The nucleation of aluminum grains in alloys of aluminum with titanium and boron
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has been v a r i o u s l y r e p o r t e d 1-4 that alloys b a s e d on the A1-Ti or A1-Ti-B s y s t e m s produce efficient g r a i n r e f i n e m e n t in c a s t i n g s of a l u m i n u m b a s e a l l o y s . It is f u r t h e r known 5'6 that the t e r n a r y a l l o y s with b o r o n p r e s e n t a r e the m o r e efficient r e f i n i n g m a t e r i a l s and c o m m e r c i a l a l l o y s of an a p p r o x i m a t e composition A1-5 wt pct Ti, 1 wt pct B, a r e widely used by the a l u m i n u m i n d u s t r y a s g r a i n r e f i n e r s or " h a r d e n e r s " , b e i n g added to the m e l t at d i l u t i o n s of the o r d e r of 1:103. While the form of the t e r n a r y s y s t e m A 1 - T i - B has not b e e n d e t e r m i n e d the b i n a r y s y s t e m s have been e x a m i n e d in m o r e or l e s s detail. 7,8 In the A1-Ti s y s t e m 9'1~ the compound A13Ti is f o r m e d by a p e r i t e c t i c r e a c t i o n : A1Ti + liquid ~- A13Ti, at 1340~ and this has a t e t r a g o n a l s t r u c t u r e , 9'11'~6 see below, section II. The liquidus c u r v e falls s t e e p l y from the p e r i t e c t i c t e m p e r a t u r e a l m o s t to pure a l u m i n u m , where t h e r e i s r e p u t e d 9 to be a p e r i t e c t i c : AlsTi + liquid ~-A1, at 665~ the l i m i t of the p e r i t e c t i c h o r i z o n t a l b e i n g placed at 0.14 wt pct T i , see below, s e c t i o n IV. In the A1-B s y s t e m r'8'~z the liquidus r i s e s v e r y steeply from an unknown r e a c t i o n which is i n d i s t i n g u i s h a b l e from the f r e e z i n g point of pure a l u m i n u m to a p e r i t e c t i c r e a c t i o n at 975~ where the compound A1B2 is f o r m e d from liquid c o n t a i n i n g ~2 wt pct B and a n o t h e r b o r i d e , probably AtBx2. The s t r u c t u r e of A1B2 is hexagonal ~3 and it is i s o m o r p h o u s with the b i n a r y compound TiB2 which also has s i m i l a r p a r a m e t e r s . In the T i - B s y s t e m 7,~,~4 t h e r e a r e a n u m b e r of i n t e r m e t a l l i c compounds but TiB2 is r e p o r t e d 14 as having the highest m e l t i n g point and that which i s g e n e r a l l y p r e s e n t in a l u m i n u m b a s e t e r n a r y a l l o y s . The p r e s e n t i n v e s t i g a t i o n has fallen into two p a r t s : a), s p e c i m e n s of the t e r n a r y c o m m e r c i a l " h a r d n e r " have b e e n e x a m i n e d in i t s u s u a l chill c a s t form and also in the slowly cooled condition, using optical a n d e l e c t r o n t r a n s m i s s i o n m i c r o s c o p y and e l e c t r o n m i c r o p r o b e a n a l y s e s , and the o r i e n t a t i o n s of a l u m i n u m I. G. DAVIES and J. M. DENNIS, formerly Research Students, Department of Metallurgy, Oxford University, Oxford, England, are with British Steel Corporation, Sheffield. A. HELLAWELLis a University Lecturer, Department of Metallurgy,Oxford University. Manuscript submitted June 11, 1969. METALLURGICALTRANSACTIONS
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