Gravity and configurational energy induced microstructural changes in liquid phase sintering
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INTRODUCTION
LIQUID phase sintering is routinely used in the fabrication of high performance materials. Example applications include dental amalgams, cemented carbides, magnets, electrical contacts, automotive components, covalent ceramics, intermetallics, and high density alloys (W-Ni-Fe). fu The liquid gives faster sintering rates than is possible by solid state techniques. Also, novel materials are possible, as is evident with the tungsten heavy alloys. These alloys have up to 98 wt pct tungsten, with nickel and iron commonly in a 7:3 weight ratio. The sintered alloys are two-phase composites consisting of interdispersed tungsten and matrix structures. In spite of the widespread applications, there are problems with compact slumping and distortion during liquid phase sintering, t2-61 Large dimensional changes take place during densification: often as large as 20 pct linear shrinkage. Accordingly, supports for the compacts are not practical except for simple shapes, since the compact is a viscous mass. Friction with a support will cause distortion or rupture. Also, in systems like the heavy alloys there is a substantial density difference between the liquid and solid phases. Such a density difference leads to solid migration and nonuniform sintered properties due to inhomogeneities in the microstructure. The microstructure obtained during liquid phase sintering depends on several processing and material parameters, tl'71 The grain size and number of solid-solid contacts are important to the grain coarsening rate, compact rigidity, densification, and final compact properties. When the solid-liquid dihedral angle is greater than zero, grain boundaries form between contacting grains. New contacts between grains are induced by gravitational segregation. Coalescence at the contacts has been proposed as a mecha-
C.M. KIPPHUT and S. FAROOQ, Graduate Research Assistants, A. BOSE, Postdoctoral Assistant, and R.M. GERMAN, Professor, are with Rensselaer Polytechnic Institute, Troy, NY 12180-3590. This paper is based on a presentation made in the symposium "Experimental Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee. METALLURGICALTRANSACTIONS A
nism of grain growth during liquid phase sintering, ts-121 Gravitational segregation will possibly alter the local grain growth rate since coarsening varies with the solid content; regions of higher solid concentration c o a r s e n faster. [13'14'151 Liquid phase sintering for systems with a large density difference between the liquid and solid will exhibit a gradient in the microstructure aligned with the gravitational force. Theoretically, the effect of a high solid volume fraction is to accelerate the rate of grain growth because the diffusion distance decreases as the solids content increases./14A6] An increase in the volume fraction of solid creates more contact between grains, in
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