Growth strains and stress relaxation in alumina scales during high temperature oxidation

A novel X-ray technique was used, exploiting synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory, to investigate the growth stresses in alpha-Al(2)O(3). Insitu measurements of Debye-Scherrer diffraction patterns from the scale were recorded during oxidation and cooling, and the elliptical distortion of the diffraction rings was analyzed to yield the in-plane strain. Fe-28Al, Fe-40Al, Fe-40Al-0.2Hf, Fe-20Cr-10Al and Ni-50Al (at. %) were studied. Data were acquired in air at temperatures between 950-1100degreesC and during cool down. In all cases, the steady stage growth strain was relatively low (<0.1%) and was either tensile or compressive depending on the alloy. A higher tensile strain often existed during the initial oxidation period when transition alumina was present. Thermal stresses imposed on NiAl by reducing the sample temperature to 950degreesC for a period of time showed noticeable stress relaxation by creep. Different degrees of relaxation were also found during cooling depending on alloy composition and scale microstructure. On all Fe-based alloys, the first formed alpha-Al(2)O(3) was highly textured with the degree of texture decreasing with further oxidation. The relationships between stress development, scale wrinkling, oxide phase changes, and the effect of reactive element addition on growth stresses are discussed. Results are compared with other reports of growth stresses in Al(2)O(3) scales.