Effect of Cr content on the oxidation behavior of alloys Co-Al-Cr and Co-Ni-Al-Cr at 900°C studied by gradient additive manufacturing

A novel high throughput oxidation testing method was developed using compositionally graded additive manufacturing to investigate the effect of Cr content on the oxidation behavior of model alloys Co-Al-Cr and Co-Ni-Al-Cr, which were fabricated using laser powder bed fusion (LPBF) with ∼ 7.5 at.% Al and gradient Cr contents ranging from 0 to 20 at.%. The results indicate that the oxidation behaviors of as-printed alloys were identical to those of alloys made by conventional casting with similar compositions after oxidation at 900 °C, confirming the effectiveness of this high throughput oxidation method. The approach dramatically accelerated the construction of isothermal oxidation diagrams by the screening of critical thresholds for the transitions of oxidation behavior. For alloys Co-Al-Cr, the selective oxidation of Cr occurred when Cr content exceeded 13.2 at.%, forming a protective Cr2O3 scale. The accelerated Al diffusivity in Co-Ni-Al-Cr alloys (versus Co-Al-Cr counterparts) significantly lowers the critical Al content required for the formation of a continuous Al2O3 scale. Consequently, these alloys prefer to form protective Al2O3 scales when Cr content exceeds 9.1 at.%. This research provides a theoretical basis for the compositional design and optimization of Co– and CoNi-based superalloys.