Oxide phases and residual stresses in scales formed at early stages of oxidation of β‐NiAl at 1473 K and the effect of implanted yttrium

Abstract

Oxide phases formed at early oxidation stages on unmodified and yttrium‐implanted β‐NiAl intermetallic compound were studied with the aid of photoluminescence spectroscopy in scales developed at 1473 K for oxidation periods of up to 4 h. Two‐stage‐oxidation exposures with 18O2‐tracer were applied. The PLS method was also used to assess the residual stresses in α‐Al2O3 scale at consecutive stages of its development. It was found that scale formation occurs in a very local way, related to inhomogeneous distribution of Cr in substrate: sometimes distances as short as 10–15 μm were sufficient to observe entirely different phases in the scale. In general, implanted yttrium was found to retard the formation of a stable and protective layer of α‐Al2O3. Oxidation for 4 h was necessary for no transient aluminum oxides (mostly θ and/or δ) to be detected, while on non‐implanted material they were not found after only 26 min exposure. Only compressive stresses were found in α‐alumina scales. They were rather high, reaching even 5 GPa. However, their dependence on the regions in the scale and the scale evolution stage was observed. The results indicate that patches frequently observed in scales on β‐NiAl are regions where the phase transformation of transient aluminas to the α phase occurs preferentially. Ridges, which fill the transformation‐related cracks, consisted only of α‐Al2O3. Combination of these results with those obtained using SIMS distribution analyses of oxygen isotopes as well as with SEM/EDX, enabled very precise conclusions to be inferred concerning the scale growth mechanism. In particular, the local nature of the scale development can be described in more detail.