In situ Synchrotron X-Ray diffraction study of high-temperature stress relaxation in chromia scales containing the reactive element yttrium

Abstract : The viscoplastic behaviour of the model yttria-coated Ni28Cr alloy was studied under high temperature oxidation conditions for various small quantities of reactive element (RE). The main purpose was to determine the way the RE acts on the non-destructive stress relaxation mechanisms (creep) occurring in thermally grown chromia scales. After building of an initial chromia microstructure by 3 h oxidizing at 1273 K, temperature jumps were applied to generate thermal stress loading, the chromia scale microstructure being considered as stable. The generated stress was then released along 3 h isothermal periods and X-ray Synchrotron diffraction was used to follow in situ its evolution. The experimental curves were then simulated in order to calculate the creep coefficients, to evaluate the activation energy of the diffusion-creep mechanism and determine the diffusion coefficients and the elementary mechanisms associated to the viscoplastic behaviour of chromia scales in relation with the introduced RE quantity.