Creep in α-Al2O3 thermally grown on β-NiAl and NiAlPt alloys

We have measured creep relaxation in α-Al2O3, thermally grown on stoichiometric β-NiAl, and on β- and γ'-phase alloys of Ni–Al–Pt at temperatures between 950–1100 °C. Creep was monitored using in-situ measurements of strain relaxation in the oxide following the sudden imposition of a stress. A stress was imposed by abruptly changing the sample temperature, exploiting the thermal expansion difference between oxide and substrate. The in-plane elastic strain was obtained using a sin2ψ X-ray diffraction technique exploiting synchrotron radiation. For oxides grown on β-NiAl and Ni–Al–Pt samples, we found that strain relaxation rates are comparable to those observed in fine grained α-Al2O3 bulk ceramics, when bulk results are extrapolated to the lower temperatures examined here. When Hf was added to the Ni–Al–Pt alloy, creep rates in the thermally grown oxide were substantially slowed. Creep rates at stress levels of 100 MPa, or less, are proportional to σn, with n ≤ 2, consistent with a diffusional creep mechanism.