The structure and mechanical behavior of the Mg-Mg 2 Ni eutectic composite

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mechanically s t i r r e d and poured into 21--in. diam by 7 in. long graphite molds. The a s - c a s t ingots were ground to remove surface contamination, placed in

EXPERIMENTAL

The eutectic alloy was prepared by dissolving 99.97 pct pure nickel in molten 99.98 pct pure magnesium contained in a steel crucible under an argon a t m o s phere. After 1 hr at 1300~ (705~ the charge was K. H. ECK~LMEYER,formerly Graduate Student, Department of Metallurgy and Materials Science, Lehigh University, Bethlehem, Pa., is now Staff Member, Sandia Laboratories, Albuquerque, N. Mex. R. W. HERTZBERG is Associate Professor, Department of Metallurgy and Materials Science, Lehigh University. Manuscript submitted July 26, 1971. METALLURGICAL TRANSACTIONS

Fig. 1 - - C r o s s - s e c t i o n of u n i d i r e c t i o n a l - s o l i d i f i c a t i o n apparatus. VOLUME 3, MARCH 1972-609

in. d i a m g r a p h i t e c r u c i b l e s , r e m e l t e d and u n i d i r e c t i o n a l l y s o l i d i f i e d at r a t e s r a n g i n g f r o m 3 to 250 c m p e r h r using the a p p a r a t u s shown in F i g . 1. T h e t e m p e r a t u r e g r a d i e n t at the l i q u i d - s o l i d i n t e r f a c e d u r i n g u n i d i r e c t i o n a l s o l i d i f i c a t i o n w a s a p p r o x i m a t e l y 100~ p e r cm (55~ p e r c m ) . M e t a l l o g r a p h i c s p e c i m e n s w e r e cut both p a r a l l e l and p e r p e n d i c u l a r to t h e s o l i d i f i c a t i o n d i r e c t i o n ( l o n g i t u dinal and t r a n s v e r s e s e c t i o n s , r e s p e c t i v e l y ) . T h e s e w e r e mounted, wet g r o u n d t h r o u g h 600 g r i t SiC a b r a s i v e , and final p o l i s h e d at a m o d e r a t e l y low s p e e d on a m i c r o c l o t h l a p c o v e r e d with a t h i c k s l u r r y of 1 ~ A1203 in a v e r y dilute s o l u t i o n of NH4OH (one d r o p of NH4OH p e r 500 m l of d e i o n i z e d w a t e r ) . A f t e r p o l i s h i n g , t h e s p e c i m e n s w e r e c l e a n e d with cotton while s u b m e r g e d in t h e NH4OH s o l u t i o n , quickly r i n s e d in hot t a p w a t e r , and blown d r y . D i f f e r e n c e s in the e l e c t r o c h e m i c a l b e h a v i o r s of the two p h a s e s m a d e the m a g n e s i u m m a t r i x e x t r e m e l y p r o n e to g a l v a n i c a t t a c k ; h e n c e , s u f f i c i e n t etching o c c u r r e d d u r i n g p o l i s h i n g that no f u r t h e r p r e p a r a t i o n w a s r e q u i r e d p r i o r to m e t a l l o g r a p h i c e x a m i n a tion. Mechanical test specimens were machined from unid i r e c t i o n a l l y s o l i d i f i e d i n g o t s with the s p e c i m e n a x e s p a r a l l e l to the s o l i d i f i c a t i o n d i r e c t i o n . T e n s i l e t e s t s w e r e p e r f o r m e d on s p e c i m e n s with 0.150 in. d i a m by 1.00 in. long gage l e n g t h s . C o m p r e s s i o n s p e c i m e n s c o n s i s t e d of 0.230 in. d i a m by 0.500 in. high c y l i n d e r s . A l l t e n s i l e and c o m p r e

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