Dendritic solidification of Cu-Ni alloys: Part I. Initial growth of dendrite structure
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freezing in a t h e r m a l l y undercooled condition where heat diffusion is controlling, and for solid a l l o y s which a r e s u p e r s a t u r a t e d where the solute diffusion alone is held to be r a t e controlling. 12''4 A difficulty a s s o c i a t e d with dendritic solidification of alloys is that both solute and heat diffusion a p p e a r to be involved in that the latent heat r e l e a s e is so r a p i d that a fixed bath undercooling cannot be m a i n tained and a t e m p e r a t u r e r i s e is frequently found. '5''6 T h e r e f o r e the v a r i o u s t h e o r i e s do not s e e m to have been tested for alloy solidification and there even e x i s t s a dispute as to whether or not there is a s i g n i f i cant constitutional undercooling during dendritic s o l i d ifieation in metal a l l o y s . 3'4''6 If significant constitutional undereooling w e r e not eliminated by dendritic growth the dendrite tips would be freezing at a t e m p e r a t u r e lower than the e q u i l i b r i u m liquidus and forming a solid containing m o r e
, SERVICE (4bore) TUBE - -
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Fig. 1--Diagram of t h e r m a l a n a l y s i s and quenching furnace. VOLUME 4, JANUARY 1973-115
solute than the equilibrium solidus composition. This is important since it will reduce the microsegregation of the resulting solid ingot.~v The conflicting evidence~v-23 has been recently discusseda and need not be repeated. However since that time Backerud and Chalmers16 have produced direct evidence for some remaining constitutional undercooling in large (on an experimental scale) ingots of aluminium alloys and showed that the amount of this undercooling increased with increased rates of heat extraction. The p r e s e n t work was u n d e r t a k e n to see if this u n d e r c o o l i n g could be c o n f i r m e d in a d i f f e r e n t s y s t e m u s i n g a d i f f e r e n t e x p e r i m e n t a l a p p r o a c h . T h i s involved s m a l l ingots s u b j e c t e d to t h e r m a l a n a l y s i s at d i f f e r e n t r a t e s of heat e x t r a c t i o n w h e r e the liquidus t e m p e r a l u r e could also be d i r e c t l y d e t e r m i n e d by an e q u i l i b r i u m method, a'17 The solute c o n t e n t of d e n d r i t e s quenched at the t h e r m a l a r r e s t at d i f f e r e n t r a t e s of heat e x t r a c t i o n was also studied. E X P E R I M E N T A L METHODS Apparatus A v e r t i c a l tube f u r n a c e was u s e d . T h i s c o n s i s t e d of a 5.0 c m a l u m i n a tube heated by a s i l i c o n c a r b i d e r e s i s t a n c e h e a t e r . The tube was sealed by w a t e r cooled b r a s s end p i e c e s and m a i n t a i n e d at a p r e s s u r e s
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