Investigation on origins of residual stresses in Ni-NiO system by X-ray diffraction at high temperature

Abstract
To characterize the respective role of oxidation stresses, thermal stresses and relaxation phenomena in the oxide scales, two high temperature chambers for X-ray diffraction have been designed allowing to determine residual stresses in situ, during oxidation of Ni, with the [MATH] technique. At room temperature, the scales are subjected to compressive stresses and compressive stresses are also analyzed in the substrate. During heating-cooling sequences, a reversible variation of the stresses is observed, without relaxation. The stresses determined at room temperature are thermal stresses and theoretical calculation fits well with experimental determination. In situ stress determinations at 900 °C show that slight tensile stresses are then generated in the scale. All these results show that the stresses found at room temperature are mainly generated during cooling, and that the role of Pilling and Bedworth ratio, often considered as the main factor for stress generation in Ni-NiO system, has little effect.