Doping strategy for achieving strong and stable bulk nanocrystalline tungsten alloy at elevated temperatures

The preparation of bulk nanocrystalline (NC) tungsten alloys with excellent thermal stability and ultrahigh strength at elevated temperatures is a challenge. In this study, bulk NC W-3at%Y (W–3Y) with an average grain size of 16 ± 4 nm was successfully prepared via mechanical alloying and high-pressure/high-temperature sintering. The NC W–3Y alloy not only has ultrahigh Vickers hardness of 22.0 ± 0.3 GPa - which is close to the hardness of ceramic tungsten carbide - but also can maintain high hardness of 16.0 ± 0.2 GPa and a small grain size of 37 ± 12 nm after annealing at 1300 °C for 50 h. The compressive strength of NC W–3Y at 900, 1000, and 1100 °C are as high as 1116, 571, and 345 MPa, respectively, much higher than those of previously reported tungsten and tungsten alloys. Doping Y into W increases the GB diffusion activation energy, lowers the GB diffusion coefficient and in turn the GB sliding rate, and thus increasing the strength at elevated temperatures. Partial alloying Y element segregated at grain boundaries (GBs) reduces the GB energy, leading to an excellent thermal stability, as revealed by our experimental observation and theoretical calculation. The utilization of high pressure has direct and indirect effects on lowering the grain growth rate of W–3Y during sintering: directly reducing diffusion coefficient, indirectly enhancing the plastic deformation, thus shortening the atomic diffusion distance required to fill the gaps between powder particles and leading to a lower sintering temperature.