Gravity segregation during remelting of dendritic alloys
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Gravity Segregation During Remelting of Dendritic Alloys T. E. STRANGMANAND T. Z. KATTAMIS
c o a r s e n v e r y f a s t . In a n u n s t i r r e d m i x t u r e a s u p e r h e a t of a b o u t 35~ w a s f o u n d n e c e s s a r y b e f o r e t h e l a s t t r a c e s o f s o l i d d i s s o l v e d . F r o m t h e p h a s e d i a g r a m , F i g . 2, R e f . 1, it c a n b e s e e n t h a t t h e l a s t s o l i d to d i s s o l v e w a s in c o n t a c t w i t h a l i q u i d of c o m p o s i t i o n H~O-43 wt p e t
NH4C1. Stability of the NH4CI crystals in a superheated liquid may be attributed to a gravity stabilized concentration gradient of solute in the liquid. This concentration gradient is established by displacement of the NH4CI dendritic debris to the bottom of the test tube during the melting process. Short term stability of the concentration gradient may be explained by the damping of vertical convection in the liquid by a gravitational restoring force. Eventually, the unstirred solution becomes homogeneous when diffusional processes have had sufficient time to operate. Confirmationof the presence of the concentration gradient was obtained by heating a partially melted H~O-NH4C1 mixture having an equilibrium liquidus temperature of 65.5~ to 97~ and then cooling to 82~ The NH4C1 crystals which remained unmelted at 97~ grew dendritically a short distance into the superheated solution when cooled to 82~ Superheating and remelting studies were extended to a Pb-46 wt pet Sn alloy, Fig. I, selected because the tin-rich liquid is lighter than the lead-rich dendritic solid. A small casting (2.5 cm diam x 4.2 cm high) was poured in a metallic mold from a well stirred melt and was rapidly solidified. The casting was subsequently remelted in a test tube submerged in a salt bath at 250~ (27~ above the equilibrium liquidus temperature of the alloy), held at this temperature for 20 min and then slowly cooled. The ingot was sectioned longitudinallyin half, polished and examined metallographically. Fig. 2(a) and (b) are photomicrographs taken near the top and the bottom of the ingot, respectively. The microstructure A consists of equiaxed dendrites of the lead-rich a-phase embedded in eutectic. Precipitation of the tin-rich fi-phase occurred within the dendrites and is resolved at higher magnification, Fig. 3. The lower part of the ingot, microstructure B, consists of spheroidized, often coalesced a-phase particles which are believed to result from coarsening of dendritic debris settling to the bottom of the test tube. Measurement of the volume fraction of the lead-rich dendritic (~-phase by quantitative metallography2-s at various distances from the bottom of the ingot indi-
DURrNG
a r e c e n t I n v e s t i g a t i o n I of c r y s t a l m u l t i p l i c a t i o n in a n u n d e r c o o l e d H~O-35 w t p c t NH4C1 a l l o y it w a s observed t h a t a c e r t a i n n u m b e r of NH4C1 c r y s t a l s a t t h e b o t t o m of t h e m e l t r e m a i n e d s o l i d w e l l a b o v e t h e e q u i l i b r i u
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