Formation and growth morphology of nodules in the high-temperature oxidation of Fe–Mn–Al–C alloy

The oxidation of a dual-phase Fe–Mn–Al–C alloy has been studied at 750 °C in dry air. A typical nodule with two distinct layers was formed after oxidation for 24 h. The external scale consists of mainly Mn2O3 and Mn3O4, while alumina oxide is observed in the ferrite layer of the inner nodule. As the duration of oxidation increased, a needle-like AlN is revealed beneath the nodule. The ferrite layer is formed due to the selective oxidation of manganese, and the inward diffusion of nitrogen through internal voids is responsible for the formation of AlN precipitates beneath the oxide scale. Qualitative and quantitative electron microscopic and transmission electron microscopic (TEM) techniques are employed to investigate the morphological development and elemental distribution in the alloy. The mechanism of the formation and growth of the nodule is explored.