International audienc

Abstract. The presence of residual stresses in thermal oxide layers has been recognized for a long time. In the present work, the mechanical fields for chromium oxide are investigated. An extended model is established to take into account the effects of temperature and thermal cycling for the calculation of oxide stress. Numerical results are given in order to predict the influence of different parameters, especially the dependence of some material parameters with temperature. This enables to make comparison with experimental results.Introduction.

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In order to clarify the mechanical features of a metal under thermal cyclic loading for the system Ni30Cr-Cr 2 O 3 , a specific study has been carried out. In the present work, the residual stresses in both the metal and the oxide layer have been investigated. An adapted method is applied to process the experimental results that were obtained by using in-situ high temperature synchrotron diffraction at European Synchrotron Radiation Facility. The sin 2 ψ analysis provides information about the stress in metal and oxide. X-ray diffraction provides also the lattice parameter between crystallographic planes in the metal. To obtain correct stress values, a correction method is also proposed taking into account different discrepancies sources to ensure the equation of mechanical balance.2 of 23 works [4][5][6] concerning NiCr alloys, it has been shown that the growth stresses are released by creep in the fine grain chromia film (grain size usually ranging from 0.2 to 0.8 µm). Strain relaxation studies in the chromia layers were carried out in-situ, after imposing a sudden temperature change introducing a supplementary stress due to the mismatch of thermal expansion coefficients of the oxide and the alloy. Relaxation rates were found to be proportional to stress power σ n , with n ≤ 2, which is consistent with a diffusion-creep mechanism [4]. For the first time, the corresponding creep coefficients have been determined in the thermally grown chromia films [5]. Moreover, the creep thermal activation was studied, and the associated elementary mechanism for the creep behavior of chromia was identified as oxygen inward transport [7]. However, at the same time during oxidation, the metallic substrate is also able to develop stresses through its mechanical balance with the chromia TGO. Thus, in order to go further in the identification of the thermomechanical behavior of the metal/oxide system and in the understanding of the underlying physical mechanisms, it appears to be mandatory to investigate simultaneously and in real time the strain evolutions in the metal and in the oxide during oxidation. Aims of the WorkThus, the aim of the present work that is based upon the coupling of in-situ X-ray diffraction with oxidation in a furnace is twofold: perform measurements in the metal with sufficient accuracy to obtain its time evolution, and compare to the mechanical balance to validate the methodology. These measurements will also enable to validate a new method considering Fourier analysis, developed during a previous work [4]. To achieve such a goal, it is mandatory to investigate with accuracy the mechanical behavior under cyclic thermal loadings [6].