Microstructural effects on distortion and solid-liquid segregation during liquid phase sintering under microgravity cond
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INTRODUCTION
LIQUID phase sintering is a means for net-shaping components from powders, but its use is primarily limited to compositions with high solid volume fractions due to the poor dimensional stability associated with excess liquid. Although high liquid contents aid densification, they contribute to distortion due to a lack of contacts between the solid particles. However, the solid/liquid ratio needed to avoid distortion is undefined and depends on the alloy system. Common liquid phase sintered systems, such as W-Ni-Fe, exhibit solubility of the solid in the liquid phase and generally display distortion at solid volume fractions around 0.75 to 0.80.[1,2] The microstructures consist of relatively large, rounded grains suspended in a liquid matrix, with the degree of grain contact governed by the dihedral angle. However, combinations of metals with little chemical similarities, such as W and Cu or Mo and Cu, resist distortion for solid volume fractions as low as 0.2.[3] Because of the lower solubility, these systems generally have a much smaller grain size and are characterized by a high solidliquid dihedral angle. The different distortion behavior associated with these systems indicates a relationship to microstructural parameters. Liquid phase sintering is affected by gravity.[1,2,4–10] For systems with density differences between the solid and liquid phases, gravity may cause solid-liquid segregation leading to compositional, microstructural, and property gradients. Studies of W-Ni-Fe alloys have demonstrated that the high density W grains settle to the base of the sample and pack to higher density.[1,2,5,6,8–10] Additional studies have looked at the settling of grains for systems with JOHN L. JOHNSON, formerly Research Associate, Department of Engineering Science and Mechanics, P/M Lab., The Pennsylvania State University, is Senior Development Engineer, Howmet Corporation, Morristown, TN 37814-1402. A. UPADHYAYA, Graduate Research Assistant, and RANDALL M. GERMAN, Brush Chair Professor in Materials, are with the Department of Engineering Science and Mechanics, P/M Lab., The Pennsylvania State University, University Park, PA 168026809. Manuscript submitted April 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
lower density differences between the solid and liquid phases.[7] Increases in grain size, contiguity, and connectivity are associated with the higher solid volume fraction in the settled region.[1,2,5,8,10] Microstructural changes accompanying segregation may affect distortion. For compositions with low volume fractions of liquid, both grain settling and distortion are minimized, indicating a relationship between these phenomena. In general, only these high solid content alloys are processed via liquid phase sintering on Earth. Extension of liquid phase sintering to lower solid volume fractions requires an understanding of the effects of gravity on grain packing within a liquid matrix. The present work analyzes the distortion of W-Ni-Fe alloys processed under microgravity conditions and rel
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