Viscoplasticity and growth strain parameters identification by full modelling optimization during the high temperature oxidation of Ni28Cr modified by the reactive element yttria or zirconium
Résumé
Chromium-based alloys have been developed to form chromia layers in oxidizing environments for high temperature applications, providing a good protection against further oxidation. The mechanical behavior and oxide layer growth of yttria-coated or zirconium-doped Ni28Cr alloy are presently studied under high temperature oxidation conditions. These kind of reactive elements can decrease oxidation kinetics and significantly increase the adhesion of chromia layer on the metallic substrate. However, during oxidation, a compressive stress generally occurs in thermally grown oxide (TGO) due to the oxide layer and its mechanical interaction with the metallic substrate. These growth stresses, combined with the thermal stresses that develop during temperature changes, may initiate and cause layer cracking or spallation, which limits the lifetime of alloys in such a challenging environment. For prediction of this lifetime, it is important to know with accuracy the thermomechanical behavior of such a system. Therefore, a thermomechanical modelling is proposed and applied to optimize numerically the unknown parameters, especially viscoplastic ones. For the raw material and modified materials by reactive elements, the creep parameter decreases with a lower temperature, which means that the viscoplastic strain decreases with a decreasing temperature. For the yttria-coated or zirconium-doped material, the main result is an increase of the activation energy of this creep parameter with the quantity of reactive element.
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