Grain Growth in Dilute Tungsten Heavy Alloys during Liquid-Phase Sintering under Microgravity Conditions
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INTRODUCTION
NET-SHAPE components can be rapidly densified from powders via liquid-phase sintering, because the liquid provides both capillary forces and a high diffusion rate. However, the high diffusivity leads to microstructural coarsening, while the excess liquid results in a loss of dimensional stability and can lead to solid-liquid separation. These phenomena narrow the range of compositions that can be liquid-phase sintered and compared to theoretical models. Common liquid-phase-sintered systems, such as W-Ni-Fe heavy alloys, generally display distortion at solid-volume fractions of approximately 0.75 to 0.80.[1–3] At this volume fraction, the component loses shape retention due to the lack of solid-solid contacts sufficient for forming a skeletal structure. Microstructures typically consist of relatively large, rounded grains suspended in a liquid matrix, with the degree of grain contact governed by the solid-volume fraction and dihedral angle. In the case of tungsten heavy alloys, a 9-g/cm3 density difference exists between the solid grains and the surrounding liquid. Gravity causes these grains to settle to the base of the sample, resulting in microstructural gradients.[1–8] Dilute tungsten heavy alloys produce a separated liquid layer at the top of the samples. JOHN L. JOHNSON, R&D Director, is with ATI Engineered Products, Huntsville, AL 35806. Contact e-mail: jjohnson@ATIEP. com LOUIS G. CAMPBELL, R&D Engineer, is with the Eaton Corporation, VI Technology, Horseheads, NY 14845. SEONG JIN PARK, Associate Research Professor, is with the Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759. RANDALL M. GERMAN, Associate Dean, is with the College of Engineering, San Diego State University, San Diego, CA 92182-1326. Manuscript submitted May 17, 2008. Article published online January 7, 2009 426—VOLUME 40A, FEBRUARY 2009
Variations in the solid-volume fraction affect grain coarsening. At low solid-volume fractions, coarsening is predicted to occur primarily via Ostwald ripening.[9] In Ostwald ripening, small grains dissolve, diffuse through the liquid, and reprecipitate on larger grains. The process is driven by free energy changes associated with the different grain sizes. Early models for grain coarsening were extrapolations of the classic Lifshitz– Slyozov–Wagner (LSW) theory[10,11] developed for dilute solid contents. Higher solid-volume fractions enhance the contribution of coalescence to grain growth.[8,12–20] In coalescence, contacting grains fuse by either grainboundary migration or grain rotation, or because the contact occurs without grain misorientation. The high solid-volume fractions typical for liquid-phase sintering give much higher grain-growth rates and broader grain size distributions than are predicted by Ostwald ripening, indicating that grain coarsening involves grain coalescence. Experimental observations of many liquid-phase-sintered systems identified a 2/3 power dependence of grain coarsening on the liquid-volume fraction.[8] Later, this 2/3 power dependen
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