Supercooling effects in Cu-10 Wt Pct Co alloys solidified at different cooling rates
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I.
INTRODUCTION
T H E cooling rate is a parameter often used to describe the as-cast microstructure in foundry practice. However, the solidification rate and temperature gradients across the solidifying interface are the parameters that actually describe the local solute diffusion, interfacial morphology, and kinetic processes. High solidification rates can be achieved without a high rate of external heat extraction in alloys which are supercooled a large extent. For supercooled liquids, since the temperature gradient in the liquid ahead of the solidification front is negative, the latent heat of fusion can be absorbed by the supercooled liquid faster than it can be transferred to the surroundings, and the solidification process occurs with high interfacial velocities. Interfacial velocities larger than 20 m / s are reported for solidification under high supercooling conditions, t~l In addition to high solidification rates, supercooling may cause diverse modes of solidification such as a change in the solidification morphology from dendritic to nondendritic, ~2,3.4j the formation of alternate phases, [5) a refinement in the grain size, t2"6-s] partitionless (massive) solidification in which the solid has the same composition as the parent liquid, 19"[~ and/or the formation of metallic glasses, tt21 There are several ways to achieve high bulk supercoolings in a melt, such as emulsification techniques, t'] melting bulk alloys in fused silica crucibles or under the cover of slag glass, i~4~and electromagnetic levitation, t2.91 Electromagnetic levitation is a technique in which bulk alloys may be cyclically melted, supercooled, and subsequently solidified. It has the advantage of reducing contamination and heterogeneous nucleation as well as allowing for better melt homogeneity and precise temperature, time, and atmospheric control. Another important effect of melt supercooling which has not been studied extensively in the past is to cause fiaetastable melt separation in some alloy systems. Whether A. MUNITZ is with the Nuclear Research Centre, Beer-Sheva, Israel. S.P. E L D E R - R A N D A L L is with Gould Lewis and Proctor Consulting Engineers, Gainesville, FL 32611. R. A B B A S C H I A N , Chairman and Professor, is with the Department of Materials Science and Engineering, University of Florida. Gainesville, FL 3261 I. Manuscript submitted June 13. 1990. METALLURGICAL TRANSACTIONS A
or not melt separation will occur depends not only on the alloy composition and the level of supercooling, but also on the solution thermodynamics of the system. For example, systems such as Cu-Co and Cu-Fe exhibit a definite thermodynamic tendency for liquid immiscibility upon supercooling as evidenced by nearly flat liquidus curves and the positive deviation of their activities from a Raoultian behavior. For the Cu-Fe system, recent experiments by the present authors []5} have determined the phase boundaries of the metastable miscibility gap and examined the process of phase separation and subsequent solidification of the heteroge
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