Liquid phase sintering of tungsten-nickel-cobalt heavy alloys

Dinçer, Onur
The aim of this thesis study was to investigate the effects of composition and sintering variables on the microstructural characteristics of W-rich W-Ni-Co heavy alloys. W-Ni-Co alloys containing 90 to 97 wt% W and different Ni/Co ratios were investigated in the study. The alloys studied were produced by applying conventional powder metallurgy methods and subjected to microstructural and mechanical characterization by using suitable techniques. All of the alloys investigated reached almost full density when sintered at and above 1475 °C, except Co-rich alloys. Co-rich alloys reached full density only when sintered at 1600 °C and could not be densified by liquid phase sintering (LPS) due to the lack of the liquid phase. The microstructures of the Ni-rich alloys were found to consist of rounded, nearly pure W grains and a Ni-Co-W binder matrix phase. A microstructure consisting of nearly pure W grains and a two-phase binder matrix was observed in all of the alloys with Ni/Co ratio 1/1. X-ray diffraction and energy dispersive spectroscopy (EDS) analysis have shown that one phase of the binder matrix of Ni/Co ratio 1/1 alloys was Ni-containing Co7W6 and the other phase was a Ni-Co-W solid solution. The binder matrix phase in the Ni-rich alloys was observed to dissolve relatively large amounts of W (up to 43 wt %). EDS line scan analysis showed that W was dissolved in the binder matrix homogenously. For these alloys, in general, the areal percentage of the binder matrix phase was determined to increase with increasing sintering temperature and decreasing alloy W content. In Ni-rich alloys, the average W grain size was found to increase with increasing sintering temperature and alloy W content, when sintered above 1525 °C. Hardness measurements have revealed that the hardness of the alloys with Ni/Co ratio 1/1 was greater than all of the remaining alloys. This beneficial behavior was attributed to the presence of a two-phase binder matrix in these alloys.
Citation Formats
O. Dinçer, “Liquid phase sintering of tungsten-nickel-cobalt heavy alloys,” M.S. - Master of Science, Middle East Technical University, 2014.