Macrosegregation and sedimentation in liquid-phase sintering
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SEDIMENTATION is often observed in liquid-phasesintered structures containing moderate-to-low solid-phase volume fractions.[1–5] When such structures contain a solid phase having density greater than the liquid phase, sedimentation is first manifested by the formation of a liquid head above the solid-plus-liquid region (the mushy zone). With increasing sintering time, the length of the liquid head increases. That is, sedimentation of the mushy zone apparently takes place. Two explanations have been put forth to explain this sedimentation. One proposes that the mushy zone creeps under its own weight (compensated by the buoyancy provided by the liquid phase).[1] A strong argument against this explanation is that the stress due to the sample weight is much too small to account for observed sedimentation rates.[2] A second explanation came from work in our laboratory.[3] We argued that sedimentation might be linked to solid particle coarsening in the mushy zone. That is, as a result of coarsening dynamics, a particle might occasionally “work itself free” from the solid skeleton, and, if it did, it would settle a distance comparable to the interparticle separation distance before reattaching itself to the skeleton. Repetition of the process leads to the observed slow sedimentation rates. Experimental evidence to support this view came from experiments in which a mushy zone skeleton was placed in vertical contact with a liquid having the same composition as the liquid in the zone. It was found that particles extricated themselves from the skeleton and floated (or sank) within the contacting liquid. However, this “extrication” mechanism also has shortcomings. In particular, the observed extrication takes place at the skeleton surface. Extrication in the sample interior is more difficult because of the greater number of interparticle contacts there. We now believe that much of this observed sedimentation Y. DU, Research Assistant, S.Z. LU, Research Associate Professor, and T.H. COURTNEY, Professor, are with the Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted October 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
is a result of long-range concentration gradients in the liquid head that are established as a result of initial particle settling and the subsequent elimination of these gradients. This has been discussed conceptually in a previous article.[4] The present article supplies experimental evidence supporting this view. Since many of the results described here devolve on an appreciation of how long-range liquid concentration gradients are established and on how their elimination results in sedimentation, it is worthwhile to review briefly the main points of the previous paper. That paper was couched in terms of an isomorphous binary system in which component A melts at a lesser temperature and has a lesser density than component B. A mixture of A and B is heated above A’s melting temperature. If the volume fraction of B is insufficient
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