Effect of barium oxide on the desulfurizing capacity of slags
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8-- \ 8
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(b)
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Fig. 4--Schematic two-dimensional representations of a grain rearrangement process to allow specimen elongation while retaining the equiaxed grain structure. Grains were pulled vertically in (b) but boundary migration occurs to reach equilibrium structure in (c) and (d) and eventually they become elongated as in (e). The actual process of migration as shown in the upper part was taken from Ref. 3, and a similar suggestion was made by Cline. 4
can be c o m p a r e d with the r e s p e c t i v e i n t e r n a l s t r u c t u r e s as shown in F i g . 3. The s p e c i m e n p u l l e d at the l o w e s t s t r a i n r a t e has an e q u i a x e d s t r u c t u r e , but that c o r r e s p o n d i n g to the i n t e r m e d i a t e r a t e shows c u r v e d b o u n d a r i e s , e s p e c i a l l y those of phase b o u n d a r i e s . At the h i g h e s t r a t e g r a i n s w e r e e l o n g a t e d , an indication that s l i p d e f o r m a t i o n is b e c o m i n g p r e d o m i n a n t . The s t r u c t u r e o b s e r v e d in s p e c i m e n s d e f o r m e d at the slow s t r a i n r a t e which c o n s i s t s of e q u i a x e d g r a i n s with m o r e o r l e s s s t r a i g h t b o u n d a r i e s could be i n t e r p r e t e d as e v i d e n c e that under such t e s t conditions the s y s t e m is a p p r o a c h i n g t h e r m a l e q u i l i b r i u m . In f a c t , g r a i n d e f o r m a t i o n f o l l o w e d by b o u n d a r y m i g r a tion could ex p l ai n the o b s e r v e d e q u i a x e d g r a i n s t r u c t u r e at l a r g e e l o n g a t i o n s . A p o s s i b l e m e c h a n i s m b o r r o w e d f r o m g r a i n growth t h e o r y 3 is given in Fig. 4. On the o t h er hand, the i n t e r g r a n u l a r f a i l u r e such as shown in F i g . 3(c) is p r o b a b l y e n c o u r a g e d both by GBS that o c c u r s in t h i s s t r a i n r a t e r e g i o n and by the l a c k of a c c o m m o d a t i n g p r o c e s s e s l e a d i n g to p o r e f o r m a t i o n at the boundary junctions. T h e r e f o r e , the e q u i a x e d g r a i n s t r u c t u r e in the high m r e g i o n at l a r g e s t r a i n s can be e x p l a i n e d only by GBS. If GBS o c c u r s e x t e n s i v e l y a c c o m p a n i e d by slip and r e c o v e r y p r o c e s s e s within the g r a i n s , the g r a i n s can m o r e o r l e s s s li d e o v e r e a c h o t h e r i n d e f i n i t e l y to p r o v i d e l a r g e e l o n g a t i o n s . Without such c o o p e r a t i v e p r o c e s s e s o p e r a t i n g e s p e c i a l l y in r e g i o n s n e a r the b o u n d a r y , a p o l y c r y s t a l could not undergo e x t e n s i v e plastic deformation. The a u t h o r is indebted to E r i c L i f s h i n f o r the e l e c tron scanning micrograph.
METALLURGICAL TRANSACTIONS
1. W. A. Backofen,I. R. Turner, and D. H. Avery:Trans. ASM, 1964, vol. 57, p. 980. 2. D. Lee:ActaMet., 1969,vol. 17, p. 1057. 3. R. E. Reed-Hill:PhysicalMetallurgyPrinciples, p. 202, D.
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