High-temperature oxidation behavior of aluminum-added Cr-Cr2Ta two-phase alloys: Mechanism of aluminum influence and critical conditions for internal-external oxidation transition

To enhance the oxidation behavior of Cr-Cr₂Ta-based two-phase alloys, this study investigated the effects of varying aluminum additions on the high-temperature oxidation behavior of Cr-Cr₂Ta two-phase alloys in air at 1200 °C. During the oxidation of alloys, the two-phase boundary is the main route for aluminum diffusion. By establishing a diffusion model for aluminum within the (Cr, Al)₂O₃ solid solution oxide layer, the critical aluminum concentration required to form a continuous α-Al₂O₃ layer in Cr-Cr₂Ta two-phase alloys was calculated. The results indicate that a rational control of aluminum content in the alloy significantly aids in enhancing its oxidation resistance. While the addition of aluminum effectively improves the oxidation resistance of Cr-Cr₂Ta alloys, excessive aluminum addition leads to cracking in the alloy matrix during oxidation, which is detrimental to maintaining the protective effect of the α-Al₂O₃ layer. Conversely, alloys with aluminum content near the critical value exhibit excellent oxidation resistance and do not experience cracking throughout the oxidation process. Additionally, a fixed aluminum‑chromium ratio of $r_{\mathrm{Al}/\mathrm{Cr}}^{\ast }$ = 0.45 ± 0.02 exists in the Cr-Cr₂Ta alloy, which ensures the formation of an Al₂O₃ layer on the alloy surface.