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The contribution of the peritectic reaction in producing grain refinement in aluminum alloys has been studied in three binary aluminum systems. It appears that titanium has a unique effect compared with zirconium or chromium; there is evidence of a refinement mechanism associated with the peritectic reaction in AI-Ti alloys which was not observed in the other two systems. Additions of boron to AI-Ti and A1-Zr alloys emphasizes the difference in effect of these two transition elements as grain refiners. The significance of the present work is discussed with reference to conflicts apparent in earlier experimental data.

T H E mechanism of grain refinement of aluminum alloys by titanium and certain other transition elements has provoked considerable controversy. It has been suggested, I following the work of lwas6 el a l . , z that the peritectic faction occurring between intermetallic compounds, e.g. TiAls, and the melt provides efficient substrates by sheathing the intermetallic crystals via the peritectic reaction in a layer of aluminum. Alternatively, the refining action has been explaineds by the formation of simple interstitial carbides or borides of the refining addition which, because of close matching with the aluminum lattice, enhance nucleation of the solid phase. Experimental evidence in support of both suggested mechanisms has been summarized by Glasson and Emley,* but much of this earlier work is difficult to assess objectively because of wide variations in purity of the base materials and melt thermal history, and also in the methods employed to determine grain size. This investigation was undertaken to determine the relative contribution of each mechanism in promoting refinement in high purity aluminum when solidified under controlled conditions. EXPERIMENTAL

WORK

The e x p e r i m e n t a l t e c h n i q u e r e s e m b l e d t h a t of C r o s s l e y and M o n d o l f o ~ but w i t h m o d i f i c a t i o n s to i m p r o v e the r e l i a b i l i t y of g r a i n s i z e e s t i m a t i o n and u s i n g b a s e m a t e r i a l s of h i g h e r p u r i t y , T a b l e I. M a s t e r a l l o y s w e r e p r e p a r e d and c a s t i n v a c u o to a v o i d c o n t a m i n a t i o n and p o s s i b l e u n c o n t r o l l a b l e e f f e c t s on r e f i n e m e n t by the u s e of f l u x e s , s C a l c u l a t e d a m o u n t s of m a s t e r a l l o y and p u r e a l u m i n u m s u f f i c i e n t to p r o d u c e a 150 g i n g o t of the r e q u i r e d c o m p o s i t i o n w e r e p l a c e d in an a l u m i n a c r u c i ble w h i c h w a s t h e n h e a t e d in a r e s i s t a n c e f u r n a c e to 100~ a b o v e the a l l o y l i q u i d u s . A f t e r s t i r r i n g , the c r u c i b l e w a s t r a n s f e r r e d to a r i g p o s i t i o n e d b e t w e e n p o l e s of an e l e c t r o m a g n e t and the a l l o y s o l i d i f i e d in a u n i d i r e c t i o n a l f i e l d of 2000 g a u s s . T h i s p r o c e d u r e m i n i m i z e d g r a v i t y s e g r e g a t i o n of