Substitution of Ni with NiB prevents shape distortion of liquid phase sintered 90W–6Ni–2Fe–2Co heavy alloys

Abstract

The shape distortion of liquid phase sintered tungsten heavy alloys (WHAs) depends on different factors. Here, the effect of partial and full substitution of Ni with its monoboride (NiB) on distortion characteristics of 90W–6Ni–2Fe–2Co (wt.%) heavy alloy is investigated systematically without changing wt. fraction of W. The phase and microstructural evolution during sintering and effect of microstructural parameters on the distortion is studied. In order to understand the phase and microstructure evolution of the matrix phase of sintered samples, free-standing samples were prepared using the composition of the matrix phase under similar sintering conditions. XRD patterns of sintered sample show characteristic W-peaks along with austenitic matrix phase, whereas, in the case of free-standing samples majority of reflections corresponding to matrix phase (fcc, lattice parameter = 0.360 nm) eliciting a peak shift to slightly higher 2θ values as opposed to those of sintered samples. The FESEM micrographs of free-standing samples reveal microstructure resembling a hypoeutectic system with decreasing inter-lamellar and proeutectic spacing with the extent of NiB substitution. In the sintered 90W–6Ni–2Fe–2Co system, shape distortion resulted in elephant-foot formation (distortion parameter: 22.9). Owing to the partial and full substitution of Ni with NiB, the shape distortion is restricted by formation of uniformly contiguous microstructure, inherent microstructural changes of the matrix phase, increase in dihedral angle, and grain connectivity in partially (90W–3Ni–3NiB–2Fe–2Co) and fully (90W–6NiB–2Fe–2Co) NiB substituted samples, respectively. As a result, the distortion parameter reduces from 22.9 to 9.3 and to 3.7 for partially and fully substituted NiB samples.