The effect of surface orientation on oxidation-induced growth strains in single crystal NiAl: An in situ synchrotron study

The evolution of oxidation-induced growth stresses in single crystal stoichiometric NiAl has been studied; it is specimen dependent and also depends on crystallographic orientation. Epitaxial issues at the NiAl/Al2O3 interface appear to govern initial formation of γ- or θ-Al2O3 and its subsequent transformation to the stable and denser α-Al2O3 polymorph. This transformation generates tensile stresses in α-Al2O3, which relax with further oxidation. Room temperature residual stresses are large, ∼3.3 GPa, and compressive. Confocal photo-stimulated microspectroscopy studies of these residual compressive stresses have revealed large spatial variabilities, which are also attributed to epitaxial issues at the NiAl/Al2O3 interfaces.