Enhancing oxidation resistance via grain boundary engineering in L12-strengthened medium entropy alloys

The concept of grain boundary engineering (GBE) has been successfully applied to L1₂-strengthened (CoCrNi)₉₄Al₃Ti₃ medium entropy alloy, with the aim of improving the oxidation resistance by increasing the ratio of special boundaries and suppressing discontinuous precipitation. Surprisingly, our results reveal that GBE treatment not only slows down the oxidation kinetics and but also alters the oxide scale from TiO₂ and multi-defect Cr₂O₃ to continuous and protective Cr₂O₃ and Al₂O₃, thereby contributing to an enhanced oxidation and anti-spalling resistance. The GBE treatment reduces the oxidation weight gain of the current alloy from 1.950 mg cm^–2 to 1.211 mg cm^–2 after 100 h of cyclic oxidation at 800 °C. The findings show that the extensive outward diffusion of Ti accelerates ion transport and promotes microporosity, thus leading to more defects being formed in the oxide film. The GBE treatment suppresses the discontinuous precipitation of the Ti-bearing L1₂ phase and breaks the random large angular grain boundaries network, inhibiting the diffusion of Ti and ultimately enhancing the oxidation properties of the alloy. The current work provides an idea of oxidation resistance enhancement for Ti-bearing L1₂-strengthened alloys without changing the alloy composition.