Thermodynamics of formation of compounds in the Ce-Mg, Nd-Mg, Gd-Mg, Dy-Mg, Er-Mg, and Lu-Mg binary systems in the tempe
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IN the b i n a r y
a l l o y s y s t e m s of m a g n e s i u m with the lanthanon e l e m e n t s , the p a t t e r n s of o c c u r r e n c e of i n t e r m e d i a t e p h a s e s with d i f f e r i n g s t o i c h i o m e t r i e s and c r y s t a l s t r u c t u r e s i n d i c a t e v a r i a t i o n in the c o m p e t i t i v e f a c t o r s which c o n t r o l p h a s e s t a b i l i t y . T h i s i s i l l u s t r a t e d in T a b l e I which i s a s u m m a r y of a v a i l able d a t a . 1-1~ E u r o p i u m and y t t e r b i u m a r e e x c l u d e d f r o m the tabulation b e c a u s e t h e i r d i v a l e n t t e n d e n c y l e a d s to a t y p i c a l a l l o y i n g b e h a v i o r . P r o m e t h i u m i s a l s o e x c l u d e d b e c a u s e d a t a a r e not a v a i l a b l e . T a b l e I l i s t s the r e m a i n i n g lanthanons in o r d e r of i n c r e a s i n g a t o m i c n u m b e r and shows that only for the s t o i c h i o m e t r y LnMg d o e s a s i n g l e s t r u c t u r e type ( C s C l - t y p e ) p e r s i s t throughout the e n t i r e s e r i e s . F o r the s t o i c h i o m e t r y LnMg2 the cubic MgCu2 s t r u c t u r e o c c u r s for the lighter lanthanon systems, La-Mg through Tb-Mg, while the hexagonal MgZnz s t r u c t u r e o c c u r s for the h e a v i e r s y s t e m s , D y - M g t h r o u g h L u - M g . An a d d i t i o n a l p h a s e h a s been r e p o r t e d 4 to o c c u r in the D y - M g s y s t e m n e a r the s t o i c h i o m e t r y DyMg-2 and to give p o w d e r p a t t e r n s t y p i c a l of an N a T l - t y p e s t r u c t u r e . Such a p h a s e w a s not o b s e r v e d in the a l l o y s of the p r e s e n t i n v e s t i g a tion and h a s been a s s u m e d to be e i t h e r m e t a s t a b l e o r i m p u r i t y s t a b i l i z e d and thus not c h a r a c t e r i s t i c of the e q u i l i b r i u m a l l o y s y s t e m . The BiLls s t r u c t u r e has b e e n found at the s t o i c h i o m e r r y LnMgs for a l l of the l i g h t e r lanthanon s y s t e m s , L a - M g through D y - M g , but no p h a s e n e a r this s t o i c h i o m e t r y has been found 4 in a l l o y s of the four h e a v i e r J. E. PAHLMAN, formerly Graduate Student, Department of Metallurgy, Iowa State University, Ames, Iowa, is now with the U. S. Bureau of Mines, Minneapolis, Minn. J.F. SMITH is Professor and Senior Metallurgist, Ames Laboratory, USAEC, Iowa State University, Ames, Iowa. Manuscript submitted February 23, 1972. METALLURGICAL TRANSACTIONS
lanthanon s y s t e m s . In c o n t r a s t no p h a s e s n e a r the s t o i c h i o m e t r y LnhMgz4 have b e e n found 5 in the L a - M g , C e - M g , or N d - M g s y s t e m s , but such p h a s e s do o c c u r in the h e a v i e r lanthanon s y s t e m s f r o m T b - M g t h r o u g h L u - M g i n c l u s i v e . T h e s e l a t t e r p h a s e s c r y s t a l l i z e with the a - m a n g a n e s e - t y p e s t r u c t u r e and a r e the m o s t m a g n e s i u m - r i c h p h a s e s in the
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