The grain refinement of high-purity aluminum by aluminum-transition metal alloys
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G. W. DELAMORE,
AND
R. W. SMITH
In view of the continuing i n t e r e s t in the s o l i d i f i c a t i o n c h a r a c t e r i s t i c s of dilute A1-Ti alloys, 1 a r e c e n t study 2 of the m e c h a n i s m ( s ) of g r a i n r e f i n e m e n t induced in high p u r i t y a l u m i n u m by v a r i o u s additions of m a s t e r alloys containing Ti, B, Cr, Mo, V and Zr is r e p o r t e d . F o r a l loys c o n t a i n i n g m o r e than 0.2 wt pct Ti, TiA13 was shown to be c o m m o n l y a nucleant, in both A1-Ti and A 1 - T i - B s y s t e m s . In addition, it was found that the nature of the m a s t e r alloys is i m p o r t a n t in d e t e r m i n i n g the d e g r e e of g r ai n r e f i n e m e n t a given alloying addition will p r o d u c e . A " s a t u r a t i o n " effect is r e p o r t e d for additions of Ti, and T i - B , i.e., f u r t h e r ad ditions of t i t a n i u m beyond a given l e v e l do not p r o v i d e any f u r t h e r r e d u c t i o n in g r a i n si ze. C E R T A I N t r a n s i t i o n m e t a l s which show p e r i t e c t i c r e actions when a l l o y e d with a lu m in u m , and which f o r m c a r b i d e s with a s i m p l e cubic s t r u c t u r e , g r a i n - r e f i n e a l u m i n u m with v a r y i n g d e g r e e s of e f f e c t i v e n e s s ; t i t a nium, t a n t a l u m and hafnium, a r e the m o s t potent g r ai n r e f i n e r s . The p r e s e n c e of s m a l l amounts of b o r o n in dilute a l u m i n u m - t i t a n i u m a l l o y s c o n s i d e r a b l y enhances grain refinement. In c o m m e r c i a l p r a c t i c e , much a l u m i n u m is c a s t by ' d i r e c t - c h i l l ' t e c h n i q u e s and is g r a i n r e f i n e d by adding an a l u m i n u m - 5 pct t i t a n i u m - 1 pct boron ' h a r d e n e r ' a l loy to the m e l t p r i o r to c a s t i n g to yield c o n c e n t r a t i o n s of a p p r o x i m a t e l y 0.005 wt pct titanium and 0.001 wt pct boron in the final ingot. Since m o s t t r a n s i t i o n m e t a l s f o r m a l u m i n i d e s with a t e t r a g o n a l s t r u c t u r e which s u b s e q u e n t l y undergo a p e r i t e c t i c r e a c t i o n with a l u m i n u m , the p r e s e n c e of the p e r i t e c t i c r e a c t i o n has been s u g g e s t e d to account for gr ai n r e f i n e m e n t in a l u m i n u m - t r a n s i t i o n m e t a l s y s t e m s . 3 It has been s u g g e s t e d that the p r i n c i p a l effect of b o r o n in the a l u m i n u m - t i t a n i u m s y s t e m is to i n c r e a s e the a l u m i n i d e - p h a s e field and to s t e e p e n the liquidus so that the a l u m i n i d e may f o r m at lower c o n c e n t r a t i o n s (Refs. 4 through 6). However, an e a r l i e r p r o p o s a l by Cibula 7 is that t r a n s i t i o n m e t a l c a r b i d e s a r e r e s p o n si ble for g r a i n r e f i n e m e n t , and that additions of b o r o n p r o v i d e t r a n s i t i o n m e t a l d i b o r i d e s on which the a l u m i num m ay e a s i l y n u cl e a t e
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