Stress, deformation, and martensitic transformation
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inescapable that the real s t r e s s e s w h i c h a c c o m pany m a r t e n s i t e f o r m a t i o n i n i r o n a l l o y s w i l l d i c t a t e a t what s t a g e t h e transformation p r o c e s s must involve d i s l o c a t i o n s . T h e e x i s t e n c e of r e a l s t r e s s e s a s o p p o s e d t o p u r e l y chemical equivalents has been proved post f a c t o by t h e e x t e n s i v e p l a s t i c d e f o r m a t i o n o b s e r v e d i n both t h e p a r e n t a n d p r o d u c t p h a s e s .~-3 B u t the involvement of dislocation motion and multiplicat i o n c a n s t i l l be q u e s t i o n e d i n t e r m s of w h a t d i s l o c a t i o n s a r e p e r t i n e n t a n d h o w a n d w h e n do t h e y m o v e . I n t h i s r e g a r d i t i s l o g i c a l t o e x p e c t only t w o p o s s i b l e r e l a t i o n s h i p s b e t w e e n a u s t e n i t e t r a n s f o r m a t i o n a n d disl o c a t i o n b e h a v i o r , w h i c h do not n e c e s s a r i l y e x c l u d e e a c h o t h e r . T h e a c t i o n of d i s l o c a t i o n s c a n be e n t i r e l y a c o n s e q u e n c e of t r a n s f o r m a t i o n , a n d f o l l o w i n g i t i n time; or, martensite formation can r e q u i r e concurrent
dislocation motion. T h e s e alternatives do not need to be involved with questions about the g e o m e t r y of the martensite-austenite interface and a b o u t martensite nucleation, but they cannot be independent of the stress state of the transforming s y s t e m . In consequence, w e shall investigate the effects of stress and strain upon the martensite reaction. S i n c e the overall p r o b l e m is quite c o m p l e x with m a n y variables and s e c o n d a r y forces such as those associated with solute pinning, s o m e simplifying t h e m e is n e e d e d at the outset. W e shall undertake a description by first postulating that martensite formation is indeed partly a p r o b l e m of dislocation motion, and that the pertinent dislocations are responsive to relatively s m a l l stresses. T h e utility of o b s e r v i n g t h e dynamic r e s p o n s e s to i m p o s e d
changes o f d e f o r m a t i o n v a r i a b l e s u p o n m e t a s t a b l e a u s t e n i t e is t h e r e f o r e a w o r k i n g a s s u m p t i o n . Dynamics a r e e m p h a s i z e d i n o r d e r t o d i s t i n g u i s h new experimental p r o c e d u r e s from t h o s e typified e a r l i e r by t h e w o r k o f E d m o n d s o n a n d Ko~ w h o o b s e r v e d p l a s t i c d e f o r m a t i o n in a u s t e n i t e a n d m a r t e n site a f t e r some transformation. Interpretations of the effects of prestrain, partial transformation, or any o t h e r c h a n g e s i n t h e s t a t i c s t a t e o f s t r a i n upon s u b s e R. H. RICHMAN and G. F. BOLLING are with the Scientific Research Staff, Ford Motor Company, Dearborn, Mich. This paper is based on an invited talk presented at a symposium on Formation of Martensite in Iron Alloys sponsored by the IMD Ferrous Metallurgy
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