Curvature changes during thermal cycling of a compositionally graded NiAl2O3 multi-layered material

The elastoplastic deformation characteristics of a plasma-sprayed, tri-layered composite plate subjected to thermal cycling from 20°C up to 800°C were studied experimentally and numerically. The tri-layered solid comprised polycrystalline Ni and Al2O3 outer layers and a 2.2 mm thick compositionally graded NiAl2O3 composite interlayer (FGM) wherein the composition varied approximately linearly along the layer thickness. The experiments involved in situ and ex situ measurements, employing a scanning laser technique, of the changes in the overall curvature of the unconstrained plate arising from the thermal mismatch between the constituent phases. The variations of curvature, accumulated plastic strains and thermal stresses at different locations in the layered solid were also assessed numerically with the aid of available continuum formulations, and the numerical predictions were compared with experiments, wherever appropriate. It is shown that when only small plastic strains exist in the Ni layer or in the Ni-rich end of the FGM layer, known formulations are capable of providing approximate predictions of the cyclic variations in curvature, the onset of plasticity and some features of the initiation of cracking (and its location). We also examine possible sources of error in the experimental measurements of curvature and in the interpretation of thermally induced deformation due to some processing conditions.