THE STRESS AND SPALLING BEHAVIOR OF THE OXIDE SCALE FORMED ON POLYCRYSTALLINE NI3AL

The residual stress in the oxide scale formed on polycrystalline Ni3Al in air in the range of 900-1200 degrees C was determined at room temp erature using optical fluorescence piezospectroscopy. The stress was f ound to be almost independent of lime or temperature of oxidation. In contrast, the spalling behavior was strongly dependent on the oxidatio n temperature, not occurring below 1100 degrees C and profusely at 120 0 degrees C. In the intermediate temperature regime (similar to 1100 d egrees C), spallation of the oxide scale occurred preferentially in th e vicinity of grain boundaries in the Ni3Al. This is attributed to a b uckling instability driven by the residual thermal mismatch stress and induced by both void formation and a non-planar interface structure r esulting from preferential diffusion along the Ni3Al grain boundaries. Copyright (C) 1996 Acta Metallurgica Inc.