Multiple oxidation mechanism of Al and Ti doping Haynes244 alloy under elevated temperature

The effects of equimolar content Al, Ti and their co-addition on the oxidation behavior of Haynes244 alloy were investigated in air at 900–1100 °C for 100 h. The results show that Al and Ti co-addition alloy possesses the optimum oxidation resistance at 1000 °C and 1100 °C, while Al-added alloy exhibits excellent performance at 900 °C. In Haynes244 alloy, the oxidation resistance is mainly attributed to the protective Cr₂O₃ layer, which prevents the outward diffusion of the alloy elements and the inward diffusion of oxygen. At 1000 and 1100 °C, Cr₂O₃ produces the volatile CrO₃, leading to the peeling of oxides layer, so that the spallation and degree of volatilization of Cr₂O₃ directly determine the oxidation performance. The diffusion rate of Cr³⁺ can be accelerated after addition of Ti by producing Cr vacancies, increasing the formation of Cr₂O₃ and degree of further volatilization. While the existence of Al₂O₃ formed at grain boundaries hinders the diffusion of Cr³⁺ and strengthens the adhesion of oxides, decreasing the volatilization and spallation. The co-addition of Ti and Al elements helps the quick formation of a dense Cr₂O₃ and obstacles destruction of the dense oxide layer caused by volatilization, collectively providing 1Ti1Al alloy excellent oxidation resistance. When temperature decreasing to 900 °C, Cr₂O₃ is difficult to largely form due to the low thermal diffusion rate of Cr and a dense Al₂O₃ layer can be only formed in the alloy doped with 2 at.% Al and prevent the further oxidization.