Hardening by ordered precipitates in some ternary ferritic alloys containing cobalt
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Spheroidal o r d e r e d p r e c i p i t a t e s have b e e n found to o c c u r d u r i n g aging of f e r r i t i c F e - C o T i and F e - C o - A 1 a l l o y s . The fine u n i f o r m l y d i s t r i b u t e d p r e c i p i t a t e s , which a p p e a r to have L2x and B2 s t r u c t u r e s r e s p e c t i v e l y , give r i s e to a 100 pct i n c r e a s e in h a r d n e s s . In the F e CD-Ti s y s t e m , the p r e c i p i t a t e c o m p o s i t i o n h a s been e s t a b l i s h e d a s a p p r o x i m a t e l y FehCoTi by e l e c t r o n p r o b e m i c r o a n a l y s i s . The o r d e r e d p r e c i p i t a t e in t h i s s y s t e m i s m e t a s t a b l e with r e s p e c t to the Fe2Ti, C14, L a v e s p h a s e , the l a t t e r growing a s the f o r m e r r e d i s s o l v e s . In the F e - C o - A 1 s y s t e m , the o r d e r e d p r e c i p i t a t e s a r e s t a b l e , s i n c e no i n t e r m e t a l l i c phase o c c u r s in the i r o n - r i c h c o r n e r of the F e - C o - A 1 phase d i a g r a m .
T H I S study i s p a r t of a w i d e r i n v e s t i g a t i o n of i n t e r m e t a l l i c p r e c i p i t a t i o n in i r o n - b a s e f e r r i t i c a n d m a r t e n s i t i c alloys.* In the p r e s e n t work, i r o n - r i c h F e - C o A1 and F e - C o - T i a l l o y s with a f e r r i t i c m a t r i x a r e studied. The B2 o r d e r e d p h a s e (CsC1 s t r u c t u r e ) is known to o c c u r at the e q u i a t o m i c c o m p o s i t i o n in each of the five b i n a r y s y s t e m s . It was t h e r e f o r e thought l i k e l y that the s a m e p h a s e , a n d p o s s i b l y o t h e r o r d e r e d p h a s e s , would p r e c i p i t a t e in the t e r n a r y a l l o y s . A p u b lished phase d i a g r a m of the F e - C o - A 1 s y s t e m s u p p o r t s t h i s idea a s it shows a t w o - p h a s e r e g i o n o c c u r r i n g a s a m i s c i b i l i t y gap below 750~ in the m i d d l e of the d i a g r a m , Fig. 1.2 T h e d i a g r a m i s s o m e w h a t s i m i l a r to that of F e - N i - A I , in which a m i s c i b i l i t y gap e x i s t s and in which s m a l l f i n e l y - d i s p e r s e d p r e c i p i t a t e s have b e e n r e p o r t e d , s~ T h e r e i s no evidence in the l i t e r a t u r e for s i m i l a r f e a t u r e s i n the F e - C o - T i s y s t e m . A l l o y s b a s e d on F e - C o a r e in s o m e c i r c u m s t a n c e s t r a n s f o r m a b l e , with the p o s s i b i l i t y of f o r m i n g m a r t e n site by quenching f r o m the y - p h a s e r e g i o n ; such m a r t e n s i t i c a l l o y s might be h a r d e n a b l e by p r e c i p i t a t i o n . F u r t h e r m o r e , cobalt a d d i t i o n s to F e - N i - T i a l l o y s a r e r e p o r t e d to i n c r e a s e the v o l u m e f r a c t i o n of fine spher o i d a l p r e c i p i t a t e s . 5 It was thought likely that s i m i l a r b e h a v i o r might o c c u r in F e - C o - T i and F e - C o - A 1 a l l o y s in the a b s e n c e of n
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