The bain strain, lattice correspondences, and deformations related to martensitic transformations
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IN 1924 in a c l a s s i c
paper c o n c e r n e d with " T h e N a t u r e of M a r t e n s i t e , " E. C. Bain i n t r o d u c e d the c o n cept that a s t r u c t u r a l change of a c e r t a i n kind m i g h t o c c u r by m e a n s of a homogeneous d e f o r m a t i o n in which the p r o d u c t phase is d e r i v e d f r o m the p a r e n t by a s i m p l e " u p s e t t i n g " p r o c e s s ? It s e e m s to have b e e n i m p l i c i t in the thinking of the t i m e that the m a r t e n s i t i c r e a c t i o n in s t e e l s is d i f f u s i o n l e s s , as evidenced by B a i n ' s c o m m e n t that " T h e outstanding c h a r a c t e r i s t i c , perhaps, of martensite is the complete alkotropic change from 7 to ot iron and the completely suppressed precipitation of carbide." Bain also appreciated that the transformation could involve only small relative displacements of neighboring atoms: "It is reasonable, a l s o , that the a t o m s t h e m s e l v e s will r e a r r a n g e with the g r e a t e s t p o s s i b l e facility by a method that will r e q u i r e the l e a s t t e m p o r a r y d i s t o r t i o n ; we cannot expect an atom to s q u e e z e through s o m e n a r r o w s p a c i n g , for i n s t a n c e . A mode of atomic shift was conceived by the author s o m e months ago and was l a t e r d i s c u s s e d with D r . A n c e l St. J o h n who, in the m e a n t i m e , had thought of the s a m e s c h e m e of things and accepted it as highly p r o b a b l e . It is not s u r p r i s i n g that this should have o c c u r r e d , as it is the only easy method of c o n s t r u c t i n g a b o d y - c e n t e r e d atomic s t r u c t u r e f r o m a f a c e - c e n t e r e d cubic c r y s t a l . " Attention was then d r a w n to F i g s . 1 and 2 (taken f r o m the o r i g i n a l in which the figure d e s i g n a t i o n s w e r e 3 and 4), and the Bain s t r a i n , as we now know it, was d i s c u s s e d . It was pointed out that " T h e r e a r e t h r e e axes in the a u s t e n i t e g r a i n on which this change m a y take place by the u p s e t t i n g p r o c e d u r e j u s t i n d i c a t e d , " and suggested that these d i f f e r e n t v a r i a n t s m u s t be o p e r a t i v e in p r a c t i c e : " U n l e s s this m o v e m e n t is c o m p e n s a t e d by e n c o u n t e r i n g soon the opposite state of a f f a i r s along any given d i r e c t i o n a r u p t u r e would e n s u e . " T h i s l a s t c o m m e n t s e e m s to a n t i c i p a t e that f e r r o u s m a r t e n s i t e p l a t e s should be twinned, which is now known to be the c a s e in a n u m b e r of t r a n s f o r m a t i o n s . In a r e c e n t c o m m u n i c a t i o nz Bain, c o m m e n t i n g on the J. S. BOWLES is Research Professor of Metallurgy, University of New South Wales, Kensington, Australia. C. M. WAYMANis Professor, Department of Metallurgyand MiningEngineering,and Materials Research Laboratory, University of Illinois at Urbana-Champaign,I11. METALLURGICALTRANSACT
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