Stresses in Cr2O3 scales developed on Ni-30Cr

High temperature X-ray diffraction has been used for measuring the expansion coefficients of Ni-30Cr alloy and Cr2O3 between 20 and 900 degrees C and for determining the stresses created in the chromia scale formed on NiCr as a function of various parameters : substrate and oxide scale thickness, grain size, yttrium doping, cooling rate. Besides, relaxation phenomena have been experimentally evidenced during isothermal or anisothermal heat-treatments after previous oxidation at 900 degrees C. After oxidation, the stresses in the oxide scale are compressive, and during further heating, their value decreases with the annealing temperature and they tend to zero at 900 degrees C, indicating that the oxide growth stresses are negligible. Simultaneously, a numerical model was developed to calculate the stress and the relaxation phenomena. It takes into account the mechanical behaviour of the substrate and of the oxide scale and parameters such as the cooling rate or the heat-treatment duration, etc...This elasto-visco-plastic modelling correlates the experimental results with the various parameters of the study.