Mechanical and tribological performance of AlCr_0.5NbTa_xTi_4–x (x = 0, 0.5, 1) refractory high-entropy alloys

The tribological behavior of AlCr_0.5NbTa_xTi_4–x (x = 0, 0.5, and 1) refractory high-entropy alloys was systematically investigated from room temperature to 800 °C. The relationship between the alloy composition, microstructure, mechanical properties, and tribological performance was analyzed. The results show that all three alloys have a body-centered cubic (bcc) single-phase structure. The Ta addition enhances solid solution strengthening and increases the melting point that would resist high-temperature thermal softening. As a result, high-temperature strength is enhanced considerably, the coefficients of friction (COF) and wear rates of the alloys are reduced considerably at elevated temperatures, demonstrating excellent friction-reducing and anti-wear properties, e.g., the COF and wear rate of AlCr_0.5NbTaTi₃ alloy at 800 °C are 0.34 and 4.40 × 10^–7 mm³ N^–1 m^–1, respectively. The excellent wear resistance at high temperatures can be attributed to the formation of an oxide-tribo layer (Ta₂O₅ and TiO₂) with ultrafine-grained layers and good high-temperature mechanical properties.