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

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 degrees 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 alpha-Al(2)O(3) as well as from the transition alumina, in this case theta-Al(2)O(3), Which formed during the early stage. Compressive stress was found in the first-formed transition alumina, but the initial stress in alpha-Al(2)O(3) was tensile, with a magnitude high enough to cause Al(2)O(3) fracture. New alpha-Al(2)O(3) 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 alpha-Al(2)O(3) layer formed at the interface, the strain level in alpha-Al(2)O(3) became compressive, reaching a steady state level around - 75 MPa at 1100 degrees C. To study a specimen's response to stress perturbation, samples with different thickness, after several hours of oxidation at 1100 degrees C, were quickly cooled to 950 degrees 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 similar to 1 x 10(-8) /s. This behavior indicates that oxide creep is the major stress relaxation mechanism.