Thermal aging effects on the microstructure and short-term oxidation behavior of a cast Ni3Al alloy

Specimens of the castable nickel aluminide alloy IC221M were annealed in still air at 900 °C to examine the effects of oxidation and thermal aging on its microstructure. The alloy, which is comprised primarily of a γ-γ′ mixture but also contains a lower melting γ+Ni5Zr eutectic, is used in applications that require casting. The as-cast microstructure is dendritic and exhibits microsegregation of the alloying elements. Microstructures of as-cast specimens annealed for periods up to 16,600 h were examined using scanning electron microscopy and energy dispersive X-ray spectroscopy with spectrum imaging. Although the material partially homogenizes with aging time, approximately 3 vol% of the γ+Ni5Zr eutectic remains after 16,600 h, and there is a tendency for the eutectic colonies to spherodize. Oxidation products were identified using X-ray diffraction and energy dispersive X-ray spectroscopy. During the initial stages of oxidation, the oxide is predominantly NiO with small amounts of Cr2O3, ZrO2, NiCr2O4, and θ-Al2O3. Initial oxidation occurs primarily in the interdendritic regions of the microstructure. With further aging, the predominant surface oxides become NiO and NiAl2O4, with a continuous film of α-Al2O3 forming immediately beneath them. Although these oxides are constrained to the near surface region, other oxides penetrate to depths greater than 10 times that of the continuous surface oxide, facilitated by oxidation of the γ+Ni5Zr interdendritic eutectic. These oxides appear in the microstructure as long, thin spikes of ZrO2 surrounded by a sheath of Al2O3.