Stress- and strain-induced formation of martensite and its effects on strength and ductility of metastable austenitic st
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U N U S U A L L Y high s t r e n g t h s can be i m p a r t e d to many s t e e l s through s p e c i a l heat t r e a t m e n t s or by c o m b i n e d t h e r m a l and m e c h a n i c a l t r e a t m e n t s . Most u l t r a h i g h s t r e n g t h s t e e l s u n d erg o an a u s t e n i t e - m a r t e n s i t e p h a s e t r a n s f o r m a t i o n , so that at l e a s t a portion of t h e i r s t r e n g t h is due to a t r a n s f o r m a t i o n p r o d u c t of low ductility which often l i m i t s the application of such steels. A u s t e n i t i c s t e e l s can be s i g n if i c a n t ly s t r e n g t h e n e d without a c o n c o m i t a n t p h a s e t r a n s f o r m a t i o n through heavy working in c o m b i n a t i o n with p r e c i p i t a t i o n h a r d ening. The low elongation v a l u e s o b s e r v e d for t h e s e s t e e l s during t e n s i l e t e s t i n g a r e due to a l o c a l p l a s t i c instability which o c c u r s b e c a u s e the m a t e r i a l is u nable to work harden at a r a t e high enough to c o m p e n sate fo r the s t r e s s i n c r e a s e due to the r e d u c t i o n in c r o s s s e c t i o n a l a r e a . I n c r e a s i n g the ductility in highs t r e n g t h a u s t e n i t i c s t e e l s thus b e c o m e s a p r o b l e m of i n c r e a s i n g the w o r k - h a r d e n i n g r a t e . T h i s can be a c h i e v e d through a d e f o r m a t i o n - i n d u c e d p h a s e t r a n s f o r m a t i o n . B r e s s a n e l l i and Moskowitz ~ studied the combined and individual e f f e c t s of c o m p o s i t i o n , t e s t t e m p e r a t u r e , and d e f o r m a t i o n r a t e on the t e n s i l e p r o p e r t i e s of type 301 s t a i n l e s s s t e e l and c l e a r l y d e m o n s t r a t e d the b e n e f i c i a l effect of a " s p e c i f i c amount of m a r t e n s i t e f o r m a t i o n " on t e n s i l e e l o n g a tion. In r e c e n t y e a r s this d e f o r m a t i o n - i n d u c e d f o r m a tion of m a r t e n s i t e was s u c c e s s f u l l y u t i l i z e d to e n h an ce the ductility of h i g h - s t r e n g t h m e t a s t a b l e a u s t e n i t i c stainless steels. 2 When the f r e e e n e r g y d i f f e r e n c e between m a r t e n s i t e and a u s t e n i t e , A F = F M -- F A , r e a c h e s a c r i t i c a l n e g DIETER FAHR, formerly Graduate Student, Department of Mate~ rials Science and Engineering, University of California, Berkeley, Calif., is now with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. Manuscript submitted October 2, 1970. METALLURGICALTRANSACTIONS
ative value, martensite starts forming spontaneously on cooling at the M s temperature. Since the martensite transformation is a diffusionless shear transformation aided by positive normal stresses, 3'4 it can be made to occur at temperatures above M s by deformation of the austenife. Above a certain temperature, Md, no deformation-induced transformation is possible. Between the M s and M d temperatures the for
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