INVESTIGATION ON ORIGINS OF RESIDUAL-STRESSES IN NI-NIO SYSTEM BY X-RAY-DIFFRACTION AT HIGH-TEMPERATURE

To characterize the respective role of oxidation stresses, thermal str esses and relaxation phenomena in the oxide scales, two high temperatu re chambers for X-ray diffraction have been designed allowing to deter mine residual stresses in situ, during oxidation of Ni, with the sin2p si technique. At room temperature, the scales are subjected to compres sive stresses and compressive stresses are also analyzed in the substr ate. During heating-cooling sequences, a reversible variation of the s tresses is observed, without relaxation. The stresses determined at ro om temperature are thermal stresses and theoretical calculation fits w ell with experimental determination. In situ stress determinations at 900-degrees-C show that slight tensile stresses are then generated in the scale. All these results show that the stresses found at room temp erature are mainly generated during cooling, and that the role of Pill ing and Bedworth ratio, often considered as the main factor for stress generation in Ni-NiO system, has little effect.