 60° segments on the one hand and the "fast" segments or prismatic edge segments on the other.