The effect of surface configuration on grain boundary sliding
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U N D E R c r e e p c o n d i t i o n s at high t e m p e r a t u r e s , g r a i n b o u n d a r y sliding (GBS) o c c u r s by the m o v e m e n t of g r a i n s along, o r in a zone a d j a c e n t to, t h e i r c o m m o n b o u n d a r y . Many i n v e s t i g a t i o n s have b e e n c a r r i e d out to evaluate the s t r a i n c o n t r i b u t e d by GBS, Egbs , to the total s t r a i n , s u n d e r v a r i o u s e x p e r i m e n t a l c o n d i t i o n s ; a review of the m o r e r e c e n t r e s u l t s i s given e l s e where.' A difficulty a s s o c i a t e d with such s t u d i e s i s that s i g n i f i c a n t l y d i f f e r e n t m e t h o d s a r e often e m p l o y e d to c a l culate E~bs. To o v e r c o m e t h i s p r o b l e m , the v a l i d i t y a n d l i m i t a t i o n s of the v a r i o u s methods w e r e r e v i e w e d , and t e c h n i q u e s s u g g e s t e d which p r o v i d e a m e a n i n g f u l m e a s u r e of the s l i d i n g c o n t r i b u t i o n , E ~ / E , 2 ,=, Sklenicka and Cadek 3 r e c e n t l y p e r f o~rum e d ~"a n i m p o r t a n t s e r v i c e by c o m p a r i n g the v a l u e s o b t a i n e d f o r Egbs/E t in ~ i r o n u s i n g s e v e r a l e x p e r i m e n t a l t e c h n i q u e s . However, they concluded f r o m t h e i r data that a method suggested e a r l i e r 2 for m e a s u r i n g EgDs - was 9 unsatzsfa.ctory, although t h i s i s in d i s a g r e e m e n t with the r e s u l t s f r o m s o m e o t h e r i n v e s t i g a t i o n s .4'~ Since it i s i m p o r t a n t that u n a m b i g u o u s m e t h o d s a r e developed for d e t e r m i n i n g Egbs , this p a p e r e x a m i n e s the r e a s o n s for t h i s a p p a r e n t dichotomy. DEFINITION OF SLIDING COMPONENTS Following the procedure introduced previously,2 the sliding vector AC at a boundary ABCD between two grains X and Y is resolved into three mutually perpendicuiar components, as shown in Fig. 1. The component along the stress axis is denotedby u, and v and w are the componentsperpendicular to the stress axis and either perpendicular to, or in the plane of, the specimen surface, respectively. The orientation of the grain boundary is defined by two angles: 8, between the surface trace of the boundary and the stress axis, and ~, between the boundary and the surface on a longitudinal section cut perpendicular to the surface. It follows from Fig. 1 that u, v, and w are related by the equation .
=
v tan ~ -
-
w tan O
+ - -
A v e r a g e v a l u e s of u, v, a n d w m a y be obtained f r o m m e a s u r e m e n t s t a k e n along a l o n g i t u d i n a l t r a v e r s e (i.e. p a r a l l e l to the s t r e s s axis), a t r a n s v e r s e t r a v e r s e (i.e. p e r p e n d i c u l a r to the s t r e s s axis), or at r a n d o m l y chosen b o u n d a r i e s ; the a v e r a g e s obtained in t h e s e ways a r e denoted by the s u b s c r i p t s l, t, and r, r e spectively. The n u m b e r of g r a i n s p e r u n i t length in the u n d e f o r m e d s p e c i
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