EVALUATION OF STRESSES IN NI-NIO AND CR-CR2O3 DURING HIGH-TEMPERATURE OXIDATION BY IN-SITU X-RAY-DIFFRACTION

A high temperature X-ray diffraction method based on the Laue (transmission) mode was developed to measure strains associated with the growth of an oxide scale on a metal. These growth strains in the attached oxide scale and metal substrate are determined simultaneously and in real time by the change in spacing of selected lattice planes relative to the unstrained lattice spacing at the same temperature. Thermally induced strains in both substrate and attached oxide can also be evaluated by monitoring the change in lattice spacing on cooling to room temperature. The method was tested by comparison with the known thermal expansivity of Au, Cr2O3 and NiO and was applied to the growth of oxide scales on polycrystalline pure Cr and Ni. The results illustrate that both the growth and thermal stresses contribute significantly to the total residual stress. The evolution of these stresses during the oxidation period and subsequent cooling is complex, especially if stress relaxation occurs or if the oxide develops a duplex microstructure. Because of this complexity, determination of the room temperature residual strains alone reveals little of the actual stress state of the system at the oxidation temperature.