Inverse segregation in unidirectionally solidified Al-Cu alloys
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Inverse Segregation in Unidirectionally Solidified AI-Cu Alloys K. P. EDWARDS
AND
J. A. SPITTLE
IT has previously been reported,' from examination of unidirectionally solidified ingots, that inverse segregation is time-dependent. This is indisputable, since, for any alloy exhibiting such segregation, solidification occurs over a temperature range. A solid + liquid region therefore exists in the ingot which, in the early stages of freezing, permits solute-rich liquid (assuming that the equilibrium partition coefficient for the solute has a value less than unity) to flow towards the chill surface. Inverse segregation is therefore time-dependent, developing as solidification progresses. However, the particular dependence on time discussed' was considered to be associated with a transition from a columnar to an equiaxed grain structure which occurred at a certain distance from the chill face. It appeared, from observations on ingots that underwent such a transition, that inverse segregation in the region of the chill face was not established prior to the transition. The origin of the inverse segregation was therefore directly attributed to the nucleation and growth of solid in the bulk liquid. It was assumed that this resulted in solute enrichment of the liquid, ahead of the macroscopic solidification front, which then infiltrated the already-formed shell adjacent to the chill surface. The technique used" for examining this apparent dependence of inverse segregation on structure transition was to cast, under identical conditions, samples having the same composition and of equal size into a mold designed to produce unidirectional solidification. Ingots of various heights were obtained by decanting the liquid from the solid after different lengths of time following the onset of freezing. These were analyzed by X-ray fluorescence to show the solute distribution as a function of the distance from the chill face. During the solidification of an ingot, the bulk liquid concentration could only approach that of the interdendritic liquid in the already formed solid + liquid shell, if the growth of solid ahead of the macroscopic solidliquid interface was associated with massive undercooling of the bulk liquid. In conventional ingots, the K. P. EDWARDSand J. A. SPITTLE are Postgraduate Research Student and Lecturer, respectively, Department of Metallurgy, University College, Cardiff, Wales, United Kingdom. Manuscript submitted September 20, 1971. 1004-VOLUME 3, APRIL 1972
c o m p o s i t i o n of the bulk liquid cannot r i s e to a v a l u e g r e a t e r than that of the i n t e r d e n d r i t i c liquid s i n c e the bulk liquid t e m p e r a t u r e is u n a b l e to fall below that of the m a c r o s c o p i c s o l i d - l i q u i d i n t e r f a c e . This p l a c e s some doubt on the above e x p l a n a t i o n of the o r i g i n of i n v e r s e s e g r e g a t i o n in ingots u n d e r g o i n g a s t r u c t u r e t r a n s i t i o n . It h a s b e e n o b s e r v e d , in u n i d i r e c t i o n a l l y solidifyi
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