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The conditions n e c e s s a r y for m e c h a n i c a l twin n u c l e a t i o n a r e c o n s i d e r e d . It is concluded that a high e l a s t i c s t r a i n e n e r g y a c c u m u l a t i o n m u s t p r e c e d e twin n u c l e a t i o n , and that twin f o r m a tion p r o v i d e s an efficient m e a n s of e x c e s s e n e r g y a c c o m m o d a t i o n . It is suggested that the s i g nificance of the r e s o l v e d s h e a r s t r e s s e s p r e s e n t is l i m i t e d to the slip d e f o r m a t i o n , which m u s t p r e c e d e , a c c o m p a n y , and follow twin f o r m a t i o n in r e a l c r y s t a l s . The r e s u l t a n t s h e a r d e f o r m a t i o n is not i n h e r e n t in twinning i t s e l f , but o r i g i n a t e s in the a s s o c i a t e d slip. In the a b s e n c e of s l i p , in a p e r f e c t c r y s t a l , twins can form by atom shuffles r e q u i r i n g no o v e r a l l shear.

T H E e a r l y concepts of twinning a s s u m e d twin n u c l e i to f o r m by a homogeneous l a t t i c e s h e a r , even when the e x i s t e n c e of twinning d i s l o c a t i o n s in the n o n c o h e r e n t b o u n d a r i e s had been postulated, t The q u e s t i o n whether twins f o r m i n s t a n t a n e o u s l y or by the s u c c e s s i v e s h e a r of the planes within the twin volume was not explored. One of the obvious p r o b l e m s p r e s e n t , h o w e v e r , was the fact that twin f o r m a t i o n o c c u r s at applied s t r e s s l e v e l s v e r y much lower than the t h e o r e t i c a l s t r e n g t h of the l a t t i c e . In the s i m i l a r d i s c r e p a n c y between the e x p e r i m e n t a l l y o b s e r v e d and the t h e o r e t i c a l flow s t r e s s l e v e l s in s l i p , a v e r y s a t i s f a c t o r y a g r e e m e n t has b e e n obt a i n e d by the application of the d i s l o c a t i o n t h e o r y . It i s u n d e r s t a n d a b l e , t h e r e f o r e , that one should expect some s i m i l a r , n o n h o m o g e n e o u s m e c h a n i s m to o p e r a t e a l s o in twin f o r m a t i o n . The a t t e m p t s to apply d i s l o c a tion f o r m a l i s m to twinning, however, have m e t at b e s t with l i m i t e d s u c c e s s . In p a r t i c u l a r t h e r e i s , as yet, no s a t i s f a c t o r y explanation for the t r a n s i t i o n f r o m slip to t w i n n i n g with d e c r e a s i n g t e m p e r a t u r e or with i n c r e a s ing s t r a i n r a t e . A d i s l o c a t i o n model for twinning r e q u i r e s e i t h e r the f o r m a t i o n of identical t w i n n i n g d i s l o c a t i o n s on e v e r y t w i n n i n g plane, or the e x i s t e n c e of a m e c h a n i s m by which a single twinning d i s l o c a t i o n f o r m e d can r e peatedly move on to the a d j a c e n t plane. In e i t h e r case the p r o c e s s m u s t be capable of o p e r a t i n g under an applied s t r e s s v e r y low c o m p a r e d with the s h e a r m o d u lus. Consequently the e x i s t e n c e of s t