GROWTH OF COMPOSITES FROM THE MELT.
MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF METALLURGY
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Conditions necessary for plane front growth of two-phase solids from a single phase melt are discussed. The general case is considered where, at equilibrium, the alloy solidifies over a range of temperatures i.e., it is not of eutectic composition. It is concluded that 1 plane front solidification is favored by low growth rate, steep thermal gradient, and essential absence of convection and 2 for steady state solidification, the structure should resemble that of directionally solidified eutectics lamellar, rod-like. Factors affecting solute redistribution along the growth direction are described quantitatively, by numerical solutions to the diffusion equation. Effects of variations in growth rate on average composition of the solidifying composite are significant. For alloys not too close to a single phase region, the initial transient at constant growth velocity is the order of DR where D is diffusion coefficient in the liquid and R is interface velocity. Two-phase lead-tin alloys, ranging in composition from 12 atomic per cent to 26 atomic per cent lead, were unidirectionally solidified in a convection-free system, with thermal gradients in the liquid of up to 480Ccm. Plane front solidification was achieved in all alloys at sufficiently steep gradient and slow growth rate. Author
- Properties of Metals and Alloys