The four stages of grain growth
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K. T. AUST,
AND
W. C. WINEGARD
A study of g r a i n growth in dilute c a d m i u m and lead base a l l o y s has r e v e a l e d that t h e r e a r e four s t a g e s of g r a i n growth a s a f u n c t i o n of a n n e a l i n g t e m p e r a t u r e and solute c o n c e n t r a t i o n . One of t h e s e s t a g e s , which can r e s u l t in a s u b s t a n t i a l change in g r a i n s i z e , i n v o l v e s i r r e g u l a r g r a i n growth or s e c o n d a r y r e c r y s t a l l i z a t i o n .
PREVIOUS
work on g r a i n growth in high p u r i t y m e t a l s has been c a r r i e d out o v e r a r a t h e r n a r r o w t e m p e r a t u r e r a n g e n e a r the m e l t i n g t e m p e r a t u r e s . ~-3 Within this range it has been shown that the t h e o r e t i c a l g r a i n growth law, D 5 (KT) ~/2, is obeyed only for g r a i n growth in u l t r a high p u r i t y m e t a l s at t e m p e r a t u r e s a p p r o a c h i n g the m e l t i n g points. F o r dilute a l l o y s , g r a i n growth r e s u l t s have u s u a l l y been d e s c r i b e d by e q u a t i o n s of the f o r m = (KT) n where the index n has been l e s s than the t h e o r e t i c a l value of 0.5, e s p e c i a l l y with ko > 1 s o l u t e s 4 p r e s e n t (ko i s the solute d i s t r i b u t i o n coefficient). A n o t h e r c o n c e p t u a l difficulty with p r e v i o u s e x p e r i m e n t a l r e s u l t s has been the fact that m e a s u r e d a c t i v a tion e n e r g i e s for the p r o c e s s of n o r m a l g r a i n growth have been found to v a r y to a l a r g e d e g r e e with solute c o n c e n t r a t i o n . 1'3'S It i s difficult to v i s u a l i z e how the p r e s e n c e of solute a t o m s , in the p a r t s per m i l l i o n r a n g e , can affect to any g r e a t extent the e n e r g y r e q u i r e d to t r a n s f e r a t o m s a c r o s s a g r a i n b o u n d a r y . In a p r e v i o u s paper s the effects of s o l i d i f i c a t i o n solute s e g r e g a t i o n on b o u n d a r y m o b i l i t y were r e p o r t e d . Since e a r l i e r i n v e s t i g a t i o n s of g r a i n growth in dilute a l l o y s were c a r r i e d out with c a s t s a m p l e s , in which s o l i d i f i c a t i o n solute s e g r e g a t i o n was p r e s e n t , it was decided to r e i n v e s t i g a t e g r a i n growth in dilute a l l o y s in the a b s e n c e of s o l i d i f i c a t i o n solute s e g r e g a t i o n . T h e s e s t u d i e s were extended over a w i d e r t e m p e r a t u r e range in hopes of m a t c h i n g e x p e r i m e n t a l data with theory.7- ~o EXPERIMENTS a) Preliminary
Investigation
The initial experimental work was undertaken in an attempt to measure an activation energy for grain growth in 99.9999 pct Cd. This value would be taken as that for grain growth in "pure" cadmium in order for a later comparison of the effect of solute on the activation energy for grain growth. A cylindrical bar of 99.9999 pct Cd was cast under an argon atmosphere into a graphite-coated
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