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 degrees 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 ci-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. (C) 2018 Elsevier B.V. All rights reserved.

The paper dwells on the research conducted into sintering mechanisms, the structure and mechanical properties of ultrafine-grained heavy tungsten W-Ni-Fe alloys. The dependence of alloy density on temperature of sintering (T-sint) is found to be nonmonotonic with a maximum equivalent to the optimal sintering temperature. Studies also encompassed the impact that the size of tungsten particles may have on the optimal Tsint. An increase in time of mechanical activation (MA) and acceleration of grinding bodies accompanied by a decrease in alloy particle size and formation of non-equilibrium solid solutions is shown to reduce the optimal Tsint of alloys. High-energy MA and Spark Plasma Sintering methods were applied to obtain samples of tungsten alloys with high mechanical properties: macroelasticity limit of up to 2,250 MPa, yield strength of up to 2,500 MPa.

The mechanisms for the sintering of ultrafine-grained 95%W–3.5%Ni–1.5%Fe heavy tungsten alloy powders have been investigated. It has been established that a decrease in the activation energy of grain boundary diffusion and the formation of a nonequilibrium solid solution of nickel and iron in the surface layer of tungsten particles upon high-energy ball milling are responsible for the decrease in optimal sintering temperature.

The structure and mechanical properties of nano- and ultradispersed mechanically activated heavy W-Ni-Fe and W-Ni-Fe-Co tungsten alloys (VNZh and VNZhK alloys, respectively) are studied. Mechanically activated nano- and ultradispersed charge powders are sintered by free sintering (thermally activated) and spark plasma sintering. The dependence of the density of the alloys made of the mechanically activated powders on the sintering temperature is found to have a nonmonotonic character with a maximum corresponding to the optimum sintering temperature. It is shown that an increase in the mechanical activation time and the acceleration of the milling bodies during mechanical activation lead to a decrease in the alloy particle size and the formation of nonequilibrium solid solutions and are accompanied by a decrease in the optimum sintering temperature of heavy tungsten alloys. Ultrahigh-strength tungsten alloys the mechanical properties of which are substantially higher than those of standard coarse-grained analogs are fabricated due to the optimization of the conditions of ball milling and high-rate spark plasma sintering of W-Ni-Fe powders.

Mechanisms of sintering and the structure and mechanical properties of nano- and ultradispersed W-Ni-Fe (WNF) and W-Ni-Fe-Co (WNFC) heavy tungsten alloys are investigated. The effect of tungsten particle sizes on the optimal sintering temperature is studied. The size of particles has been changed by the mechanical activation (MA) of the source W-Ni-Fe coarse-grained (CG) charge and by adding ultradispersed particles obtained using plasmochemical synthesis. Nanodispersed powders and ultradispersed powders (UDPs) have been sintered using the techniques of free sintering and pulse plasma sintering (PPS). It has been revealed that the dependence of the alloy density on heating temperature is nonmonotonic, with the maximum corresponding to the optimum sintering temperature. It has been shown that an increase in the time of MA and acceleration of grinding bodies in the process of MA accompanied by a decrease in the size of alloy particles and formation of nonequilibrium solid solutions lead to a reduction in the optimal sintering temperature. It has been shown that, using planetary high-energy milling methods and high-rate spark plasma sintering, it is possible to obtain ultrastrong tungsten alloys whose mechanical properties (macroelasticity stress and yield stress) substantially exceed analogous properties of commercial alloys.

Sintering mechanisms, structure and mechanical properties of nano and superdispersed tungsten pseudoalloys W-Ni-Fe (WNF) and W-Ni-Fe-Co (WNFC) have been investigated. The influence of tungsten particle size on the optimum sintering temperature has been studied. The particle size has been changed by mechanical activation of the original coarse-grained blend and by adding superdispersed particles obtained by sol-gel method and plasmochemical synthesis. Free sintering and spark plasma sintering of nano and superdispersed powders have been used. The dependence of pseudoalloy density on heating temperature has been found to be nonmonotonic with the maximum corresponding to the optimum sintering temperature. It has been shown that an increase in mechanical activation time and the strain rate, accompanied by pseudoalloy particle reduction and formation of nonequilibrium solid solutions, leads to the reduction of the optimum sintering temperature. It has also been shown that using planetary highenergy milling methods and high-rate spark plasma sintering it is possible to create superstrong tungsten alloys whose mechanical properties (macroelastic stress, yield stress) exceed substantially similar properties of commercial alloys.

We have studied the structure and mechanical properties of nanodisperse tungsten-based heavy alloys of the W-Ni-Fe system. The temperature dependence of the density of compacted alloys exhibits a nonmonotonic character with a maximum that corresponds to the optimum temperature of sintering. The effect of the regime of solid-state pulsed spark plasma sintering (SPS) on the structure and mechanical properties of mechanically activated W-Ni-Fe heavy alloys has been studied. It is established that, using preliminary mechanical activation in a planetary ball mill and the subsequent high-rate SPS, it is possible to obtain superhard tungsten-based heavy alloys with mechanical properties that substantially exceed those of the analogous standard alloys.