Wear and oxidation resistances of AlCrFeNiTi-based high entropy alloys

Based on the AlCoCrFeNi high entropy alloy, by replacing Co with Ti, new cobalt-free high entropy alloys AlCrFeNiTi and its derivative AlCrFeNiTiMn_0.5 were designed and prepared by arc melting, and as-cast microstructures, wear and oxidation behaviors were investigated. The results show that two alloys are all composed of an ordered body centered cubic (BCC) and a disordered BCC phases, and typical dendritic structures including dendritic (DR), interdentritic (ID) and ‘eutectic phase’ areas were observed, implying the feature of near-eutectic high entropy alloys for AlCrFeNiTi and AlCrFeNiTiMn_0.5. Adhesive wear mechanisms for these two alloys are indentified mainly as delamination wear, while oxidative wear is gradually dominated in AlCrFeNiTi. The wear resistance declines with addition of a half mole of Mn into AlCrFeNiTi due to rising wear coefficients of alloys. The oxidation kinetics of AlCrFeNiTi and AlCrFeNiTiMn_0.5 alloys oxidized at 900 °C meet the exponential rate law, and the oxidation rate of AlCrFeNiTi is lower than that of AlCrFeNiTiMn_0.5, suggesting a better oxidation resistance. Comparing with AlCrFeNiTi, a new Mn₂O₃ subscale was formed between the outer TiO₂ scale and the underlying Al₂O₃ subscale on surfaces of AlCrFeNiTiMn_0.5, which is consistent with theoretical analysis from the view of forming ability level by the first principles calculations.