Cyclic oxidation behavior of Ni3Al-basedsuperalloy

Cyclic oxidation behavior of a polycrystalline Ni3Al-based superalloy under as-cast and solution treated conditions was investigated in detail. SEM equipped with energy dispersive X-ray spectrometer and high-resolution transmission electron microscopy was used for microstructure observation and mechanism analysis. Results showed that oxidation process occurred along eutectic area (gamma-gamma') and dual phase area (gamma + gamma)interfaces firstly for as-cast alloy and oxide scales along (gamma-gamma')/(gamma + gamma') interfaces spalled gradually with oxidation process proceeding. As for the solution treated alloy, though oxidation process also occurred along (gamma-gamma')/(gamma + gamma') interfaces, no obvious spalling of oxide scales was observed under the present conditions. Due to the precipitation of Hf-containing carbides which can increase the adhesion of scales and its segregation to Al2O3 grain boundaries during cyclic oxidation to hinder outward migration of Al, leading to slower growth rate of scales. During the long-time (8-200 h) cyclic oxidation process of the solution treated alloy, O atoms continuously diffused towards the matrix along the pores in scales, resulting in thickening of Al2O3 layer. Moreover, thickening of Al2O3 layer also hindered the outward diffusion of Ni and a small amount of NiO was observed on surface of specimen after long-term cyclic oxidation, meaning the improvement of oxidation resistance.