Transmission electron microscopy observations on the phase composition and microstructure of the oxidation scale grown on as-polished and yttrium-implanted β-NiAl

Phase transformations and microstructural evolution of thermally grown oxide scale on polycrystalline β-NiAl at 1100 °C up to 6 h, with and without (e.g., as-polished) yttrium implantation, were examined by glancing angle X-ray diffraction, photostimulated luminescence, scanning and transmission electron microscopy. Site-specific TEM specimens were prepared by using focused ion beam in-situ lift-out technique. The oxide scale developed on as-polished β-NiAl consisted of the islands of 390 nm-thick flat regions (e.g., patches) in 916 nm-thick scales. Regardless of microstructure, the oxide scale consisted of α-Al2O3 with very little trace of θ-Al2O3, and had uniform compressive residual stress. The oxide scale on Y-implanted β-NiAl had a two-layer microstructure: the outer layer was mainly α-Al2O3 and the inner layer was made up of α-, δ-, and θ-Al2O3 phases. Clearly, the Y addition retarded the θ-to-α Al2O3 phase transformation. The oxide scale on Y-implanted β-NiAl, in general, consisted of a 722 nm-thick layer with islands of 470 nm-thick patched regions, some of which contained Y-rich nodules that protruded with thickness up to 1200 nm. Except for islands of patch-regions, the oxide scale developed on Y-implanted β-NiAl was thinner (722 nm) than that on as-polished β-NiAl (916 nm).