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P. CHIOTTI

The p h a s e r e l a t i o n s of the y t t r i u m zinc s y s t e m have been r e e x a m i n e d . A t o t a l of eight c o m p o u n d s have b e e n i d e n t i f i e d by d i f f e r e n t i a l t h e r m a l a n a l y s i s , m e t a l l o g r a p h i c and X - r a y d i f f r a c t i o n t e c h n i q u e s . T h e t h r e e c o m p o u n d s YZn, YZn2 and YZns.s m e l t c o n g r u e n t l y a t 1105, 1080 and 890~ r e s p e c t i v e l y . T h e c o m p o u n d s YZns, YZn~.67, YZn4.4G, YZr~ and YZn~2 a l l d e c o m p o s e p e r i t e c t i c a t l y at 905, 896, 882, 872 and 685~ r e s p e c t i v e l y . T h r e e e u t e c t i c s w e r e found at 875~ and 23.2 wt p c t Zn (29.12 at. pct), 1015~ and 51 wt p c t Zn (59 at. pct) and a t 863~ and 82 wt p c t Zn (86 at. pct). A p o l y m o r p h i c t r a n s f o r m a t i o n w a s o b s e r v e d in the YZne compound that p r o c e e d s at 750~ in the y t t r i u m r i c h r e g i o n but d r o p s to a p p r o x i m a t e l y 630~ in the zinc r i c h r e g i o n . The t h e r m o d y n a m i c p r o p e r t i e s of the i n t e r m e t a l l i c compounds have b e e n d e t e r m i n e d using the dewpoint m e t h o d to m e a s u r e the v a p o r p r e s s u r e of zinc. The s t a n d a r d f r e e e n e r g y r e l a t i o n s v a r y f r o m ZXG~ (YZn) = - 2 4 , 0 2 0 + 8.76T to ZxG~ (YZnl~) = - 8 5 , 9 4 0 + 48.61T c a l / m o l e .

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o r i g i n a i w o r k on the p h a s e d i a g r a m and t h e r m o d y n a m i c r e l a t i o n s h i p s of the y t t r i u m - z i n c s y s t e m w a s r e p o r t e d s e v e r a l y e a r s ago b y Chiotti e t a t . ~ V a r i ous s t u d i e s at this and o t h e r l a b o r a t o r i e s have s i n c e i n d i c a t e d that r e v i s i o n s of this e a r l y w o r k w e r e in o r d e r . The p u r p o s e of this s t u d y was to r e v i e w the r e s u l t s of o t h e r w o r k e r s , m a k e a d d i t i o n a l m e a s u r e m e n t s w h e r e i n d i c a t e d and p r e s e n t the r e v i s e d p h a s e d i a g r a m and t h e r m o d y n a m i c p r o p e r t i e s of the s y s t e m . EXPERIMENTAL T h e m a t e r i a l s u s e d w e r e B u n k e r H i l l s l a b zinc of 99.99+ p c t p u r i t y and A m e s L a b o r a t o r y y t t r i u m . V a c u u m f u s i o n of the y t t r i u m gave 1800 O, 64 N and 69 H in p p m by weight. S p e c t r o g r a p h i c a n a l y s i s g a v e 800 F e , 300 Ni, 120 Ti, 40 A1, 36 Ca, 24 Si, 20 C r , 15 Cu and 10 Mg in ppm by weight with Ta and W u n d e t e c t e d . T a n t a l u m c o n t a i n e r s w e r e u s e d and found s u i t a b l e e x c e p t at high zinc c o n t e n t s and high t e m p e r a t u r e s . A s in p r e v i o u s r a r e e a r t h - z i n c s t u d i e s u n d e r such c o n d i t i o n s f a i l u r e of the t a n t a l u m was o b s e r v e d due to p e n e t r a t i o n of the t a n t a l u m g r a i n b o u n d a r i e s by the liquid a l l o y s . T h i s l i m i t e d the s t u d y of the YZnt