The determining role of Al addition on tribology properties and oxidation behavior at elevated temperatures of TiZrHfNb refractory high-entropy alloy

This study developed a series of Al_x(TiZrHfNb)_100-x (x = 0, 3, 5, 7, 12 at. %) refractory high-entropy alloys (RHEAs) with an aim to improve the poor oxidation and wear resistance at elevated temperature of the base alloy TiZrHfNb. These alloys were prepared by arc melting, and the mechanical properties, oxidation and wear resistance at elevated temperatures were studied systematically. The results indicate that all the alloys are single solid solution phase with body-centered cubic (BCC) structure. The addition of Al significantly improves the strength of the RHEAs due to the large solid solution strengthening effect, and the yield strength and Al content obey the liner relation. The oxidation and wear test results indicate that the Al element will preferentially react with O atoms and accumulate on the surface to form a dense oxide layer, which prevents other elements reacting with O atoms and makes the alloy have better oxidation resistance at high temperatures, especially in alloys with 12% Al. The friction coefficient and wear rate of the RHEAs dramatically decrease with the increase of Al, which can be attributed to the combined effect of mechanical properties and oxidation resistance. This study provides an effective method for improving the oxidation resistance and wear resistance of RHEAs in a relatively high temperature range.