Effect of Si additions on microstructure and mechanical properties of refractory NbTaWMo high-entropy alloys

To improve the high-temperature strength and decrease the density of NbTaWMo alloys, addition of light element Si producing the second-phase silicide is employed. Refractory NbTaWMoSi_x (x = 0, 0.25, 0.5, 0.75) high-entropy alloys are produced by spark plasma sintering. The phase evolution, microstructure, compressive mechanical properties, and high-temperature hardness are investigated in this study. It reveals that there is only a disordered body-centered cubic (BCC) phase in the matrix NbTaWMo alloy. After adding the Si element, NbTaWMoSi_x alloys demonstrate the presence of multiphase structure: disordered BCC phase and silicide phase. As the content of Si is increased, the proportion of silicide is increased, which improves the hardness and strength. The addition of Si at certain concentration (x = 0.25 and 0.5) has positive effect on ductility of the alloys. The alloys all demonstrate brittle fracture due to the brittle phase of BCC and silicide phase. The high-temperature hardness of NbTaWMoSi_x alloys is increased with the addition of Si, but the same alloy presents a slightly decreasing phenomenon as the temperature improves.