An estimate of the nucleation time in the martensitic transformation
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T H E n u c l e a t i o n of m a r t e n s i t e , for which t h e r e a r e two s c h o o l s of t h o u g h t - t h e l a t t i c e softening ~'2 and the O l s o n - C o h e n 3-5 d i s l o c a t i o n c o n c e p t s - c a n be e i t h e r r a p i d ( a t h e r m a l m a r t e n s i t e ) o r slow ( i s o t h e r m a l m a r t e n s i t e ) . T h e p u r p o s e of t h i s note i s to d e s c r i b e a f i r s t a t t e m p t to obtain the t i m e i n t e r v a l f o r a t h e r m a l m a r t e n s i t e n u c l e a t i o n . F i r s t , a technique i s d e s c r i b e d t h a t a l l o w s the d e t e r m i n a t i o n of n u c l e a t i o n t i m e s for m a r t e n s i t e f o r m a t i o n ; t h i s t e c h n i q u e u s e s the p a s s a g e of a t e n s i l e p l a s t i c wave t h r o u g h the a l l o y a s a t r i g g e r m e c h a n i s m for m a r t e n s i t e t r a n s f o r m a t i o n . Second, the t e c h n i q u e is a p p l i e d to a r e a l s i t u a t i o n r e p o r t e d in the l i t e r a t u r e and an e s t i m a t e d n u c l e a t i o n t i m e i s c a l c u l a t e d t h e r e from. TECHNIQUE A h y p o t h e t i c a l e x p e r i m e n t i s d e s c r i b e d h e r e i n which w i l l y i e l d a t i m e for m a r t e n s i t e n u c l e a t i o n . A c e r t a i n c l a s s of a l l o y s is r e q u i r e d ; a l l o y s w h e r e i n c o m p r e s s i v e s h o c k w a v e s do not induce m a r t e n s i t e t r a n s f o r m a t i o n at a r e a s o n a b l e p r e s s u r e , while t e n s i l e w a v e s do. M e y e r s and G u i m a r ~ e s 6 r e c e n t l y o b s e r v e d t h i s b e h a v i o r for an F e - 3 1 pct N i - 0 . 1 pct C a l l o y ( M s = 223 K). W h i l e the c o m p r e s s i v e s h o c k wave induced only a c e l l like d i s l o c a t i o n s u b s t r u c t u r e , t e n s i l e r e f l e c t e d w a v e s caused transformation. These results are corroborated by o t h e r i n v e s t i g a t i o n s for s y s t e m s with M s = 245 K 7 and 240K. 8 T h e s e a l l o y s , s u b j e c t e d to e x c l u s i v e l y c o m p r e s s i v e s h o c k w a v e s of the s a m e o r d e r of m a g n i t u d e a s M e y e r s and G u i m a r ~ e s ' s y s t e m s , did not exhibit 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 . H o w e v e r , M e y e r s and G u i m a r ~ e s 6 u s i n g an e s p e c i a l l y d e s i g n e d e x p e r i m e n t a l s e t - u p , w e r e a b l e to show that the t e n s i l e r e f l e c t e d wave g e n e r a t e d m a r t e n s i t e . The s t a t e of s t r e s s i n d u c e d by s h o c k l o a d i n g u n i a x i a l s t r a i n - c a n be d e c o m p o s e d into a h y d r o s t a t i c and a d e v i a t o r i c c o m p o n e n t . The d e v i a t o r i c c o m p o n e n t s a r e s i m i l a r f o r the d i r e c t c o m p r e s s i v e and r e f l e c t e d p u l s e . The d e v i a t o r i c ( s h e a r ) c o m p o n e n t s of the w a v e s MARC A. MEYERS, formerly Assistant Professor, Department of Metallurgical Engine
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