Segregation of molybdenum atoms to the liquid-solid interfaces in liquid phase sintering of W-8 pct Mo-7 pct Ni-3 pct Fe
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I. INTRODUCTION
GROWTH of W grains during liquid phase sintering of W-Ni-Fe alloys has been studied extensively, wherein a diffusion-controlled mechanism has been assumed.[1,2] Alloying of Mo to W-Ni-Fe alloys results in reduced grain size[3,4] and transition of the controlling mechanism of grain growth from diffusion of W atoms in the liquid phase to interfacial reaction (solution and precipitation) of W atoms.[4,5] The reduction of grain growth rate is explained by the analysis based on competitive dissolution between W and Mo in the matrix.[5] In this analysis, the dissolution of Mo into the liquid phase reduces the concentration of W atoms in the liquid phase, which in turn reduces the kinetic rate constant for a diffusion-controlled process. However, this explanation can only partially account for the reduced grain size, because experimental evidence shows that the concentration of W atoms dissolved in the liquid phase is not effectively reduced by the presence of Mo atoms in the liquid phase. For example, the concentration of W in the liquid phase is about 8 mole pct for W-7 pct Ni-3 pct Fe[6] and 6.6 mole pct for W-8 pct Mo-7 pct Ni-3 pct Fe.[4] The transition of controlling mechanism of grain growth from diffusion of W atoms in the liquid phase to interfacial reaction has been observed based on fitting experimental data with the classical kinetic model of grain growth[5] and the kinetic model of W atoms precipitating from the liquid phase.[4] However, the origin of such a transition has not been well presented. For example, reduction in concentration of W atoms in the liquid phase has been suggested to be the origin of such a transition.[5] Nevertheless, for a process originally controlled by diffusion of W atoms in the liquid phase, the decreased concentration of W atoms in the liquid phase should further reduce the mass transfer rate of W atoms in the liquid phase. As the mass transfer rate of a CHEN-SHENG HSU, Lecturer, and PI-CHUEN TSAI, Associate Professor, are with the Department of Materials Science and Engineering, Huwei Institute of Technology, Yulin 632, Taiwan. SHUN-TIAN LIN, Professor, is with the Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan. Manuscript submitted April 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
bottleneck step is further reduced, the entire mass transfer process should be controlled by the bottleneck step (diffusion of W in the liquid phase) to a greater extent, instead of shifting to a different bottleneck step—the interfacial reaction. Consequently, reduced interfacial reaction rate should be the origin, which not only reduces the grain growth rate but also shifts the bottleneck step from diffusion of W in the liquid phase to interfacial reaction. This study is accordingly carried out to illustrate this concept by focusing on the role of Mo in the sintering of W-Mo-Ni-Fe alloys.
II. EXPERIMENTAL PROCEDURES The heavy alloy investigated in this study was W-8 pct Mo-7 pct Ni-3 pct Fe by weight. Elemental
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