Altering the time cycle of heat treatment by preannealing prior to grain growth
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F O R a b o u t 20 y e a r s , m a n y s t u d i e s '-* h a v e b e e n u n d e r t a k e n t o o b t a i n a b e t t e r u n d e r s t a n d i n g o f the n a t u r e a n d c h a r a c t e r i s t i c s of g r a i n b o u n d a r i e s w h i c h p l a y a n i m p o r t a n t r o l e in d e t e r m i n i n g the p r o p e r t i e s of m e t a l s a n d c e r a m i c s . S i n c e the m a j o r w o r k s o f A u s t a n d R u t t e r 4-7 on g r a i n b o u n d a r y m i g r a t i o n i n b i c r y s t a l s , of B o l l i n g a n d W i n e g a r d s'~ on g r a i n g r o w t h a n d G o r d o n a n d V a n d e r m e e r~°-lz on r e c r y s t a l l i z a t i o n in p o l y c r y s t a l s , a n d o f W e s t b r o o k et a l . ~-~6 on m i c r o h a r d n e s s m e a s u r e m e n t s n e a r g r a i n b o u n d a r i e s , both p r a c t i c a l a n d t h e o r e t i c a l i n t e r e s t in t h i s m e t a l l u r g i c a l a r e a h a v e considerably expanded. T h e principal r e s u l t s o f t h e s e c l a s s i c a l s t u d i e s c a n be s u m m a r i z e d by the f o l l o w i n g statements: 1) T h e r a t e of b o u n d a r y m i g r a t i o n i s m u c h h i g h e r in z o n e - r e f i n e d m e t a l s t h a n in t h o s e o f c o m m e r c i a l purity. 2) S p e c i f i c i m p u r i t i e s s t r o n g l y a f f e c t the m i g r a t i o n k i n e t i c s of r a n d o m g r a i n b o u n d a r i e s . 3) Q u e n c h e d - i n d e f e c t s w h i c h c a n a n n i h i l a t e o u t a t i n t e r n a l s u r f a c e s a r e b e l i e v e d t o be r e s p o n s i b l e f o r the g r a i n b o u n d a r y s o f t e n i n g o r h a r d e n i n g . In a d d i t i o n t o t h e s e g e n e r a l c o n s i d e r a t i o n s , e x p e r i m e n t s h a v e s h o w n t h a t a n u m b e r of o t h e r f a c t o r s a r e q u i t e i m p o r t a n t in c o n t r o l l i n g t h e m i g r a t i o n r a t e during g r a i n g r o w t h a n d reerystallization, such a s t h e s o l i d i f i c a t i o n r a t e ,~7'~8 the type a n d a m o u n t of p r i o r d e f o r m a t i o n , ~8''~ t h e p a r t i t i o n c o e f f i c i e n t of the i m p u r i t i e s b e t w e e n t h e g r a i n b o u n d a r i e s a n d i n t e r i o r s , 2°~2z the p r e s e n c e of i n c l u s i o n s , 23'24 the p r e s e n c e o f n e a r l y f r e e s u r f a c e s ~8''9'2S'26 a n d r e s i d u a l s t r a i n s . 27,es F r o m t h e t h e o r e t i c a l s t a n d p o i n t , the g r a i n b o u n d a r y m i g r a t i o n p r o b l e m in p u r e a n d i m p u r e m a t e r i a l s h a s b e e n a p p r o a c h e d f r o m d i f f e r e n t a s p e c t s . 29-33 With r e g a r d t o g r a i n g r o w t h i n z o n e r e f i n e d m e t a l s on the one J. P . DROLET formerly, Graduate Student at Laval University; now at the Max-Planck Institut for Metallforschung, Stuttgart, West Germany. A. GAL1BOIS is Associate Professor, Department of Mining and Metallurgy, Laval University, Quebec, Quebec, Can. This paper is based on a present
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