The effect of solidification rate on structure and high-temperature strength of the eutectic NiAl-Cr
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The eutectic NiA1-Cr, c o n s i s t i n g of c h r o m i u m rods in a NiA1 m a t r i x , was d i r e c t i o n a l l y s o l i d i fied at r a t e s v a r y i n g f r o m ~ to 30 in. per hr. The i n t e r - r o d s p a c i n g and e l e v a t e d t e m p e r a t u r e t e n s i l e p r o p e r t i e s were m e a s u r e d and the s t r u c t u r e d e t e r m i n e d for each s o l i d i f i c a t i o n r a t e . The spacing, ;~, obeyed the r e l a t i o n ~2V = C o n s t a n t , where V is the solidification r a t e . At V g r e a t e r than 1 in. per h r , a cell or colony s t r u c t u r e f o r m e d and the cell size d e c r e a s e d with i n c r e a s i n g s o l i d i f i c a t i o n rate. At 600~ the t e n s i l e s t r e n g t h i n c r e a s e d s u b s t a n t i a l l y with dec r e a s i n g i n t e r - r o d spacing, r e a c h i n g 100,000 psi at the h i g h e s t V. At 800 ~ and 1000~ the s t r e n g t h f i r s t i n c r e a s e d then d e c r e a s e d at the highest s o l i d i f i c a t i o n r a t e s r e a c h i n g m a x i m u m s t r e n g t h s of 63,000 psi and 47,000 p s i , r e s p e c t i v e l y . At 1200~ the s t r e n g t h d e c r e a s e d slightly with i n c r e a s i n g solidification r a t e . F r a c t u r e o c c u r r e d by s h e a r along g r a i n b o u n d a r i e s and cell walls. The d e c r e a s e d s t r e n g t h at the highest s o l i d i f i c a t i o n r a t e s m a y be r e l a t e d to the i n c r e a s e d cell b o u n d a r y a r e a .
D I R E C T I O N A L L Y s o l i d i f i e d eutectic a l l o y s exhibit n a t u r a l composite s t r u c t u r e s , c o n s i s t i n g of r o d s in a m a t r i x or of a l t e r n a t i n g plates of the two p h a s e s . The r a t e at which the e u t e c t i c i s solidified d e t e r m i n e s the d i s t a n c e between the r o d s or the t h i c k n e s s of the p l a t e s , with the higher s o l i d i f i c a t i o n r a t e s p r o d u c i n g the finer spacing. 1-3 As the s p a c i n g d e c r e a s e s , the r o o m - t e m p e r a t u r e strength i n c r e a s e s . 4-6 The e u t e c t i c s a r e p a r t i c u l a r l y i n t e r e s t i n g for possible h i g h - t e m p e r a t u r e a p p l i c a t i o n s b e c a u s e of the i n h e r e n t s t a b i l i t y of the fine two-phase m i c r o s t r u c t u r e at e l e v a t e d t e m p e r a t u r e s . Thus, the r e l a t i o n s h i p b e t w e e n s t r e n g t h and s p a c i n g that e x i s t s at low t e m p e r a t u r e s m a y a l s o hold for e l e v a t e d t e m p e r a t u r e s . Such a r e l a t i o n s h i p does indeed exist and this r e p o r t d e s c r i b e s the r e s u l t s of a d e t e r m i n a t i o n of the h i g h - t e m p e r a t u r e s t r e n g t h of the eutectic NiA1-Cr a s a function of the s o l i d i f i c a t i o n r a t e . The NiA1-Cr eutectic alloy was chosen for this study b e c a u s e both p h a s e s a r e r e s i s t a n t to h i g h - t e m p e r a t u r e oxidation. In addition, a single t e n s i l e
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