Low-temperature Spark Plasma Sintering of fine refractory composite powders core-shell: A case of the powders W@Ni
The mechanisms of fast low-temperature Spark Plasma Sintering (SPS) of W + 10% wt. Ni powders were investigated. The powder compositions were obtained in two methods: (i) by mixing W and Ni powders in a specified ratio (hereinafter referred to as W + Ni); (ii) by Ni deposition on the surface of submicron W particles allowing the formation of particles with a core W - shell Ni structure (hereinafter referred to as W@Ni). To reduce the concentrations of oxygen and oxides, the powders were annealed in hydrogen. The solid-phase sintering was performed at various temperatures (1000-1150°C), pressures (40-80 MPa), heating rates (50-500°C/min), and isothermal holding times (0-20 min). The sintering temperatures corresponded to the onset of intense dissolution of W in Ni. The samples had high relative density and small grain sizes. The activation energy of SPS of the mixed powders was close to the one of the grain boundary diffusion. The key mechanism for the compaction of W@Ni particles in the SPS process is Coble creep. The increasing of the sintering temperature was shown to lead to an increase in the solubility of W in Ni and, consequently, to an increase in the number of secondary Ni4W particles formed during cooling down. The grain growth has a minor effect on the mechanical properties of the W alloy compared to the changes in its phase composition.