Transformation plasticity in iron-nickel alloys
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A c o n s i d e r a b l e i n c r e a s e in d u c t i l i t y and s t r a i n r a t e at flow s t r e s s e s well below t h o s e r e q u i r e d for y i e l d ing of " s t a b l e " m i c r o s t r u c t u r e s has been noted in c e r t a i n m e t a l s and alloys when d e f o r m e d during a phase t r a n s f o r m a t i o n . ~-~ T h i s p h e n o m e n o n is t e r m e d " t r a n s f o r m a t i o n p l a s t i c i t y , " following the suggestion by W a s s e r m a n n in 1937 ~ to d e s c r i b e the p l a s t i c flow b e h a v i o r d u r i n g 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 in F e 30 pct Ni. F u r t h e r m o r e , on r e p e a t e d c y c l i n g through the t r a n s f o r m a t i o n r a n g e u n d e r load, one can obtain a b n o r m a l l y l a r g e n e c k - f r e e e l o n g a t i o n s in e x c e s s of 500 pct for plain c a r b o n s t e e l and 300 pct for c o m m e r cially p u r e t i t a n i u m . 2 This s u p e r p l a s t i c b e h a v i o r o c c u r r i n g d u r i n g s t r u c t u r a l i n s t a b i l i t i e s is c o n t r a s t e d to s u p e r p l a s t i c i t y in f i n e - g r a i n m a t e r i a l s (grain s i z e ~ 1/1), t e r m e d " m i c r o g r a l n p l a s t i c i t y , " which o c c u r s u n d e r conditions of " s t a b l e " m i c r o s t r u c t u r e s . The c o m m o n f e a t u r e of both t y p e s of s u p e r p l a s t i c i t y is the p o s s i b i l i t y of achieving l a r g e n e c k - f r e e e l o n g a t i o n s at r e l a t i v e l y low flow s t r e s s as a c o n s e q u e n c e of a high, q u a s i v i s cous s t r a i n r a t e s e n s i t i v i t y . 9'1~ This q u a s i v i s c o u s b e havior for both types of s u p e r p l a s t i c i t y o c c u r s at v e r y low s t r a i n r a t e s . An e x c e l l e n t s u r v e y of the field, p r i o r to 1962, has been g i v e n by Underwood. n Since then, n u m e r o u s s t u d i e s have been r e p o r t e d both on the m e c h a n i s m s as well a s the extent of o c c u r r e n c e of these p h e n o m ena as r e v i e w e d by W e i s s and Kot. 9 S y s t e m s e x t e n s i v e l y i n v e s t i g a t e d for t r a n s f o r m a t i o n p l a s t i c i t y include a l l o t r o p i c t r a n s f o r m a t i o n in p u r e m e t a l s 7'~2'13 and the a u s t e n i t e to p e a r l i t e , 3-8 b a i n i t e 3'1s or m a r t e n s i t e 3'~6-~9 r e a c t i o n s in i r o n alloys. A l m o s t all the s t u d i e s p e r f o r m e d on the high t e m p e r a t u r e t r a n s f o r m a t i o n s in p u r e i r o n and i r o n a l l o y s w e r e m a d e for a complete t r a n s f o r m a t i o n cycle, i . e . , d e f o r m i n g d u r i n g both the d i r e c t and r e v e r s e t r a n s f o r m a t i o n . The extent of t r a n s f o r m a t i o n p l a s t i c i t y has not been studied as thoroughly for the a n s t e n i t e ~-~m a r t e n s i t e t r a n s f o r m a t i o n s . A n k a r a e t a l . ~9 have i n v e s t i RICHARD A. KOT, formerly Graduate Student, De
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