Oxidation-induced strains in thermally grown alumina scales on Ni-base alloys: An in situ synchrotron radiation study

The stresses developed during the formation of alpha-Al2O 3 on Ni-base alloy bond coats can lead to premature failure of thermal barrier systems, which have been used for almost three decades to extend the life of gas turbine systems. Conventional X-ray diffraction techniques are difficult and have not been totally satisfactory for measuring the stresses in situ in Al2O3 scales.; In this work, in situ evolution of growth strains in Al2O3 scales formed on a variety of Ni-base alloys, as well as thermal mismatch strains developed during cooling have been studied using 21.6 keV X-ray synchrotron radiation. The lattice strains in the Al 2O3 scales were obtained by analyzing the elliptical distortions of Debye-Scherrer (DS) diffraction rings which develop in response to the presence of in-plane stresses in the oxide.; A model NiCrAlY bond coat alloy was investigated in air at four oxidation temperatures: 950, 1000, 1050 and 1100°C. While the growth stresses in alpha-Al 2O3 were tensile at all temperatures, stress relaxation is observed only at 1050 and 1100°C. Stresses in the TGOs in these samples at room temperature were also measured by piezospectroscopy and laboratory XRD, with good agreement between all three methods. A quasi-peritectoid phase transformation in the bond coat leads to a greatly increased room temperature compressive stresses in the samples oxidized at 1050 and 1100°C.; The evolution of growth stresses in single crystal NiAl at 1100°C is dependent on crystallographic orientation. Epitaxial issues at the NiAl/Al 2O3 interface appear to govern initial formation of gamma- or theta-Al2O3 and its subsequent transformation to the stable and denser alpha-Al2O3 polymorph. This transformation generates tensile stresses in alpha-Al2O 3, which relax with further oxidation.; The present work also independently confirms an error in the sign of one of the elastic constants reported in the literature. The value of c14 reported in the literature is -22 GPa but this has recently been disputed; in situ strain measurements have confirmed that c14 should be +22 GPa. A consequence of using the wrong sign of c14 is that errors in excess of 25% can occur in the measured residual stresses of alpha-Al 2O3 scales or alpha-Al2O3 ceramics.