The role of thermal stabilization in martensite transformations
- PDF / 712,232 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 97 Downloads / 164 Views
The v a r i a t i o n of the k i n e t i c s of 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 with carbon content and m a r t e n s i t e habit plane has been i n v e s t i g a t e d in s e v e r a l F e - N i b a s e d a l l o y s . T r a n s f o r m a tion in an F e - 2 5 wt pct Ni-0.02 wt pct C alloy exhibits p r e d o m i n a n t l y a t h e r m a l f e a t u r e s , but some a p p a r e n t l y i s o t h e r m a l t r a n s f o r m a t i o n also o c c u r s . In a d e c a r b u r i z e d alloy, on the other hand, the o b s e r v e d k i n e t i c f e a t u r e s , such as the dependence of M s on cooling r a t e , were c h a r a c t e r i s t i c of an i s o t h e r m a l t r a n s f o r m a t i o n . In c o n t r a s t , F e - 2 9 . 6 wt pct N i - 1 0 . 7 wt pct Co a l l o y s with c a r b o n contents of 0.009 wt pct C and 0.003 wt pct C t r a n s f o r m by b u r s t k i n e t i c s to {259}-7 plate. At both these c a r b o n l e v e l s , the M b t e m p e r a t u r e s of the F e - N i - C o alloys a r e independent of cooling r a t e . It is proposed that the change in k i n e t i c b e h a v i o r of the F e - 2 5 pct Ni alloy with the d i f f e r e n t c a r b o n contents is due to the o c c u r r e n c e of d y n a m i c t h e r m a l s t a b i l i z a t i o n in the higher c a r b o n alloy. D y n a m i c t h e r m a l s t a b i l i z a t i o n is r e l a t i v e l y u n i m p o r t a n t in the F e - N i - C o alloys which t r a n s f o r m by b u r s t k i n e t i c s to {259}-7 plate m a r t e n s i t e .
1. INTRODUCTION
M s . While these f e a t u r e s suggest an a t h e r m a l n u c l e a -
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 s in i r o n - n i c k e l b a s e d a l l o y s exhibit t h r e e types of kinetic b e h a v i o r : a t h e r m a l , i s o t h e r m a l , and b u r s t k i n e t i c s . The k i n e t i c s a r e s o m e t i m e s modified by s u r f a c e t r a n s f o r m a t i o n which is i s o t h e r m a l ~'2 and can o b s c u r e the b u l k k i n e t i c b e h a v i o r . D e s p i t e much study, there is s t i l l c o n s i d e r a b l e u n c e r t a i n t y c o n c e r n i n g the f a c t o r s which d e t e r m i n e the k i n e t i c s . The m o r e r e c e n t d i s c u s s i o n s of m a r t e n s i t e k i n e tics ~'~ have a t t e m p t e d to r a t i o n a l i z e a t h e r m a l and b u r s t k i n e t i c s as m o d i f i c a t i o n s of a b a s i c a l l y i s o t h e r m a l n u c l e a t i o n m e c h a n i s m . Thus, it has been s u g g e s t ed 6'7 that b u r s t k i n e t i c s may r e s u l t f r o m an i s o t h e r m a l t r a n s f o r m a t i o n in which the f o r m a t i o n of one (or a few) plate(s) t r i g g e r s the b u r s t , so p r e v e n t i n g continued i s o t h e r m a l t r a n s f o r m a t i o n . It has b e e n pointed out, however, s that in this case the M b t e m p e r a t u r e should depend on cooling r a t e . An i m p o r t a n t factor a s s o c i a t e d with b u r s t k i n e t i c s a p p e a r
Data Loading...
