The volume diffusion of carbon in Fe-17 Wt Pct Cr-12 Wt Pct Ni
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Fig. 2 - - T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h of w a t e r - q u e n c h e d c o p p e r - b e a r i n g s t e e l s h o w i n g p r e c i p i t a t e s l y i n g on d i s l o c a tions within the martensite laths.
This n a t u r a l aging phenomenon can provide an e x p l a nation both for the high value r e c o r d e d for as-quenched strength and for the nucleation of the v e r y l a r g e numb e r of p r e c i p i t a t e s o b s e r v e d . Although the i n c r e a s e in h a r d n e s s at room t e m p e r a t u r e shown in Fig. 1 m a y be a t t r i b u t e d in p a r t to the s e g r e g a t i o n of carbon atoms, the m a j o r contribution is b e l i e v e d to be due to copper a t o m s . The equilibrium concentration of v a c a n c i e s at 925~ is about 5.6 • 10 -7 (2.3 • 10 -7 for 850~ taking the activation energy for vacancy f o r m a t i o n as 1.5 eV. n Although not all v a c a n c i e s would be r e t a i n e d on quenching,12 the number r e tained would be l a r g e c o m p a r e d with the equilibrium concentration at room t e m p e r a t u r e (~ 1.1 • 10-2s). A s suming that copper atoms a r e p a i r e d with v a c a n c i e s in a s - q u e n c h e d m a t e r i a l as suggested by Hornbogen, is the activation e n e r g y for the m i g r a t i o n of copper atoms would be s m a l l , and, following the calculations of F e d e r i g h i . 14 it can be shown that, in a s - q u e n c h e d m a t e r i a l , the i n c r e a s e in diffusion r a t e due to e x c e s s v a cancies is about 5 • 10 ~8if all v a c a n c i e s were r e t a i n e d on quenching. It has been shown conclusively by Goodman e t a l . ~ that the p r e c i p i t a t e s f o r m e d on aging a r e not pure copper as suggested p r e v i o u s l y ~5,'e and we m a y p r e sume that the c l u s t e r s f o r m e d at room t e m p e r a t u r e s i m i l a r l y a r e not composed e n t i r e l y of copper a t o m s . The as-quenched m a t e r i a l is then in a c l u s t e r - h a r d ened state r a t h e r than being a homogeneous, s u p e r s a t u r a t e d solid solution, and this p r o v i d e s an explanation for the high as-quenched y i e l d s t r e s s o b s e r v e d by Goodman e t a l . x Because of the g e n e r a l l y low mobility of copper atoms in the alloy at r o o m t e m p e r a t u r e and the e x haustion of e x c e s s v a c a n c i e s , the c l u s t e r s f o r m e d at room t e m p e r a t u r e or low aging t e m p e r a t u r e s a r e too s m a l l to be detected even by the FIM technique. However, these c l u s t e r s p r o b a b l y provide the nuclei for the subsequent growth of E-phase p r e c i p i t a t e s at higher aging t e m p e r a t u r e s , and the p r e c i p i t a t i o n p r o c e s s in F e - C u a l l o y s (at least) is p r o b a b l y not by a c l a s s i c a l p r o c e s s of nucleation and growth, but r a t h e r by a p r o METALLURGICAL TRANSACTIONS
1. S. R. Goodman, S. S. Brenner, and J. R. Low, Jr.: Met. 7kans., 1973, vol. 4, p. 2363. 2. S. R. Goodm
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