The properties and microstructure of directionally cast alloys
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e x c e l l e n t w o r k h a s b e e n p u b l i s h e d r e c e n t l y in w h i c h the relationship between mechanical properties o f unidirectionally solidified (UDS) aluminum a l l o y s a n d t h e v o l u m e f r a c t i o n o f i n t e r d e n d r i t i c e u t e c t i c a r e disc u s s e d . 1 O t h e r s have shown t h e effects o f cooling r a t e s on t h e s o l u t e r e d i s t r i b u t i o n i n d e n d r i t i c s o l i d i f i c a t i o n . 2'a T h i s note p o i n t s out s o m e s i m p l e o b s e r v a t i o n s made o n a s e r i e s o f a l u m i n u m a l l o y s that h a d b e e n u n i d i r e c t i o n a l l y c a s t into t e n s i l e t e s t s p e c i m e n s . T h e m e c h a n i c a l p r o p e r t i e s o f A 1 - 4 . 5 p c t Cu in t h e c h i l l e d a n d u n c h i l l e d cast conditions h a v e been evaluated. 4 A substantial net g a i n i n m e c h a n i c a l p r o p e r t i e s w a s o b s e r v e d in c h i l l e d sections. T h e m e c h a n i c a l p r o p e r t i e s o f A1-Si a l l o y s u n d e r d i f f e r e n t solidification m o d e s h a v e also been reported. ~ T h e p u r p o s e of t h i s s t u d y w a s t o p r e p a r e a n d e x a m i n e d i r e c t i o n a l l y s o l i d i f i e d m i r r o r b l a n k s . It w a s s p e c u l a t e d that t h i s t y p e of s u b s t r a t e w o u l d be m o r e s t a b l e (optically) u n d e r situations of t h e r m a l cycling t h a n m i r r o r s having equiaxed s t r u c t u r e s . T h e d i r e c t i o n of the r e s e a r c h w a s t o c a s t b a r s f o r t e n s i l e t e s t i n g and s e v e r a l t e s t - s i z e m i r r o r b l a n k s for t h e r m a l c y c l e stability. T h e p r o c e d u r e u s e d to o b t a i n t h e t e s t b a r s was s i m i l a r t o that of o t h e r i n v e s t i g a t o r s . T o f a c i l i t a t e t e n s i l e t e s t i n g , t e n s i l e b a r s were f o r m e d in w a x a n d t h e n i n v e s t m e n t cast in a c r y s t o b a l i t e - p l a s t e r m i x t u r e . M o l d s were h e a t e d to 1 2 0 0 ° F , a n d c a s t i n g s were p o u r e d o n a water-cooled, copper c h i l l block. All a l l o y s were h e a t e d t o a p p r o x i m a t e l y 100° t o 2 0 0 ° F a b o v e t h e liquidus. T h e c a s t t e n s i l e t e s t b a r s w e r e 0 . 2 5 0 i n . d i a m by 1.0 i n . gage l e n g t h . T h e y w e r e t e s t e d a s c a s t a n d i n the T - 6 condition on an Instron tensile testing machine. A l l s a m p l e s W e r e p u l l e d a t 0.2 i n . p e r m i n . U l t i m a t e t e n s i l e s t r e n g t h s a n d e l o n g a t i o n to f a i l u r e w e r e r e c o r d e d ; a l s o , a s e r i e s o f m i c r o h a r d n e s s t e s t s was made to d e t e r m i n e o r i e n t a t i o n e f f e c t s . T h e g e n e r a l o b j e c t i v e in c a s t i n g e a c h a l l o y was to o b t a i n a n u m b e r of r a n d o m - o r i e n t e d a n d d i r e c t i o n a l (columnar) s t r u c t u r e s for
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