Grain boundary sliding, and the effects of particles on its rate
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AND
M. F. ASHBY
GraIn boundary s l id in g can be c o n v e n i e n t l y st u d i ed by loading h e l i c a l s p r i n g s wound of w i r e with a ' % a m b o o " s t r u c t u r e . Such t e s t s on p u r e s i l v e r c o i l s show that i d e n t i c a l l y - s t r e s s e d b o u n d a r i e s s l i d e at w i d e l y d i f f e r e n t r a t e s , and that the s l i d i n g r a t e in any one boundary i s v e r y a n i s o t r o p i c . A g r a i n b o u n d a r y p r e c i p i t a t e of h a r d p a r t i c l e s (SiO2 o r A12Os) sl o w s down the s l i d i n g , and r e n d e r s a ll b o u n d a r i e s i d e n t i c a l , and i s o t r o p i c (as f a r a s g r a i n b o u n d a r y s l i d i n g is c o n c e r n e d ) . T h i s can be u n d e r s t o o d if the g r a i n b o u n d a r y p a r t i c l e s a r e thought of a s h a r d p e g s , a r o u n d which s i l v e r m u s t diffuse if sl i d i n g is to continue.
WE h a v e m e a s u r e d the s li d i n g which o c c u r s at p l a n e , o r a l m o s t - p l a n e , g r a m b o u n d a r i e s when a s m a l l s h e a r s t r e s s i s a p p l i e d to t h e m . T h e s l i d i n g i s r a p i d a b o v e 0.6 TM ( w h e r e T M is the m e l t i n g t e m p e r a t u r e of t h e g r a i n s ) ; 1'2 it is in this t e m p e r a t u r e r a n g e that we have w o r k e d . We h a v e found that h a r d , i n e r t p a r t i c l e s of SiO2 o r A1203 p r e c i p i t a t e d into t h e b o u n d a r y c a u s e the sliding to slow down, and a l t e r i t s c h a r a c t e r in o t h e r w a y s . I n s t e a d of studying s li d i n g at one b o u n d a r y in a b i c r y s t a l , we s t u d ie d s l i d i n g by s t r e s s i n g h e l i c a l c o i l s , m a d e of w i r e with a " b a m b o o " s t r u c t u r e , that i s, with p l a n e g r a i n b o u n d a r i e s n o r m a l to the w i r e a x i s , F i g . 1. T h i s s c h e m e o f f e r s a n u m b e r of a d v a n t a g e s , a s will appear below. 1) E X P E R I M E N T A L M E T H O D We t e s t e d t h r e e g r o u p s of s i l v e r c o i l s . The f i r s t w as m a d e of p u r e s i l v e r , the s e c o n d c o n t a i n e d SiOz p a r t i c l e s , and the t h i r d c o n t a i n e d Al~O3 p a r t i c l e s . A l l o y s w e r e m a d e up f r o m 99.999 pet p u r e c o m p o n e n t s , with the c o m p o s i t i o n s shown in T a b l e I. T h e a l l o y s w e r e drawn to w i r e , and wound to c o i l s , which w e r e a n n e a l e d at 5~ below t h e i r m e l t i n g point f o r 12 h r to r e c r y s t a l l i z e t h e m to a b a m b o o s t r u c t u r e . F i n a l l y , the a l l o y c o i l s w e r e i n t e r n a l l y o x i d i z e d by h e a ti n g in a i r o r in oxygen to p r e c i p i t a t e oxide p a r t i c l e s . The oxide p r e c i p i t a t e s p r e f e r e n t i a l l y a s d i s c r e t e p a r t i c l e s onto the grain boundaries. The c o i l s w e r e d e s i g n e d and l o a d e d so that t h e m a x imum shear s t r e s s was always very s
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