The effect of annealing on the microstructures and oxidation behaviors of AlCoCrFeNi complex concentrated alloys

Abstract

This paper reports the effects of annealing at 1050 °C in an inert atmosphere on the microstructures and high temperature oxidation behaviors of Alx(CoCrFeNi)1−x CCAs, where x = 8, 12, 15, 20, and 30 (at%) complex concentrated alloys. In all cases, annealing stabilized multi-phase microstructures consisting of a combination of BCC, B2, and/or FCC phases with the final phase constitution being dependent on the Al content in the alloy. All the alloys exhibited brief periods of transient oxidation, followed by various degrees of parabolic oxide growth. The as-cast alloys, which were observed to predominantly be Cr2O3 formers, oxidation resistance increased with increasing Al content. After annealing, all of the alloys transitioned into Al2O3 formers but with increased mass gain up to 20 at% Al. Comparison of the experimental results with thermodynamic models of alloy phase equilibria and oxidation products suggests that an increase in the volume fractions of Al-rich phases near the alloy surfaces as a result of annealing, increases oxidation resistance by providing Al reservoirs for the formation of Al2O3. The oxidation mechanisms are discussed relative to existing M-Cr-Al oxide formation models.