Impact of mechanical activation on sintering kinetics and mechanical properties of ultrafine-grained 95W-Ni-Fe tungsten heavy alloys

This paper is a study of sintering mechanisms, structure, and mechanical properties of ultrafine-grained 95W-Ni-Fe tungsten heavy alloys. Powder particle sizes were controlled by mechanical activation (MA) of original coarse-grained components and by addition of ultrafine particles. W-Ni-Fe alloys were obtained by sintering in hydrogen and Spark Plasma Sintering (SPS) in a vacuum. The dependence of ultrafine-grained (UFG) alloy density on sintering temperatures has been found to be non-monotonic with a maximum corresponding to the optimal sintering temperature. It has been demonstrated that the sintering activation energy of UFG alloys is significantly lower than that of coarse-grained alloys. It has been shown that the optimal SPS temperature for mechanically activated nanopowders goes down by 350-400 °C in comparison with the optimal sintering temperature in hydrogen for coarse-grained 95W-Ni-Fe powder composition. The reason for a lower optimal sintering temperature lies in a decreased activation energy of grain-boundary diffusion and formation of a non-equilibrium solid solution of nickel and iron in the surface layer of tungsten α-W particles during high-energy MA. High-energy MA and SPS were used to obtain samples of UFG tungsten alloys with high mechanical properties: macro-elastic limit - up to 2250 MPa, yield stress - up to 2500 MPa.