A MgZn 2 type laves phase in the Mn-Cu-Si System
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Laves
phase
Table I. Alloys Investigated in the Mn-Cu-Si System
NominalComposition
AND K. P. GUPTA is often found in a binary
system
if
the relative atomic size and electron concentrations are favorable. Nonexistence of Laves phase in certain binary systems having favorable relative atomic size (RA/R B ~, 1 . 2 2 5 , w h e r e RA a n d RB a r e t h e C N 1 2 a t o m i c r a d i i of e l e m e n t s A and B) and the stabilization of t h e L a v e s p h a s e i n t h e s a m e s y s t e m s o n a d d i t i o n o f silicon has been attributed to silicon lowering the elect r o n t o a t o m r a t i o (e/a) b e l o w a l i m i t i n g v a l u e of 8.1 In the Mn-Cu binary system there is no Laves phase; b o t h RA/RB a n d e/a r a t i o s a r e u n f a v o r a b l e i n t h i s case. However, a Laves phase has been reported in the Mn-Cu-X systems where X = Ge, Ga, and In. 2 Surprisingly, however, so far no Laves phase has been reported in the Mn-Cu-Si system. Since silicon is k n o w n t o b e a s t r o n g s t a b i l i z e r of m a n y c o m p l e x p h a s e s as well as the Laves phases, a search for a Laves phase was made in the Mn-Cu-Si system. Since silicon stabilized Laves phases usually occur a t t h e A(B, Si)z c o m p o s i t i o n 3'4 ( s i l i c o n v a r y i n g b e t w e e n 12 a t . p e t 4 t o 4 0 a t . p e t 5 ) , s i x t e e n M n - C u - S i a l l o y s w e r e induction melted in pure alumina crucibles under argon atmosphere with silicon and manganese varying bet w e e n 1 1 . 5 t o 25 a t . p e t a n d 33 t o 6 2 . 5 a t . p e t , r e s p e c tively, Table I. The melting losses were very small, always within 0.3 pet. Hence the alloys were not chemically analyzed. Alloys annealed at 700 ~ + I~ for 72 h i n e v a c u a t e d a n d s e a l e d s i l i c a c a p s u l e s w e r e water quenched and analyzed by metallographic and X-ray diffraction techniques. Alloy powders for X-ray d i f f r a c t i o n s p e c i m e n s w e r e a n n e a l e d a t 7 0 0 ~ f o r 15 min, water quenched, and diffraction patterns were taken using an asymmetrical f o c u s i n g c a m e r a (20 c m diem), a Debye Scherrer Camera (114.6 mm diam) and u n f i l t e r e d C r K r a d i a t i o n a t 30 k V P a n d 15 m a . Of the sixteen alloys prepared, except for the two alloys P and Q, which were two phase alloys, all conr a i n e d t h r e e p h a s e s , T a b l e I . F o r i d e n t i f i c a t i o n of t h e p h a s e s t h e d i f f r a c t i o n p a t t e r n s of a l l a l l o y s w e r e c o m p a r e d w i t h s t a n d a r d p a t t e r n s of f e e , b e e , a n d M g Z n 2 t y p e L a v e s p h a s e s . T h e d i f f r a c t i o n p a t t e r n s of a l l a l 1oys, except for alloy N, showed the presence of a fee p h a s e , t h e f e e s o l i d s o l u t i o n p h a s e of t h e M n - C u s y s t e m . A l l t h e m a j o r d i f f r a c t i o n l i n e s of t h e a l l o y s B through N matched well with the Laves phase lines. F o r m o r e p o s i t i v e i d e n t i f i c a t i o n of t h e L a v e s
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