Microstructure, Mechanical Properties and Tribological Properties of Al_0.2Co_1.5CrNi_1.5Ti_0.5Mo_x High-Temperature Wear-Resisting High Entropy Alloy

The Al_0.2Co_1.5CrNi_1.5Ti_0.5Mo_x (x=0 to 0.4) high entropy alloy was prepared by vacuum arc melting, and the influence of Mo element on the microstructure, mechanical properties and tribological properties of the high entropy alloy was analyzed. Moreover, the wear mechanism of the high entropy alloy was researched. The Al_0.2Co_1.5CrNi_1.5Ti_0.5 high entropy alloy consists of FCC phase and ordered AlNi₃ phase. The addition of Mo caused the segregation of Cr and Mo, and promotes the formation of hard σ phase through the in-situ reaction of Cr and Mo atoms. With the increase of Mo content, the content of σ phase also increases gradually, and the addition of Mo makes the high entropy alloy produce obvious lattice distortion. The lattice distortion effect induced by the Mo with larger atomic radius and the precipitated phase strengthening effect of the hard σ phase endow the high entropy alloy with excellent mechanical and tribological properties. As the molar content of Mo increased from 0 to 0.4, the hardness of high entropy alloy was increased from 520 HV to 730 HV by 40.4%, and the yield strength increased by 32.1% from 1 060 MPa to 1 400 MPa. However, the hard σ phase is a brittle phase and cannot deform as the FCC phase with better plasticity at the same time, which leads to a significant decrease in the plasticity and toughness of high entropy alloys. The tribological performance of the high entropy alloy at RT~900 ℃ is characterized by ball-on-disc high temperature tribometer. The addition of Mo elements significantly improved the tribological performance of the high entropy alloy, especially when the Mo content was 0.4 mol, the wear rate at RT and 800 ℃ was 2.62×10⁻⁶ mm³/(N·m) and 6.23×10⁻⁷ mm³/(N·m), respectively, which improved by 83.4% and 89.7% compared with that of high entropy alloy without Mo. And the wear resistance is better than Inconel 718 and Stellite 6 alloys. The improvement of wear resistance was attributed to the increased micro-hardness and yield strength of the high entropy alloy at room temperature, and the wear mechanism is abrasive wear and delamination wear. The formation of the glaze layer and good high temperature softening resistance play a key role in the improvement of high temperature friction. A smooth and dense oxidized glaze layer was formed on the worn surface at high temperature, which has obvious anti-wear effects. The thickness of the glaze layer was 2 μm at 800 °C, and the thickness of the glaze layer increased to 5 μm at 900 °C. The glaze layer enhances the high temperature wear resistance of high entropy alloys, and reduces the high temperature friction coefficient. The wear mechanism changes from oxidative wear to adhesive wear. At medium temperature, the hardness of the high entropy alloy decreases to a certain extent. Moreover, the glaze layer formed on the wear surface is incomplete. Under the combined influence of the two, the wear of the high entropy alloy is aggravated and the wear resistance is reduced.