The effect of gravity on solution-reprecipitation during liquid phase sintering
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I. INTRODUCTION
IT is apparent from the literature concerning liquid phase sintering that continuous gravity-aligned gradients in the refractory phase can be found in sintered material. Observations of such gradients have been extensively reported for tungsten-based heavy alloys; [1–5] however, similar gradients have also been documented for other materials systems, including Fe-Cu and Mo-Ni.[6,7] It has been proposed that these microstructural gradients have their origin in the internal pressure gradients that result from the density difference, Dr, between the solid and the liquid phases,[3] particularly for the tungsten heavy alloys where Dr is near 10 g ? cm23 at the sintering temperature. It is unlikely that gravitational pressures alone can cause significant solid state deformation of refractory grains;[8] however, during liquid phase sintering in systems where the solid has some solubility in the liquid, solution-reprecipitation provides an alternative mechanism by which solid phase grains can change their shape and coordination at observable rates. Relatively small pressure differences along the liquidsolid interfaces in these systems cause dissolution of the refractory phase, particularly along regions of contact between solid grains, and redeposition of this refractory material elsewhere along the liquid-solid interface. This process, often referred to as “the heavy alloy mechanism,” results in greater densification than can be achieved solely by the rearrangement of solid (generally spheroidal) grains in the liquid. Microstructures after densification by solutionreprecipitation exhibit significant “grain shape accommodation,” where solid particles are flattened along solid grain contacts, thus achieving higher packing densities than closepacked spheres (Figure 1). As solution-reprecipitation is driven by small pressure differences, the relatively small pressure gradients caused by gravity can result in the formation of gravity-aligned gradients in the concentration of solid grains. In this article, C. SAN MARCHI, Postdoctoral Research Associate, L. FELBERBAUM, Research Engineer, and A. MORTENSEN, Professor, are with the Laboratory of Mechanical Metallurgy, Department of Materials, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland. Manuscript submitted June 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
we provide an analysis of gravitational and capillary forces during solution-reprecipitation to show that although microstructural gradients in composition can theoretically result from this effect, their magnitude is generally small. II. ANALYSIS Several analyses of liquid phase sintering by solutionreprecipitation have been published, beginning with Kingery et al.,[9,10] and followed by more recent contributions.[11–14] These analyses rely on several assumptions; in particular, the dihedral angle, f, is assumed to be zero. Microstructural coarsening always occurs during solution-reprecipitation, and there is some unresolved controversy in the literature concerning the role of so
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