Kinetics of grain coarsening during sintering of Co-Cu and Fe-Cu Alloys with low liquid contents
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INTRODUCTION
IN some alloy systems consisting of spherical grains dispersed in liquid matrices, the grain coarsening behavior is found to agree with the theoretical predictions based on a solution-reprecipitation process (Ostwald ripening) controlled by solute diffusion through the matrix. 1,2 When the liquid matrix content is l o w - - as often found in metallic and ceramic systems prepared by liquid phase sintering--the grains become anhedral, accommodating their shape to the neighboring grains. 3 Also in these systems, the grain coarsening behavior-- as determined by the form of the rate law and variations in the growth rate with liquid content and temperature--is found to agree in general with diffusion controlled Ostwald ripening. 4-7 Attempts have been made to describe the grain coarsening of the system with small matrix content by assuming a liquid layer around each grain with its thickness proportional to the average grain size. 4'8 But the nature of the liquid layer is ambiguous, particularly when grain boundaries--with dihedral angles greater than zero--are formed between the grains. 'Coalescence' has been proposed 6'9-~6as a mechanism for grain coarsening in liquid matrices. The suggestion by Whalen and Humenik 12 is based on obviously false interpretation of the grain size distribution. The coalescence models ~3 where the coarsening rate is determined by grain encounters cannot apply to systems with small liquid content, since all grains are in contact with each other. Courtney's treatment u'~5'~6 based on fusion (sintering through the liquid matrix) of particles may be also inadequate because the existence of dihedral angles and the difference in grain sizes are ignored. Experimental evidences for coalescence mechanism are also inadequate. On the other hand, Buist and his colleagues7 have suggested that movement of the boundary between the grains in SU SOK KANG, formerly a student in the Department of Materials Science at the Korea Advanced Institute of Science and Technology, is now with the Hongneung Machinery Company, Daejeon, Korea. DUK N. YOON is a Professor in the Department of Materials Science at the Korea Advanced Institute of Science and Technology, P. O. Box 150 Chongyangni, Seoul, Korea. Manuscript submitted April 20, 1981. METALLURGICAL TRANSACTIONS A
contact might be determined by the dissolution at the surface of the smaller grains in contact with the liquid and deposition on the larger. Jones t7 has proposed essentially the same mechanism for the lateral migration of grain boundary grooves at the solid-liquid interface during directional solidification, and Kingery and Franqois TM have also proposed a similar mechanism for grain growth in porous compacts. These models are similar to that of normal Ostwald ripening in that the rates are determined by the solution and reprecipitation which arise from the difference in the curvatures of the grain surfaces which are in contact with the matrix. In this work, the mechanism of grain coarsening is investigated as a function of liquid cont
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