Growth of ternary composites from the melt: Part I
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of the p l a n a r i n t e r f a c e , when three p h a s e s w e r e f o r m ing, could be given by a s i m p l e constitutional s u p e r cooling c r i t e r i o n that will be seen to be a s p e c i a l case of the a n a l y s i s given h e r e i n . A c o n s i d e r a b l e amount of e x p e r i m e n t a l work has been c a r r i e d out on " p s e u d o b i n a r y " e u t e c t i c s , p a r t i c u l a r l y in s y s t e m s in which the fibrous phase is a c a r bide. A " p s e u d o b i n a r y " eutectic composition in a t e r n a r y alloy is one that l i e s on the saddle point of a eutectic line of two-fold saturation. Bibring e t a l . a have studied the s t r u c t u r e and p r o p e r t i e s of C o - C r and N i - C r solid solutions r e i n f o r c e d by m o n o c r y s t a l l i n e f i b e r s of TaC and produced by u n i d i r e c t i o n a l s o l i d i f i cation. Thompson and Lemkey 9 formed c o m p o s i t e s in the t e r n a r y s y s t e m C r - C o - C consisting of a cobalt m a t r i x containing some d i s s o l v e d chromium and an aligned fibrous d i s p e r s i o n of (Cr, Co)7C3. They unid i r e c t i o n a l l y solidified alloys not only of pseudobinary composition but also slightly off-pseudobinary c o m p o sitions along the monovariant trough. In addition to these c a r b i d e a l l o y s , the pseudobinary s y s t e m s N i A I - C r , l~ Ni3AI-Ni3Cb,II and Cd3As2-NiAs12 have a l s o been studied in d e t a i l . Several e x a m p l e s have r e c e n t l y been given of the improved high t e m p e r a t u r e p r o p e r t i e s that a r e obtained when t e r n a r y alloys a r e solidified with e u t e c t i c like composite s t r u c t u r e s . Bibring e t a l . s r e p o r t high t e m p e r a t u r e tensile p r o p e r t i e s of aligned Co-TaC a l loys m o r e than twice as good as those of the nonaligned s t r u c t u r e s . Thompson e t a l . ~3 show that c r e e p of the C o - C r - C pseudobinary in the range 1800 ~ to 2200~ is s u b s t a n t i a l l y above that of existing c o b a l t base a l l o y s .
QUALITATIVEDESCRIPTIONOF INTERFACE STABILITY In this paper, we examine interface stability in ternary alloys using the "constitutional supercooling criterion" exactly as was done previously by Mollard and Flemings for binary alloys. Assumptions employed are the same; these include no convection in the liquid, lateral composition gradients in the liquid ahead of the liquid-solid interface are negligible, and equilibrium pertains at the liquid-solid interface with negligible effect of radius of curvature. ~4'15 The problem can now be readily described qualitatively. To aid in this, consider the aluminum-rich corner of the AI-Cu-Ni phase diagram shown in Fig. 1. A line of two-fold saturation extends from the AI-Ni VOLUME 3, DECEMBER 1972-3133
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