Microstructural Investigation of the Thermally Grown Oxide on Grain-Refined Overdoped NiAl–Zr

Abstract

The thermally grown oxide on sputter-deposited NiAl–0.09Zr was studied using transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) to microstructurally assess its oxidation resistance. Sputter deposition resulted in a refined grain size of 0.3 μm that was compared to extruded NiAl–Zr with a grain size of approximately 25 μm. Thermogravimetric oxidation of the sputter-deposited material showed a shorter transient regime with lower mass gain than an extruded NiAl–0.1Zr alloy and improved spallation resistance through 50 h of isothermal oxidation at 1000 °C. After 5 h the thermally grown oxide on the sputter-deposited alloy exhibited a three-layer structure consisting of external θ-Al2O3 whiskers, intermediate equiaxed α-Al2O3 grains (< 100 nm) + ZrO2 precipitates and internal columnar α-Al2O3 grains, in contrast to the extruded alloy which showed sparse α growth. After 50 h of oxidation, the three-layer structure was retained, but the top θ-Al2O3 layer was transformed to α-Al2O3. TKD after 50 h showed the top and bottom oxide layers to be composed of high-misorientation α-Al2O3 grains approximately three times smaller than the extruded sample. Monoclinic and tetragonal ZrO2 precipitates were identified in the fine-grained middle region. These features show that grain refinement significantly increases Zr diffusion to the reacting surface, while simultaneously mitigating the effects of overdoping. This increased Zr diffusion is believed to have expedited the formation of a continuous α-Al2O3 layer, resulting in a shorter transient oxidation period.