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

Abstract 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 -Al 2 O 3 with very little trace of θ -Al 2 O 3 , 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 -Al 2 O 3 and the inner layer was made up of α -, δ -, and θ -Al 2 O 3 phases. Clearly, the Y addition retarded the θ -to- α Al 2 O 3 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).