Strain rate sensitivity and ductile-brittle behavior of polycrystalline Fe-Si alloys with 2.5, 3.5, and 4.5 wt pct Si
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t r a n s i t i o n f r o m d u c ti le to b r i t t l e b e h a v i o r is a p r o b l e m in the d e v e l o p m e n t and a p p l i c a t i o n of m o s t f e r r i t i c s t e e l s . In m a n y c a s e s the r e q u i r e m e n t of a r e a s o n a b l e d u c t i l i t y s e t s a l i m i t to the i m p r o v e m e n t s that can be obtained in o t h e r p r o p e r t i e s . A good e x a m ple i s o f f e r e d by the e l e c t r i c a l s h e e t s i l i c o n s t e e l s . In t h e s e a high s i l i c o n content is d e s i r a b l e s i n c e t h i s i m p l i e s high r e s i s t i v i t y and s m a l l l o s s e s due to eddy c u r r e n t s , but the d e c r e a s e in d u c t i l i t y which a c c o m p a n i e s an i n c r e a s e d s i l i c o n c o n te n t puts an u p p e r l i m i t on the l a t t e r at about 3 wt pct. F o r this type of s t e e l a t h o r o u g h knowledge of the f a c t o r s which a f f e c t the t r a n s i t i o n f r o m d u c ti le to b r i t t l e b e h a v i o r is i m p o r t a n t . A m o n g t h e s e f a c t o r s , t e m p e r a t u r e , s t r a i n - r a t e and g r a i n size (in addition to s i l i c o n content) should be m e n t i o n e d in the f i r s t p l a c e . T he p r e s e n t p a p e r w i l l m a i n l y deal with the e f f e c t s of s i l i c o n content and s t r a i n - r a t e , the e f f e c t s of g r a i n s i z e i and t e m p e r a t u r e 2 b e i n g f a i r l y w e l l known f r o m p r e v i o u s w o r k . The c o m p o s i t i o n s of the a l l o y s studied have been c h o s e n so as to e m b r a c e the s i l i c o n content at which the t r a n s i t i o n f r o m d u c ti le to b r i t t l e b e h a v i o r o c c u r s at n o r m a l v a l u e s of t e m p e r a t u r e and s t r a i n rate. On the b a s i s of p r e s e n t and p r e v i o u s r e s u l t s a gene r a l d i s c u s s i o n of the d u c t i l e - b r i t t l e b e h a v i o r of p o l y c r y s t a l l i n e s i l i c o n f e r r i t e w i l l be given. A s a b a c k ground f o r the d i s c u s s i o n of t e m p e r a t u r e and s t r a i n r a t e d e p e n d e n c e , the a t h e r m a l and t h e r m a l c o m p o n e n t s of the y i el d (proof) s t r e s s have been d e t e r m i n e d . I n f o r m a t i o n c o n c e r n i n g the d e f o r m a t i o n m e c h a n i s m s h as been obtained by d e t e r m i n i n g the d i s l o c a t i o n v e l o c i t y exponent and by studying the d i s l o c a t i o n s t r u c t u r e of B. ARONSSON is Director of the Swedish Institute for Metal Research, Stockholm, Sweden. L. GRANAS, formerly with the same Institute, is now Metallurgist with Surahammars Bruks AB, Surahammar, Sweden. Manuscript submitted May 18, 1970. METALLURGICAL TRANSACTIONS
d e f o r m e d s p e c i m e n s with t r a n s m i s s i o n e l e c t r o n m i croscopy.
EXPERIMENTAL
METHODS
The alloys were prepared by melting mixtures of Orkla electrolytic iron (main impurities, in ppm: oxygen 300, sulfur 80, cobalt 50, carbon 20,
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