Stress development and relaxation in Al2O3 during early stage oxidation of β-NiAl

Abstract

Using a glancing synchrotron X-ray beam (Advanced Photon Source, Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from thermally grown oxides on NiAl samples were recorded during oxidation at 1000 or 1100°C in air. The diffraction patterns were analyzed to determine strain and phase changes in the oxide scale as it developed and evolved. Strain was obtained from measurements of the elliptical distortion of the Debye-Scherrer rings, where data from several rings of a single phase were used. Results were obtained from α-Al2O3 as well as from the transition alumina, in this case θ-Al2O3, which formed during the early stage. Compressive stress was found in the first-formed transition alumina, but the initial stress in α-Al2O3 was tensile, with a magnitude high enough to cause Al2O3 fracture. New α-Al2O3 patches nucleated at the scale/alloy interface and spread laterally and upward. This transformation not only puts the alpha alumina in tension, but can also cause the transition alumina to be in tension. After a complete α-Al2O3 layer formed at the interface, the strain level in α-Al2O3 became compressive, reaching a steady state level around –75 MPa at 1100°C. To study a specimen’s response to stress perturbation, samples with different thickness, after several hours of oxidation at 1100°C, were quickly cooled to 950°C to impose a compressive thermal stress in the scale. The rate of stress relaxation was the same for 1 and 3.5 mm thick samples, having a strain rate of ∼1×10−8/s. This behavior indicates that oxide creep is the major stress relaxation mechanism.