Investigation Tribological Properties of Al-Cr-Nb-Ti-Zr Eutectic Refractory High Entropy Alloy at High Temperature

Recently, a series of new refractory high entropy alloys with eutectic structure have been prepared by researchers through composition regulation, specifically the addition of elements such as Al and Cr. Refractory high entropy alloys produced by this new strategy exhibit superior structural stability and significantly improved resistance to high-temperature oxidation, as compared to their conventional counterparts. Besides, previous studies have reported that the tribological properties of high entropy alloys could be significantly enhanced by incorporating eutectic structures. Based on the above, the design strategy of these novel refractory high entropy alloys may offer a new approach to simultaneously address the problems of inadequate high temperature resistance to oxidation and wear in conventional refractory high entropy alloys. Therefore, it is necessary to further enrich the research on the high temperature wear behavior of these new refractory high entropy alloys. In this work, eutectic refractory high entropy alloy Al_15+xCr₂₀Nb₁₅Ti_40−xZr₁₀ (x = 0,13) were prepared by vacuum arc-melting. The effects of Al and Ti elements content changes on the phase structure, microstructure, microhardness were studied by XRD, SEM, EDS and nanoindentation. The high-temperature tribological properties of the eutectic refractory high entropy alloys Al_15+xCr₂₀Nb₁₅Ti_40−xZr₁₀ were investigated by ball-on-disk machine, 3D optical microscope, XPS and SEM. Based on the SEM and EDS results, these two alloys had the same phase structure and were composed of B2 phase rich in Nb and Ti and C14 Laves phase rich in Cr and Zr. SEM results showed that with the increase of Al content, the as-cast microstructure of the alloy underwent a transition from hypoeutectic (primary B2 phase+B2/Laves eutectic phase, x = 0) to eutectic (B2/Laves eutectic phase, x = 13), accompanied by the increase of the microhardness, the microhardness of Al₁₅Cr₂₀Nb₁₅Ti₄₀Zr₁₀alloy was 558 HV and Al₂₈Cr₂₀Nb₁₅Ti₂₇Zr₁₀alloy was 713 HV. One contributing factor to the increase in microhardness was the significant difference in atomic radius between Al atom and other refractory metal elements in the alloy (Ti, Zr, Nb, Cr), leading to pronounced lattice distortion in both the B2 phase and Laves phase, and a higher proportion of Al atoms in Al₂₈Cr₂₀Nb₁₅Ti₂₇Zr₁₀ resulted in more serious lattice distortion effect. On the other hand, the relatively uniform and complete layered B2/Laves eutectic phase of Al28Ti27 could undergo cooperative plastic deformation under external stress, resulting in higher microhardness. As the wear test temperature increased, the wear mechanism of both alloys gradually changed from abrasive wear to oxidation wear, and the wear rate showed a gradually decreasing trend. Wear tests at different temperatures found that the fully eutectic alloy Al₂₈Cr₂₀Nb₁₅Ti₂₇Zr₁₀ exhibited superior wear resistance. Compared to the tribological properties at different temperatures of the two alloys, the wear rate of Al₂₈Cr₂₀Nb₁₅Ti₂₇Zr₁₀ alloy decreased by 75% at room temperature compared to Al₁₅Cr₂₀Nb₁₅Ti₄₀Zr₁₀. This was attributed to the higher hardness of the fully eutectic alloy, resulting in better load-bearing capacity. When the temperature rised to 600 ℃, the wear rate of Al₂₈Cr₂₀Nb₁₅Ti₂₇Zr₁₀ was (3.408±1.314)×10⁻⁵ mm³/(N·m), which was half of the wear rate of Al₁₅Cr₂₀Nb₁₅Ti₄₀Zr₁₀, indicating that Al₂₈Cr₂₀Nb₁₅Ti₂₇Zr₁₀ had better wear resistance in a wide temperature range below 600 ℃. At high temperatures, Al₂₈Cr₂₀Nb₁₅Ti₂₇Zr₁₀ possessed higher proportion of Al₂O₃ in the oxidized tribo-layer at high temperatures that resulted in a denser tribo-layer, which was conducive to improving the wear resistance of the alloy.