Cellular precipitation in Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys
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l y z e d in t e r m s of the m o d i f i e d T u r n b u l l t h e o r y , %'2 f r o m which d i f f u s i v i t i e s , DB, a n d a c t i v a t i o n e n e r g y t e r m s a r e d e r i v e d . It is shown that the r e s u l t s for D B fall within the v a l u e s expected of a g r a i n b o u n d a r y diffusion c o n t r o l l e d r e a c t i o n , although the a c t i v a t i o n e n e r g y o b t a i n e d i s c l o s e r to that of v o l u m e diffusion. EXPERIMENTAL The d i r e c t i o n a l l y solidified l a m e l l a r e u t e c t i c m a t e r i a l is the s a m e p r e v i o u s l y u s e d for s t u d i e s 13 of m e c h a n i c a l p r o p e r t i e s . The i n v e s t i g a t i o n of the eutectic m a t e r i a l is s u p p l e m e n t e d by o b s e r v a t i o n s on cast N i - 4 4 C r a l l o y , this b e i n g the c o m p o s i t i o n of the n i c k e l - r i c h phase in the grown e u t e c t i c , is The c a s t m a t e r i a l was p r e p a r e d f r o m high p u r i t y sponge n i c k e l (99.999) and " E l e c t r o m e t L a b " grade c h r o m i u m , which c o n t a i n s l e s s than 50 ppm O. After a r c c a s t i n g in an a t m o s p h e r e of a r g o n , the m a t e r i a l
Fig. 1--Phases in directionally solidified Ni-51Cr lamellar eutectic aged 9 hr at 700~ eel: chromium-rich lamellae; 3'1: nickel-rich lamellae; c~2: chromium-rich grain boundary precipitates; 2/2: nickel-rich Widmanst~tten precipitates, + % Cellular precipitation (Cr + Ni). VOLUME 1, JUNE 1970-1623
was homogenized (also in an a t m o s p h e r e of a r g o n ) for 7 days at 1200~ and b r i n e quenched. H o m o g e n e i t y was v a r i f i e d by probe s c a n n i n g which showed c o m position v a r i a t i o n s of l e s s than 0.5 pct. Both the l a m e l l a r eutectic and c a s t m a t e r i a l s were then aged at 625 ~ 700 ~ 760 ~ and 850~ for t i m e s r a n g i n g f r o m 30 m i n to 100 hr. The aged s p e c i m e n s were m e t a l l o g r a p h i c a l l y polished and etched and the r a d i u s of the l a r g e s t cell in the c a s t N i - 4 4 C r alloy d e t e r m i n e d . As shown in the text, the c e l l s fan out in a h e m i s p h e r i cal fashion in the c a s t m a t e r i a l , w h e r e a s in the e u tectic alloy the c e l l s grow between the c o n f i n i n g b o u n d a r i e s of the a d j a c e n t c h r o m i u m - r i c h l a m e l l a e . In the l a t t e r c a s e , the length of the cell was d e t e r m i n e d . When these data a r e plotted vs t i m e , a s t r a i g h t line is obtained, where the slope is equal to the growth r a t e G, Fig. 5. The i n t e r l a m e l l a r s p a c i n g , S, was det e r m i n e d from s u r f a c e r e p l i c a photographs. In each photograph e x a m i n e d , s p a c i n g was m e a s u r e d only on c e l l u l a r colonies showing fine, u n i f o r m s t r u c t u r e , see, for e x a m p l e , Fig. 1, and the m i n i m u m of the v a l u e s
obtained for S were used, to e n s u r e that the l a m e l l a e were e s s e n t i a l l y
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