CURVATURE CHANGES DURING THERMAL CYCLING OF A COMPOSITIONALLY GRADED NI-AL2O3 MULTILAYERED MATERIAL

The elastoplastic deformation characteristics of a plasma-sprayed, tri -layered composite plate subjected to thermal cycling from 20 degrees C up to 800 degrees 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 interlay er (FGM) wherein the composition varied approximately linearly along t he layer thickness. The experiments involved in situ and ex situ measu rements, employing a scanning laser technique, of the changes in the o verall curvature of the unconstrained plate arising from the thermal m ismatch between the constituent phases. The variations of curvature, a ccumulated plastic strains and thermal stresses at different locations in the layered solid were also assessed numerically with the aid of a vailable continuum formulations, and the numerical predictions were co mpared with experiments, wherever appropriate. It is shown that when o nly small plastic strains exist in the Ni layer or in the Ni-rich end of the FGM layer, known formulations are capable of providing approxim ate predictions of the cyclic variations in curvature, the onset of pl asticity and some features of the initiation of cracking (and its loca tion). We also examine possible sources of error in the experimental m easurements of curvature and in the interpretation of thermally induce d deformation due to some processing conditions.