An investigation of the effects of austenite strength and austenite stacking fault energy on the morphology of martensit
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F E R R O U S m a r t e n s i t e f o r m s f r o m a u s t e n i t e by a diff u s i o n l e s s , s h e a r - t y p e t r a n s f o r m a t i o n . The product p h a s e is not unique, however, and s e v e r a l m o r p h o l o gies have b e e n identified in s t e e l s . T h e s e m o r p h o l o gies a r e g e n e r a l l y divided into t h r e e types: lath m a r t e n s i t e , typically f o r m e d a s p a c k e t s of s h e e t s or heavily dislocated, bcc, untwinned, n e e d l e - l i k e units; plate m a r t e n s i t e , typically f o r m e d as i n d i v i d u a l bct p l a t e s c o n t a i n i n g many fine twins; and e p s i l o n m a r t e n s i t e , an hcp p h a s e . Many p r e v i o u s i n v e s t i g a t i o n s have sought to d e t e r mine which v a r i a b l e s c o n t r o l the morphology of the m a r t e n s i t e that f o r m s in a given s t e e l . V a r i a b l e s r e p o r t e d to affect morphology include: M s t e m p e r a t u r e , 1"4 s u b s t i t u t i o n a l solute content, 1,e,5-~ i n t e r s t i t i a l solute content, 1'~~ a u s t e n i t e s h e a r s t r e n g t h , s,'~ aust e n i t e s t a c k i n g fault e n e r g y , 13-~6 quench r a t e above M s , 1 5 t h e r m o m e c h a n i c a l p r o c e s s i n g , ~s and h y d r o s t a t i c p r e s s u r e . ~9 Some of t h e s e a r e of a s e c o n d a r y n a t u r e , and the f a c t o r s c o n t r o l l i n g m a r t e n s i t e morphology a r e u s u a l l y r e d u c e d to one or a c o m b i n a t i o n of the following: M s t e m p e r a t u r e , c a r b o n content, a u s t e n i t e yield s t r e n g t h , and a u s t e n i t e s t a c k i n g fault e n e r g y . However, these p r o p e r t i e s a r e i n t e r r e l a t e d and a r e difficult to i s o l a t e for study. As a r e s u l t , c o m p a r i s o n s a m o n g v a r i o u s e x p e r i m e n t a l s t u d i e s a r e difficult. Since the 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 o c c u r s by a s h e a r m e c h a n i s m , i r r e s p e c t i v e of the morphology produced, it is e s p e c i a l l y i m p o r t a n t to define the effects of a u s t e n i t e SFE and a u s t e n i t e s t r e n g t h on m a r t e n s i t e morphology b e c a u s e these p r o p e r t i e s s t r o n g l y influence s h e a r p r o c e s s e s in the a u s t e n i t e . A u s t e n i t e s t a c k i n g fault e n e r g y (SFE) c o n t r o l s slip c h a r a c t e r by c o n t r o l l i n g the a b i l i t y of d i s l o c a t i o n s to c r o s s - s l i p . The effect of SFE on m a r t e n s i t e morphology has b e e n
studied by Kelly and Nutting, 14 B r e e d i s , z~ B r e e d i s and Kaufman, 2~ and Holden, 21 et a l . The g e n e r a l t r e n d obs e r v e d is that the morphology changes f r o m plate to lath to e p s i l o n m a r t e n s i t e as the SFE d e c r e a s e s . The effect of a u s t e n i t e s t r e n g t h on m a r t e n s i t e morphology has b e e n studied by Davies and Mcgee, 6'7'12 who found a t r a n s i t i o n f r o
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