The use of controlled solidifcation and melting experiments to determine liquidus and solidus boundaries
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AND
E. D.
GIBSON
Theoretical steady-state solute profiles at melting and solidifying interfaces predict that the solid-liquid interface temperature should correspond to the liquidus temperature upon m e l t ing and the solidus temperature upon freezing. Experiments have been c a r r i e d out with SnBi and Sn-Sb alloys which show that it is experimentally possible to achieve the predicted steady-state profiles on melting and freezing in concentrated alloys. A technique was d e v e l oped to m e a s u r e solid-liquid interface temperatures and the solidus and liquidus p h a s e boundaries were determined in t h e s e two systems. The results a r e compared t o literature values.
IN
controlled solidification and melting experiments on m e t a l alloys at slow growth r a t e s w i t h a flat solidliquid interface a l o c a l equilibrium is essentially obtained at the interface. Consequently, from a knowledge of the phase d i a g r a m and from interface tempera t u r e measurements one should be able to determine interface compositions. Alternately, if one is able to control the interface composition in a known m a n n e r and then m e a s u r e the interface temperature, it should be possible t o determine the liquidus and solidus phase boundaries of alloys. If a long rod of binary alloy of composition Co is solidified u n d e r steady-state conditions, one may obtain the composition profile shown in Fig. l ( a ) .1'2 In o r d e r to experimentally achieve this composition profile the following four conditions must be obtained. 1) The solid-liquid interface must be planar. 2) The alloy composition, Co, must initially be constant throughout. 3) The rate of solidification must be constant and a sufficient length of alloy must have solidified in o r d e r to be beyond the initial transient. 4) Liquid convection must be sufficiently suppressed to maintain an effective distribution coefficient of one for the conditions of the experiment. Under these conditions the interface temperature will correspond to the solidus temperature at composition Co. Similarly, if one m e l t s a long rod of an alloy, a steady-state composition profile as shown in Fig. l(b) will be obtained if conditions 1 through 3 above are realized. 3 In this steady-state melting case the interface temperature will correspond t o the liquidus temperature at composition Co. The p u r p o s e of this work has been t o determine if it is experimentally feasible t o achieve t h e s e steady-state solute profiles in concentrated alloys and at the same time accurately m e a s u r e the interface temperature.
and melted by r a i s i n g and lowering the cooling cylinder and furnace in tandem. The objectives of the heat t r a n s f e r a r r a n g e m e n t were to obtain very high t e m p e r a t u r e gradients in o r d e r t o avoid interface instability, and to obtain l i n e a r heat flow in o r d e r to maintain a flat interface and eliminate convection near the i n t e r face due to horizontal temperature gradients. Furnace temperature control was wi
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