The effect of Ni on the decomposition of austenite in Fe-Cu alloys
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1. INTRODU COl?ION h n u m b e r of w o r k e r s have studied the f a c t o r s which influence the a u s t e n i t e ~ f e r r i t e r e a c t i o n in c a r b o n f r e e i r o n b a s e alloys i n c l u d i n g F e - Cu. In g e n e r a l , these s t u d i e s have c o n c e n t r a t e d e i t h e r on the c h a r a c t e r i z a t i o n of t r a n s f o r m a t i o n p r o d u c t s as a function of cooling r a t e s , 1-6 or on p r e c i p i t a t i o n (e.g. of Cu) f r o m quenched and aged ferrite.7,8 Few s t u d i e s have been r e p o r t e d c o n c e r n i n g the i s o t h e r m a l d e c o m p o s i t i o n of such alloys s i n c e , in g e n e r a l , the speed of the a u s tenite ~ f e r r i t e r e a c t i o n p r o h i b i t s the a c q u i s i t i o n of r e a c t i o n k i n e t i c data. As a r e s u l t the t e m p e r a t u r e r a n g e within which each t r a n s f o r m a t i o n product is f o r m e d is not well defined and some doubt e x i s t s as to the exact o r d e r of r e a c t i o n s with d e c r e a s i n g t e m perature. The advent of high speed d i l a t o m e t e r s , u s i n g h e l i u m gas as a quenchant e n a b l e s the k i n e t i c s of the a u s t e n i t e f e r r i t e r e a c t i o n to be studied in alloys of this type. In this p a p e r r e a c t i o n k i n e t i c data, obtained by a high speed d i l a t o m e t r i c study of a b i n a r y i r o n - c o p p e r a l loy, will be p r e s e n t e d and c o r r e l a t e d with the r e s u l t s of a p a r a l l e l m i c r o s t r u c t u r a l e x a m i n a t i o n of the r e a c tion p r o d u c t s f o r m e d by i s o t h e r m a l t r a n s f o r m a t i o n . S i m i l a r l y the effects of t e r n a r y additions of n i c k e l on the k i n e t i c s and r e s u l t a n t m i c r o s t r u c t u r e s is e x a m i n e d . 2. E X P E R I M E N T A L The a l l o y s i n v e s t i g a t e d had the following c o m p o s i t i o n s (wt pct):* *Thecarboncontentof the alloyswas lessthan0.002 pct. nominal Fe-2 Fe-2 Fe-2 Fe-2
pct pct pct pct
Cu Cu-2 pct Ni Cu-5 pct Ni Cu-7 pct Ni
t e r i a l s and m e l t e d in a n a r g o n a r c f u r n a c e . D i l a t o m e t r y was p e r f o r m e d on a T h e t a I n d u s t r i e s high speed d i l a t o m e t e r e m p l o y i n g induction h e a t i n g and h e l i u m gas quenching. Hollow c y l i n d r i c a l s p e c i m e n s (25 x 3 mm) were used to obtain the highest p o s s i b l e quench r a t e ( a p p r o x i m a t e l y 300~C / s ) . S p e c i m e n s for e l e c t r o n m i c r o s c o p y were p r e p a r e d in a t w i n - j e t e l e c t r o p o l i s h e r u s i n g an e l e c t r o l y t e cons i s t i n g of 10 pct p e r c h l o r i c acid plus 20 pct g l y c e r o l in ethanol. A l l e l e c t r o n m i c r o s c o p y was c a r r i e d out on a P h i l l i p s EM300 o p e r a t i n g at 100 kV. 3. REACTION KINETICS 3.1. F e - 2 C u The d i l a t o m e t r i c a l l y d e t e r m i n e d T T T curve for the F e - 2 C u alloy is shown in F i
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