Microstructural changes as a result of temperature gradients in the Al-Al 3 Ni eutectic
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fusion, it is difficult, if not i m p o s s i b l e , to s e l e c t a s y s t e m which w i l l d e m o n s t r a t e s i g n i f i c a n t t h e r m a l d i f f u s i o n e f f e c t s only. T h e r e f o r e , e x p e r i m e n t a l s t u d i e s m u s t d i s t i n g u i s h m o r p h o l o g i c a l c h a n g e s o c c u r r i n g in the p r e s e n c e of a t h e r m a l g r a d i e n t f r o m t h o s e o c c u r r i n g u n d e r i s o t h e r m a l c o n d i t i o n s . T h i s a s p e c t of the p r o b l e m may, in f a c t , be p a r t l y r e s p o n s i b l e f o r c o n t r a d i c t o r y r e s u l t s o c c a s i o n a l l y r e p o r t e d a s to the e f f e c t that t h e r m a l g r a d i e n t s have on m i c r o s t r u c t u r a l s t a b i l i t y in a g i v e n s y s t e m (e.g., c o m p a r e R e f s . 19 v s 21, 17 v s 22 and 1 v s 23 and 24). In t h i s p a p e r we r e p o r t on the effect that t h e r m a l g r a d i e n t s have on m i c r o s t r u c t u r a l s t a b i l i t y in the A1-A13Ni e u t e c t i c . T h i s s y s t e m w a s s e l e c t e d s i n c e , of a l l of the e u t e c t i c l i k e s y s t e m s , i t s m i c r o s t r u c t u r a l s t a b i l i t y u n d e r i s o t h e r m a l c o n d i t i o n s has b e e n m o s t t h o r o u g h l y s t u d i e d . 25-~9 A d d i t i o n a l l y , the s t a b i l i t y of this alloy under a thermal gradient has also been prev i o u s l y s t u d i e d but with c o n t r a d i c t o r y r e s u l t s . F r o m h i s e x p e r i m e n t a l r e s u l t s , M c L e a n 19 d e d u c e d that c h a n g e s in m i c r o s t r u c t u r e r e s u l t i n g f r o m e x p o s u r e to a t h e r m a l g r a d i e n t could be e n t i r e l y e x p l a i n e d a s due to c o n c u r r e n t i s o t h e r m a l e f f e c t s . J o n e s and May, 21 on the o t h e r hand, o b s e r v e d an i n c r e a s e in the c o a r s e n i n g r a t e of d i s p e r s e d AI~Ni f i b e r s which t h e y a t t r i b u t e d to f i b e r c o a l e s c e n c e r e s u l t i n g f r o m f i b e r s moving up the t e m p e r a t u r e g r a d i e n t . P a r t of the m i c r o s c o p i c e v i d e n c e for t h i s , h o w e v e r , was the p r e s ence of e l o n g a t e d o r highly e l l i p t i c a l f i b e r c r o s s - s e c t i o n s and it h a s b e e n shown p r e v i o u s l y that such s t r u c t u r e s a l s o d e v e l o p d u r i n g i s o t h e r m a l c o a r s e n i n g . 27 F o r the above r e a s o n s , we have conducted what we f e e l i s a m o r e e x t e n s i v e e x p e r i m e n t a l s t u d y of the s t a b i l i t y of t h i s s y s t e m u n d e r e x p o s u r e t o a t h e r m a l g r a d i e n t . A s w i l l be shown, our r e s u l t s a r e m o r e in a g r e e m e n t with t h o s e of M c L e a n than t h o s e of J o n e s and M a y . Yet our d a t a do s u g g e s t that t h e r e i s a m e a -
ISSN 0360-2133/80/0211-0323500.75/0 @ 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICALSOCIETY OF AIME
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