Stress-Assisted and strain-induced martensites in FE-NI-C alloys
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M A R T E N S I T I C t r a n s f o r m a t i o n s can be affected in a v a r i e t y of ways by the application of s t r e s s and by p l a s t i c s t r a i n . Patel and Cohen z d e m o n s t r a t e d quantit a t i v e l y how the initial nucleation of m a r t e n s i t e is a s s i s t e d by applied s t r e s s e s . A m e c h a n i c a l work t e r m consisting of the product of the applied s t r e s s field and the t r a n s f o r m a t i o n s t r a i n is added to the f r e e ene r g y change that p r o v i d e s the d r i v i n g force for the t r a n s f o r m a t i o n . L o w - t e m p e r a t u r e p l a s t i c deformation ( i . e . , cold work) of the p a r e n t austenite p r i o r to the m a r t e n s i t e t r a n s f o r m a t i o n influences both the n u c l e a tion and the growth of m a r t e n s i t e . Nucleation is a s s i s t e d by s m a l l amounts of p l a s t i c deformation, while extensive cold working s u p p r e s s e s nucleation. 2-s It a p p e a r s that the nucleation kinetics of the spontaneous t r a n s f o r m a t i o n of austenite to m a r t e n s i t e can be e x plained only by a s s u m i n g that the p l a s t i c s t r a i n o f the austenite n e c e s s a r y to a c c o m m o d a t e the t r a n s f o r m a tion s t r a i n of each m a r t e n s i t e p l a t e l e t s t i m u l a t e s subsequent additional t r a n s f o r m a t i o n . 6,7 The effect of p r i o r cold working of the austenite m a t r i x on s u b s e quent m a r t e n s i t e plate growth is not so c l e a r . It a p p e a r s that m a r t e n s i t e growth o c c u r s l e s s e a s i l y in c o l d - w o r k e d austenite; m a r t e n s i t e that f o r m s in coldworked austenite tends to occur with s m a l l e r , l e s s r e g u l a r l y shaped p l a t e l e t s than in annealed austenite. 8 When m e t a s t a b l e austenite is p l a s t i c a l l y deformed, the f o r m a t i o n of m a r t e n s i t e can occur c o n c u r r e n t l y P. C. MAXWELL, formerly a graduate student at Stanford University, is now Assistant Professor, Escuela de Ingenieria Metalurgica y Ciencia de los Materiales, Universjdad Central de Venezuela, Caracas, Venezuela. A. GOLDBERG, is Senior Scientist, Lawrence Livermore Laboratory, University of California, Livermore, California. 94550. J. C. SHYNE, is Professor and Chairman of the Materials Science and Engineering Department, Stanford University, Stanford, California, and consultant, Lawrence Livermore Laboratory. Manuscript submitted February 19, 1973. METALLURGICAL TRANSACTIONS
with the p l a s t i c s t r a i n . In fact, the f o r m a t i o n of m a r t e n s i t e can i t s e l f be a m e c h a n i s m for p l a s t i c d e f o r m a tion. M a r t e n s i t e f o r m e d during p l a s t i c d e f o r m a t i o n of austenite influences subsequently m e a s u r e d p r o p e r t i e s . 9-18 T h e r e f o r e , if m a r t e n s i t e f o r m s during m e chanical testing, the m e c h a n i c a l b e h a v i o r must n e c e s s a r i l y be affected, x6-
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