Effects of Hf and C on microstructure and mechanical properties of Re0.06HfxTa1.6W0.45(TaC)y refractory medium-entropy alloys sintered by spark plasma sintering

The synergistic optimization of high temperature strength and room temperature plasticity is the core challenge of refractory high-entropy alloys (RHEAs) in practical engineering applications. In this study, four kinds of Re_0.06Hf_xTa_1.6W_0.45(TaC)_y refractory medium-entropy alloys (RMEAs) were prepared by powder metallurgy process. The formation mechanism of oxides and carbides in refractory alloys prepared by powder metallurgy was studied. The effects of Hf and C contents on the microstructure evolution and mechanical properties of the alloys were investigated. The results show that the microstructure of the prepared Re_0.06Hf_xTa_1.6W_0.45(TaC)_y alloys is mainly composed of BCC matrix, carbides and dispersed nano-sized oxides. The carbides can be classified into two types: M₂C and MC. The introduction of Hf element in the form of HfH₂ can avoid the increase of oxygen during the sintering process and form dispersed hafnium oxide, which significantly improves the strength and high-temperature softening resistance of the alloy. Hf content has a key influence on the microstructure of carbides. Higher Hf content can induce the formation of MC carbides and improve the high temperature mechanical properties of the alloy. Due to the strengthening of carbides and dispersed oxides, the four Re_0.06Hf_xTa_1.6W_0.45(TaC)_y alloys exhibit good high temperature strength and outstanding high temperature softening resistance at 1450 °C. The yield strength at 1450 °C is more than 400 MPa, and no strain softening occurs. The compressive fracture strain of the alloy at room temperature is more than 28 %, which has good formability and damage tolerance. The prepared refractory alloy exhibits good high-temperature mechanical properties and room-temperature plasticity, and has application potential in the field of ultra-high temperature structural materials.