Effect of vanadium and zirconium on the formation of CrAI 7 primary crystals in 7075 aluminum alloy
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DONALD J . BEERNTSEN P r i m a r y c r y s t a l s of CrAfT which f o r m d u r i n g the s e m i c o n t i n u o u s c a s t i n g of 7075 a l u m i n u m alloy can, u n d e r s o m e c a s t i n g conditions, grow l a r g e enough to c o n s t i t u t e s i g n i f i c a n t d e f e c t s in the f i n a l wrought p r o ducts. The f o r m a t i o n of such l a r g e p r i m a r y c r y s t a l s r e q u i r e s an alloy c o m p o s i t i o n in the liquid + CrAfT phase field, plus q u i e s c e n t growth s i t e s in the m o l t e n alloy which a r e at t e m p e r a t u r e s j u s t above the p e r i t e c t i c . C o n t r o l of the c h r o m i u m c o n t e n t to v a l u e s n e a r the m i n i m u m r e q u i r e d in 7075 alloy (0.18 wt pct) is not u s u a l l y s u f f i c i e n t to avoid the f o r m a t i o n of p r i m a r y c r y s t a l s , b e c a u s e other e l e m e n t s p r e s e n t in the c o m m e r c i a l alloy also c o n t r i b u t e to t h e i r f o r m a tion. Steele and C o l l i n s ~ d e r i v e d a q u a n t i t a t i v e " c h r o m i u m e q u i v a l e n c e " equation to d e s c r i b e the c o n t r i bution of o t h e r e l e m e n t s to the f o r m a t i o n of s u c h c r y s t a l s in a 7075-type alloy. F r o m e q u i l i b r a t e d a l loys they q u a n t i t a t i v e l y e x t r a c t e d and a n a l y z e d CrA17 c r y s t a l s , and c a l c u l a t e d the Cr, Ti, Mn and F e c o n t e n t s of the liquid with which the c r y s t a l s had b e e n in e q u i l i b r i u m . U s i n g a l e a s t s q u a r e s p r o c e d u r e to m i n i m i z e e r r o r in the p h a s e b o u n d a r y v a l u e they e x p r e s s e d t h e i r r e s u l t s as follows (in wt. pct): Cr + 0.797 T i + 0.090 Mn + 0.142 Fe = 0.244. With our own d a t a we s i m p l y c a l c u l a t e d c r y s t a l / liquid d i s t r i b u t i o n coefficients and a v e r a g e d and n o r realized t h e m . A b o u n d a r y value was then obtained by applying the r e s u l t i n g e l e m e n t c o e f f i c i e n t s to the liquid a n a l y s e s and a v e r a g i n g . (See below.) When this p r o c e d u r e was applied to Steele and C o l l i n s ' raw data (using the m o s t r e l i a b l e 13 of t h e i r 14 alloys) the following was obtained: Cr + 0.66 T i + 0.21 Mn + 0.073 F e = 0.246. This type of a n a l y s i s does not give a b o u n d a r y value of m i n i m u m e r r o r . However, at e q u i l i b r i u m the d i s t r i b u t i o n coefficient is a d i r e c t r e f l e c t i o n of the f r e e e n e r g y of the s y s t e m and as such a c c u r a t e l y r e f l e c t s the r e l a t i v e p r e f e r e n c e of the i n d i v i d u a l e l e m e n t s for the r e s p e c t i v e p h a s e s . This is p r e f e r a b l e f r o m an o p e r a t i n g s t a n d p o i n t b e c a u s e it is p a r t l y t h r o u g h m a n i p u l a t i o n of t h e s e s e c o n d a r y e l e m e n t s that the p o t e n t i a l for f o r m a t i o n of p r i m a r y c r y s t a l s can be m i n
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