Microstructural evolution of mechanically alloyed and spark plasma sintered Ni-W alloy matrix composites

Microstructural evolution of Ni(W) solid solution alloys fabricated via mechanical alloying (MA) of Ni - 30 wt.% W powders and spark plasma sintering (SPS) at 1000 degrees C for 3 min were presented in this study. Due to contaminations during MA, fabricated alloys are termed as Ni(W) solid solution alloy matrix composites. Effects of modifications on the processing conditions such as covering the graphite dies with hexagonal boron nitride spray and/or annealing of the as-MA'd powders on the microstructure of the SPS'ed Ni(W) alloy matrix composites were investigated via analytical transmission electron microscopy (TEM) studies. X-ray diffraction (XRD) and/or TEM investigations revealed that all the samples comprise Ni(W), WC and WO2 phases. A slightly reduced amount of 13.24 wt.% WC by covering the graphite dies, significantly reduced to 3.90 wt.% with the annealing of the MA'd powders. It is found that covering the graphite dies has led significant grain growth, whereas annealing of the powders had no significant effect on the grain size of SPS'ed samples. A 96.7% relative density value was increased to 97.3% with the covering of graphite dies, which further increased to 99.4% for the sample fabricated via SPS of the annealed powders with covered graphite dies. Although the amount of WC is decreased, a microhardness value of 4.28 GPa for the sample fabricated via SPS of as-MA'd powders, slightly increased to 4.35 GPa for the samples sintered by spray covered graphite dies, which further increased to 4.52 GPa with the applied annealing, revealing the enhancement of the microhardness values by increased W solubility in Ni lattices. (C) 2013 Elsevier B. V. All rights reserved.