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The effect of powder processing on the microstructure and sinterability of the heavy alloy W-Ni-Cu was investigated. The heavy alloy powders were prepared by the ball milling and hydrogen reduction of metal oxide powders. As the milling time increased, the size of the powder mixture decreased and at 5 h of milling was found to be about 2.5 μm. Microstructural analysis revealed that the powder mixture was changed to W and NiCu alloys with an average particle size of about 200 nm after hydrogen reduction at 800°C for 2 h. The reduction kinetics of the oxide powder mixture was evaluated by the amount of peak shift with heating rates using TGA in a N2-10% H2 atmosphere. The activation energy of the reduction reaction, calculated from the slope of the Kissinger plot, was measured to be 42.8 kJ/mol for CuO, 57.9 kJ/mol for NiO, and 50.1~112.6 for kJ/mol WO3. The relative densities of the heavy alloy sintered at 1100oC and 1200oC using oxide powder were 81.4% and 96.0%, while the specimen using metal powder as a raw material showed a relatively low value of 67% and an inhomogeneous microstructure. It was explained that the changes in sintered microstructure with different powder synthesis methods are mainly due to the powder characteristics, such as the size of the particles of the initial mixed powder.
Heo et al. (Thu,) studied this question.