Effect of High Temperature Exposure in Air on the Deformation Mechanisms of Intermetallic γ-TiAl Alloys

Intermetallic γ-TiAl-based alloys are promising lightweight alternatives to nickel superalloys for aircraft engine components that operate at temperatures up to 900°C. This due to their low density, high specific strength/moduli and good creep properties. However, exposure to air at these elevated temperatures results in a significant in yield stress and a drop in ductility. The underlying mechanisms that are responsible for this embrittlement remain poorly understood. One hypothesis is that this embrittlement is a direct consequence of the bulk and/or interfacial oxygen (O) diffusion. The first objective of this study is to determine the extent to which oxygen can penetrate into the material. To this end, samples heat treated in a controlled environment of 83% Ar-17%¹⁶O₂ were first observed using SEM-based BSE and EBSD to analyze the microstructure of the surface layer. Nanoindentation was then used to monitor the change in hardness from the surface layer to the matrix. To accurately determine the depth of O penetration, Secondary Ion Mass spectrometry (SIMS) profiling was carried out on samples that had been heat-treated in a controlled environment of 99% Ar-1%¹⁸O₂. The second objective was to investigate the mechanisms of plastic deformation (pinning of dislocations on oxygen-rich precipitates) within the O-diffused surface layer by using transmission electron microscopy (TEM), for comparison with the O-free regions far from the surface. Lamellae were obtained in these distinct regions by site-specific lift-out using a FIB-SEM. Comparative analysis of microstructure and deformation mechanisms in the O-diffused surface layer and O-free regions far from the surface is expected to provide new insights that may offer a way to eliminate embrittlement in intermetallic γ-TiAl alloys.