The effect of Mo addition on the liquid-phase sintering of W heavy alloy
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I.
INTRODUCTION
IN the early stage of liquid-phase sintering of a powder mixture, the solid grains and the liquid matrix are not in chemical equilibrium. But as the solid grains grow by Ostwald ripening to an average size larger than the powder with the highest melting point in the initial mixture, the solid and the liquid will be in chemical equilibrium except for the small size effect. In many alloys, such as the W-NiFe heavy alloy, the equilibrium shape of the grains is spherical because of an isotropic interfacial energy between the solid and liquid. If the volume fraction of the liquid matrix is sufficiently high, the grains are indeed nearly spherical,t141 but with low liquid volume fractions the grains accommodate their shape to their neighbors and become anhedral.[1. 3] When the grains are spherical, their Ostwald ripening is diffusion controlled, as verified experimentally,t1-7] because the isotropic interfacial energy implies an atomically rough (or diffuse) interface and hence rapid interfacial reaction. Even during Ostwald ripening the grains have nearly the equilibrium shape because the mass transport between the surface segments of the same grain is faster than between the neighboring grains. The grains undergoing Ostwald ripening maintain geometrical self-similarity with constant
HEE-DONG PARK, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is with the Steel Products and Welding Research Team, Technical Research Laboratory, Pohang Iron and Steel Company, Kyungbuk 790-785, Korea. WOON-HYUNG BAIK, Principal Researcher, is with the Advanced Technology and Research Center, Agency for Defense Development, Daejon 305-600, Korea. SUK-JOONG L. KANG and DUK-YONG YOON, Professors, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejon 305-701, Korea. Manuscript submitted October 4, 1994. 3120~VOLUME 27A, OCTOBER 1996
normalized size distributions 11,3,81 as predicted theoretically.[9.1o.~i] It has been suggested that the coalescence of grains in contact can contribute to the overall coarseningY 2-z~ At low liquid volume fractions, the neighboring grains will be separated by thin liquid layers if the energy 7g of the grain boundary is larger than twice the solid-liquid interfacial energy %~. If, on the other hand, the grains in contact have either a low misorientation angle or a high coincidence orientation, they will form a grain boundary, since yg can be smaller than 2%~. In the W-Ni-Fe alloy and the model MoNi alloy,12~.22] most of the grains were separated by liquid films, but some grain pairs formed grain boundaries. In FeCur] and Co-Cu,[2] grain boundaries appeared more frequently. But even in these alloys, the dihedral angles at the junctions between the grain boundaries and the liquid matrix are so small that the movement of the grain boundaries is likely to be controlled by the movement of the junctions, as proposed by Kim and Yoon.t231 The junction move
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