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

A high temperature X-ray diffraction method based on the Laue (transmi ssion) mode was developed to measure strains associated with the growt h of an oxide scale on a metal. These growth strains in the attached o xide scale and metal substrate are determined simultaneously and in re al time by the change in spacing of selected lattice planes relative t o the unstrained lattice spacing at the same temperature. Thermally in duced strains in both substrate and attached oxide can also be evaluat ed by monitoring the change in lattice spacing on cooling to room temp erature. The method was tested by comparison with the known thermal ex pansivity 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 t otal residual stress. The evolution of these stresses during the oxida tion period and subsequent cooling is complex, especially if stress re laxation occurs or if the oxide develops a duplex microstructure. Beca use 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.