Transformation-induced structures and martensite nucleation
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Transformation-Induced Structures and Martensite Nucleation R. N. C A R O N A N D G. KRAUSS
tures introduced by cyclic transformation provide u n i q u e n u c l e a t i o n s i t e s f o r t h e f o r m a t i o n of m a r t e n site. T h e v a c u u m - m e l t e d a l l o y s t u d i e d c o n t a i n e d 32.2 N i , 0 . 0 0 4 C, 0 . 0 7 2 A1, 0.10 Ti~ wt p c t . H o m o g e n i z a t i o n a n d annealing were performed at ll00~ and the reverse t r a n s f o r m a t i o n of m a r t e n s i t e t o a u s t e n i t e w a s p e r f o r m e d b y i m m e r s i o n i n m o l t e n s a l t a t 475~ f o r 1 min. The low-temperature transformation kinetics were followed by electrical resistance measurements of 0 . 0 5 8 5 i n . d i a m s w a g e d w i r e s in a 2 - m e t h y l b u t a n e bath cooled by liquid nitrogen additions. Nickel lead w i r e s w e r e s p o t w e l d e d to 4 i n . l o n g s a m p l e s . A s p a c i n g of 2 1 in. w a s m a i n t a i n e d b e t w e e n t h e p o t e n t i a l leads. Thin foils were prepared by electropolishing 0 . 0 0 3 - i n . t h i c k s h e e t i n a n e l e c t r o l y t e of 775 m l a c e t i c a c i d , 150 g s o d i u m c h r o m a t e , a n d 75 g c h r o m i u m trioxide. T h e e f f e c t of a c y c l e of t r a n s f o r m a t i o n on t h e a t h e r mal transformation kinetics is shown in Fig. 1 and s u m m a r i z e d in T a b l e I. A s i g n i f i c a n t a n d r e p r o d u c i b l e i n c r e a s e i n M s t e m p e r a t u r e , a n a v e r a g e of 16~ i n t h r e e r u n s f o r t h e 32.2 Ni a l l o y , i s s h o w n b y t h e r e s u i t s of t h e s e e x p e r i m e n t s . A n o t h e r i m p o r t a n t o b s e r v a t i o n , e m p h a s i z e d b y t h e s u p e r p o s i t i o n of t h e t w o c u r v e s of F i g . 1, i s t h e m a r k e d r e d u c t i o n in t h e a m o u n t of m a r t e n s i t e f o r m e d b y b u r s t i n g a f t e r t h e s a m p l e h a s b e e n e x p o s e d t o a c o m p l e t e c y c l e of m a r t e n s i t i c transformation. F r o m t h e s u m m a r y of t h e d a t a f o r s e v e r a l F e - N i a l l o y s , a d e f i n i t e t r e n d of i n c r e a s e d 1
THREE t y p e s of d e f e c t s t r u c t u r e s a r e i n t r o d u c e d i n t o t h e a u s t e n i t e of F e - 3 0 p c t Ni a l l o y s w h e n m a r t e n s i t e i s t r a n s f o r m e d to a u s t e n i t e b y h e a t i n g t o j u s t above the A f, the temperature at which transformat i o n of m a r t e n s i t e t o a u s t e n i t e i s c o m p l e t e : 1) h i g h d e n s i t i e s of t a n g l e d d i s l o c a t i o n s , ~ 2) s u b s t r u c t u r e of o r i e n t a t i o n d i f f e r e n t f r o m t h a t of t h e v i r g i n a u s t e n i t e , o f t e n l e n t i c u l a r in c r o s s s e c t i o n , up to 5 # in w i d t h , a n d s o m e t i m e s t w i n n e d r e l a t i v e to t h e v i r g i n a u s t e n i t e , 1 a n d 3) s u b m i c r o s c o p i c t w i n s of t h i c k n e s s on t h e o r d e r of a f e w h u n d r e
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