Austenite ferromagnetism and martensite morphology
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alloys t h e r e a r e two c l e a r l y identifiable m a r t e n s i t e m o r p h o l o g i e s which we choose to call l e n t i c u l a r and packet m a r t e n s i t e s . This d i s t i n c t i o n is d r a w n on the b a s i s of t h e i r m e t a l l o g r a o h i c a p p e a r a n c e in c o n v e n t i o n a l light m i c r o s c o p y . L e n t i c u l a r m a r t e n s i t e has the c h a r a c t e r i s t i c lens shape often with a m i d - r i b and i s i n t e r n a l l y twinned; this s t r u c t u r e is o b s e r v e d in F e - 3 1 pct Ni I and F e - l . 0 wt pet C 2 alloys. P a c k e t m a r t e n s i t e is composed of groups of p a r a l l e l plates, each of which has a high d i s l o c a t i o n d e n s i t y , and only r a r e l y c o n t a i n s twins; this s t r u c t u r e i s found i n F e - 1 0 to 28 pct Ni a l l o y s , s A t t e m p t s have been made to c o r r e l a t e m a r t e n s i t e m o r p h o l o g y with v a r i a b l e s such a s c o m p o s i t i o n and t e m p e r a t u r e . Owen, Wilson, and Bell 4 pointed out that for F e - C and F e - N i - C alloys the m o r p h o l o g i c a l change could be c o r r e l a t e d with the Z e n e r o r d e r i n g t e m p e r a t u r e , T z , of the m a r t e n s i t e . If M s , the m a r t e n s i t e s t a r t t e m p e r a t u r e , is above T z cubic, packet m a r t e n s i t e f o r m s , while if M s is below T z the m a r t e n s i t e is l e n t i c u l a r and t e t r a g o n a l . This c o r r e l a t i o n between cubic s t r u c t u r e and packet morphology has r e c e n t l y b e e n v e r i f i e d for a s e r i e s of F e - N i - C alloys with a M s = 200~ ~ However Z e n e r o r d e r i n g cannot account for all r e s u l t s since c a r b o n f r e e alloYS, where T z = 0~ should always f o r m packet m a r t e n s i t e and yet F e - 3 1 pct Ni f o r m s l e n t i c u l a r m a r t e n s i t e . F o r c a r b o n - f r e e a l l o y s , for e x a m p l e F e - N i a l l o y s , it has b e e n suggested %8 that the m o r p h o l o g i c a l t r a n s i tion is g o v e r n e d by the t e m p e r a t u r e of t r a n s f o r m a t i o n . With a d e c r e a s e in M s to below 0~ it was proposed that slip b e c o m e s difficult and twinning is p r e f e r r e d , thus giving r i s e to the t r a n s i t i o n f r o m packet to l e n t i c u l a r m a r t e n s i t e . On the o t h e r hand, Brook and E n t w i s t l e 7 suggested that nickel c o n t e n t - - n o t t e m p e r a t u r e - d e t e r m i n e s the habit plane and morphology. In c o n s i d e r i n g 0nly c a r b o n - f r e e m a r t e n s i t e s , the m a i n evidence for a m o r p h o l o g i c a l t r a n s i t i o n is p r o vided by the F e - N i alloys. F o r this alloy s y s t e m one could a l s o s u g g e s t an a s s o c i a t i o n between m a r t e n s i t e m o r p h o l o g y and m a g n e t i c c h a r a c t e r ( p a r a - or f e r r o magnetic) of the a u s t e n i t e ; packet m a r t e n s i t e f o r m s f
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