Effects of Hf and C on microstructure and mechanical properties of Re_0.1Hf_xTa_1.6W_0.4(TaC)_y refractory medium-entropy alloy

The trade-off between high-temperature strength and room-temperature ductility represents a critical concern for practical application of refractory high-entropy alloys (RHEAs). Here, a set of five Re_0.1Hf_xTa_1.6W_0.4(TaC)_y refractory medium-entropy alloys (RMEAs) were fabricated using the vacuum arc melting technique, and the effect of Hf and C on the microstructures and mechanical properties of these alloys was investigated. These RMEAs exhibit excellent high-temperature strength and favorable plasticity at room-temperature. The microstructures of the Re_0.1Hf_xTa_1.6W_0.4(TaC)_y alloys are hypoeutectic or eutectic structures, consisting of BCC solid solution and primary and secondary carbides. The carbides can be classified into two types: M₂C and MC. Hf and C have an important influence on the grain size, carbide type and spatial configuration of the alloy, and in turn, influence the mechanical properties of the alloy. For instance, the content of MC-type carbides is proportional to the addition of Hf element, whereby a higher proportion of Hf results in reduced plasticity at room-temperature and enhanced softening resistance at elevated temperatures in the alloy. The carbides and BCC matrix exhibit synergistic strengthening behavior. The ultimate compressive strength of the five Re_0.1Hf_xTa_1.6W_0.4(TaC)_y alloys at 1450 °C is >950 MPa, while maintaining a fracture strain exceeding 11% at room-temperature. Notably, the Re_0.1Hf_0.1Ta_1.6W_0.4(TaC)_0.25 alloy demonstrates a compressive strength of 1110 MPa at 1450 °C, which is 2.3 times greater than that of the NbMoTaW alloy at 1400 °C. Additionally, the fracture strain of the Re_0.1Hf_0.1Ta_1.6W_0.4(TaC)_0.25 alloy at room-temperature is 16.9%, which is 8 times higher than that of the NbMoTaW alloy. The Re_0.1Hf_xTa_1.6W_0.4(TaC)_y alloy exhibit excellent mechanical properties at both room and high temperatures, making it a potential candidate for applications in fields that require high strength at ultra-high temperatures.