Viscoplastic characteristics of thermally grown chromia films obtained from in situ 2D synchrotron X-ray diffraction

Abstract

This work takes place in the general context of a better understanding of materials degradation mechanisms in severe environments. More particularly, its purpose is to correlate microstructural elements to growth stress magnitude evolution and stress release mechanisms for chromia thin films thermally grown on model NiCr alloys. X-ray elastic strains have then been measured in situ, as the chromium oxides develop and evolve, by using synchrotron X-ray diffraction, coupled with an induction furnace. 2D diffraction patterns were continuously recorded by applying different temperature variation procedures between 1000°C and 700 °C. Thermal stresses imposed on Cr2O3/NiCr system by abruptly reducing the sample temperature, i.e. by exploiting the thermal expansion difference between oxide and substrate, showed noticeable subsequent stress relaxation by creep. The main results obtained from these experiments were the kinetic of the growth stress from the isothermal measurements, and the study of the stress release mechanism after the temperature jumps to lower values. In complement, the oxide microstructure development during the course of oxidation was also investigated from both the peaks intensity and peaks width evolution. The creep coefficients could be estimated for the first time for chromia under the thin film form. It was proposed that the stress release by diffusion creep in the chromia films is very likely governed by a mechanism involving diffusion of oxygen ions at the chromia grain boundaries.