Effect of Ti/Ta ratio on the microstructure and mechanical properties of Hf₂₀Mo₁₅Nb₂₅Ta_20-xTi_20+x refractory high-entropy alloys

The limited deformation capacity at room temperature remains a significant challenge in refractory high-entropy alloys (RHEAs). In this study, the fine-grained Hf₂₀Mo₁₅Nb₂₅Ta_20-xTi_20+x (x = 0, 5, 10, 15) alloys were prepared by mechanical alloying and subsequent spark plasma sintering (SPS), based on precise component regulation. The impact of altering the Ti/Ta ratio (R, R = (20+x)/(20-x)) on the pre-alloyed powders and the as-sintered alloys were investigated. An increase in R-value led to larger powder particle sizes and lower powder yields, which was attributed to the improvement in the plasticity of the pre-alloyed powders. After SPS sintering, the as-sintered alloys were comprised of two BCC solid solution matrices and nanoscale precipitated phases. As the R-value increases, the stability of the microstructure decreases slightly, while the yield strength shows a slight improvement, and the plastic strain experiences a relatively significant enhancement. The as-sintered Hf₂₀Mo₁₅Nb₂₅Ta_20-xTi_20+x alloys all exhibited an excellent balance of strength and plasticity. Theoretical calculations revealed that the yield strength of the alloys was the result of the combined effect of several strengthening mechanisms, predominantly substitutional solid solution strengthening. Furthermore, a systematic discussion was conducted on the reasons for the improvement in the plasticity of the as-sintered alloys. This study presents an effective approach to enhancing the room temperature deformation capacity of RHEAs, thereby facilitating their broader application.