| To understand the role of a nickel(Ni) activator during sintering of tungsten(W), the densification behaviors of pure W and Ni-doped W compacts have been observed. The densification of the Ni-doped W compacts occurred rapidly at 1000℃. The W compacts containing 0.2% Ni by weight (more than the solubility limit) were densified up to 90% relative density at 1500℃, while the W compacts containing 0.037% Ni(less than the solubilty limit) were densified to 75% at 1290℃ and stopped being densified above 1290℃. In the plateau of the temperature between 1290℃ and 1800℃, neither grain growth nor densification was observed. The starting temperature of the plateau was lowered when the compacts were sintered longer time at each temperature. Above 1800℃, the 0.037% Ni-doped W compacts started to be densified again and their densification behavior was similar to that of the pure W compacts. The roles of the Ni activator on the densification of the W compacts were interpreted in two different ways. One is to provide a fast diffusion path for W atoms, which can result rapid densification due to rapid polygonization of the elongated W grains. The other is to form an elongated W grain structure formed by the grain boudary migration during alloying of Ni into W phase. The elongated grain structure is considered to be a prerequisite microstructure for subsequent rapid shrinkage. The activation energy of the alloying by grain boundary migration induced by diffusion of Ni in W was estimated to be about 234 kJ/mol; this value is within the range of previously estimated values of activation energy for densification of W-Ni powder compacts. |
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