High-Temperature Oxidation of NiAlCr–Ca and NiAlCr–Sr Alloys in Air

In this study, interrupted oxidation behavior of synthetic NiAlCr–Ca (Ca = 0.3, 1.4, 2 at.%) and NiAlCr–Sr (Sr = 0.4 at.%) alloys in the air at 1027 °C for 192 h was investigated. Parabolic rate constants ( k p ) showed that the Sr-containing alloy exhibited the best oxidation resistance among the alloys investigated in this study. The oxide scale formed on the Sr-containing alloy was composed of α-Al 2 O 3 phase with Sr-rich nodules. Increasing the Ca concentration in the alloys was found to reduce the oxidation resistance due to the formation of non-protective Ca-rich complex oxides and consumption of α-Al 2 O 3 scale by the reaction between Al 2 O 3 and CaO. The Ca-rich complex oxides were initially formed on the Ca-rich interdendritic region and grew with time. Very little scale spallation was observed for the Sr-containing alloy, while it was notable for 0.3 at.% Ca-containing alloy. Spallation was attributed to the coefficient of thermal expansion (CTE) mismatch arisen from the formation of CaAl 4 O 7 phase, a compound with a very low CTE.