Microstructure and mechanical properties of unidirectionally solidified Al-Si-Ni ternary eutectic
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R. C. SHARMA,
AND
K. V. PRABHAKAR
A1-10.98 pct Si-4.9 pet Ni t e r n a r y eutectic alloy was u n i d i r e c t i o n a l l y solidified at growth r a t e s f r o m 1 . 3 9 # m / s e c to 6.95 # m / s e c . B i n a r y A1-Ni and A1-Si eutectics p r e p a r e d f r o m the s a m e p u r i t y m e t a l s were also solidified under s i m i l a r conditions to c h a r a c t e r i z e the growth conditions under the conditions of p r e s e n t study. NiAla phase a p p e a r e d as f i b e r s in the b i n a r y A1-Ni eutectic and silicon appeared as i r r e g u l a r p l a t e s in the b i n a r y A1-Si eutectic. However, in the t e r n a r y A1-Si-Ni eutectic alloy both NiAI 3 and silicon p h a s e s a p p e a r e d as i r r e g u l a r p l a t e s d i s p e r s e d in ~-A1 phase, without any r e g u l a r r e p e t i t i v e a r rangement. The s i z e and spacing of NiAla and $i p l a t e l e t s in cone shaped colonies d e c r e a s e d with an i n c r e a s e in the growth r a t e of the t e r n a r y eutectic. Examination of s p e c imen quenched during unidirectional solidification indicated that the t e r n a r y eutectic grows with a n o n - p l a n a r interface with both Si and NiA13 phases p r o t r u d i n g into the liquid. It is concluded that it will be difficult to grow r e g u l a r t e r n a r y eutectic s t r u c t u r e s even if only one phase has a high entropy of melting. The t e n s i l e strength and modulus of unidir e c t i o n a l l y solidified A1-Si-Ni eutectic was lower than the chill cast alloys of the s a m e composition, and d e c r e a s e d with a d e c r e a s e in growth r a t e . Tensile modulus and strength of t e r n a r y A l - S i - N i eutectic alloys was g r e a t e r than b i n a r y A1-Si eutectic alloy under s i m i l a r growth conditions, both in the chill c a s t and in u n i d i r e c t i o n a l l y solidified conditions. of eutectic U N I D I R Esolidification CTION A L alloys has r e c e i v e d c o n s i d e r a b l e attention in the p a s t few y e a r s because the alignment of f i b e r s or p l a t e l e t s in some of these e u t e c t i c s produces a t t r a c t i v e d i r e c t i o n a l physical or mechanical p r o p e r t i e s . / - s Most of the work has been done on b i n a r y alloys of eutectic and near eutectic c o m positions, and only r e c e n t l y work has been initiated in the area of ternary eutectic alloys.4-s Much of the work so far has been on experimental observations of microstructures obtained on unidirectional solidification of ternary eutectics. Rinaldi, Sharp and Flemings6 have developed quantitative parameters for plane front solidification of ternary eutectic alloys and found reasonable a g r e e m e n t with e x p e r i m e n t s in A1-Cu-Ni s y s t e m . However, t h e r e is no s a t i s f a c t o r y t h e o r y to p r e d i c t the shapes and a r r a n g e m e n t of the p h a s e s obtained in t e r n a r y eutectic solidification. The formulation of this type of t h e o r y will be f a c i l i t a t e d when e x p e r
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