Frequency analysis for investigation of the thermomechanical mechanisms in thermal oxides growing on metals

Abstract

International audienc

Determination of Residual Stresses in an Oxidized Metallic Alloy under Thermal Loadings

“…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].…”

“…There is presently no experimental verification of the absence of stress gradient. However, previous works on the same material have proved with different techniques (XRD, Raman spectroscopy) with different gauge volumes that the stress is of the same magnitude for different depth in the material [4][5][6]10]. Because of the sin 2 ψ curve obtained, it is experimentally proved for almost all the oxidation time that the system presents transverse isotropy.…”

“…The energy of 20 keV (optimum for photons flux on BM02) is well adapted to optimize the film response. A XRD diagram has been recorded using an area detector (FReLoN, ESRF, Grenoble, France), which is large enough to extract the stresses evolution in both the film and the metal, and fast enough to obtain data in real time, as in previous experiments [4].…”

“…">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].…”

“…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.…”