The isothermal decomposition of austenite in a high purity Iron-Chromium binary alloy
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r e c e n t y e a r s , the n a t u r e of the g a m m a to alpha t r a n s f o r m a t i o n and the s t r u c t u r e s produced have been e x t e n s i v e l y studied a s a function of cooling r a t e in i r o n ~,2 and many v e r y low carbon, i r o n - b a s e d b i n a r y alloys; 3-7 i n c l u d i n g F e - C r . 8-n U n f o r t u n a t e l y , the r e s u l t s of these i n v e s t i g a t i o n s have i n t r o d u c e d c o n t r o v e r s i e s c o n c e r n i n g both the t r a n s f o r m a t i o n modes and the t e r m s applied to each s t r u c t u r a l v a r i a n t . Since t h e s e v - a r e a c t i o n s a r e v e r y r a p i d , 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 a u s t e n i t e in i r o n b i n a r y a l l o y s has r a r e l y b e e n studied. However, E n t i n12 has shown that f a i r l y low c a r b o n F e - C r and F e - C r - N i a l l o y s did exhibit C - c u r v e k i n e t i c s for the ~ ~ a t r a n s f o r m a t i o n , and that c h r o m i u m additions s u b s t a n t i a l l y r e t a r d e d the r e a c t i o n . The p u r p o s e of this p a p e r is to r e p o r t s o m e o b s e r v a t i o n s made i n a d i l a t o m e t r i c study of a s e r i e s of F e - C r alloys in the v-loop c o m p o s i t i o n r a n g e , and to p r e s e n t the r e s u l t s of a s u b s e q u e n t i n v e s t i g a t i o n into the t r a n s f o r m a t i o n b e h a v i o r of an F e - 1 0 pct C r alloy under isothermal conditions. 2. E X P E R I M E N T A L
m e t e r which had f a c i l i t i e s for p r o g r a m m e d heat t r e a t m e n t with a r a n g e of heating and cooling r a t e s up to 20~ m i n -1. S a m p l e s 40 m m in length were cut f r o m 3 m m d i a m rod. M e a s u r e m e n t of the s p e c i m e n length change was by an inductive method, and a l l e x p e r i m e n t s were p e r f o r m e d u n d e r v a c u u m . 2.3. I s o t h e r m a l D i l a t o m e t r y F u l l y a n n e a l e d s p e c i m e n s of 10 m m length and 2 m m d i a m w e r e a u s t e n i t i z e d for 10 m i n at 1050~ u s i n g r a d i a n t heating, then i n e r t g a s - q u e n c h e d to the i s o t h e r m a l t r a n s f o r m a t i o n t e m p e r a t u r e . The t e m p e r a t u r e and e x p a n s i o n of the s a m p l e were c o n t i n u o u s l y recorded. 2.4. M e t a l l o g r a p h i c E x a m i n a t i o n The f e r r i t i c and m a r t e n s i t i c c o m p o n e n t s of the m i c r o s t r u c t u r e w e r e found to be most c l e a r l y d e l i n e a ted u s i n g a light e l e c t r o e t c h of 10 pct h y d r o c h l o r i c acid i n alcohol at 10 volts, followed by a second etch u s i n g a solution of the c o m p o s i t i o n : 20 m l s a t u r a t e d s o l u t i o n of p i c r i c acid in alcohol, + 1 t o 2 gCuC12 + a s m a l l addition of a wetting agent.
2.1. M a t e r i a l The a l l o y s were p r e p a r e d by a r g o n - a r c m e l t i n g using high-purity base materials: zone-refined iron and e x - i o d i d e c h r o m i
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