Abnormal growth of faceted (WC) grains in a (Co) liquid matrix
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INTRODUCTION
THE shape of the grains dispersed in liquid matrices in materials prepared, for instance, by liquid-phase sintering is either nearly spherical with smoothly curved surface or faceted with flat planes. These grains coarsen during the heat treatment at temperatures above the melting point of the matrix phase. While those alloys with spherical grains undergo normal Ostwald ripening, those with faceted grains often show abnormal growth. The purpose of this work is to examine the mechanism of the abnormal growth of these faceted grains in liquid matrices, as related to the grain surface structure and the growth mechanisms which have been extensively studied for the crystal growth in melts and solutions. In such metallic systems as W-Ni-Fe, t~,21 W-Ni,[3] MoNi,[4] Co_Cu, tS] and Fe-Cu, t61 but also in some oxides such as ZnO,t7.s] MgO,[ 9] and Zr203, [t~ and in carbides such as VC-Co Illl and Mo2C-Co, t~21 prepared by liquid-phase sintering, the grains in liquid matrices are nearly spherical if the liquid volume fractions are sufficiently high. The spherical grains represent the equilibrium shape, which in turn implies isotropic interfacial energy. This conclusion was confirmed by observing the spherical liquid droplets trapped in the grains, t7,~31The observed spherical shape of liquid droplets trapped in the grains implies that the spherical grains represent the equilibrium shape and that the interracial energy is isotropic. The isotropic surface energy in turn implies an atomiYOUNG JOON PARK, Postdoctoral Candidate, and DUK YONG YOON, Professor, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejeon 305-701, Korea. NONG MOON HWANG is Principal Investigator with the Korea Research Institute of Standards and Science, Taejeon 305-600, Korea. Manuscript submitted February 15, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
cally rough (or diffuse as sometimes referred to) surface. Because the atom attachment or detachment at a diffuse surface is sufficiently rapid to maintain local equilibrium, the grain growth is diffusion controlled, as is indeed verified experimentally in several of these systems,t3,5,6,11,12,14-161 The spherical grain shape is therefore consistent with diffusion-controlled coarsening. As theoretically predicted,t~7-231 the material systems with spherical grains were found to reach the steady state of an invariant with the normalized size distribution,t3,5,6,24JThe geometrical similarity was thus maintained. In particular, no abnormal grain coarsening has been observed in these alloys. Some crystals were also believed to grow from their melts with diffuse surface structures. The growth of tin, cyclohexanol, and succinonitrile crystalst251was observed to occur at finite observable rates, even at very small undercoolings in the order of 10 -3 K, and the growth rates increased linearly with the degree of undercooling.[25-28] In these cases, it was believed that there was little barrier to the atom attachment to the diff
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