Analytical Modeling Of Residual Stresses In Oxidation Of Metals At High Temperature

During high temperature isothermal oxidation of alloys or metals oxide layers are formed on exposed surfaces. Such formation of oxide layers generates stresses in the oxide/metal composite. Specimen deflection test and specimen extension test have been extensively used to predict growth stresses developed in oxide/metal composite during oxidation time. In deflection test, a single major surface of the specimen is exposed to oxidation. Therefore, residual stresses are relived through bending of the oxide/metal composite. The presence of stresses can be deduced with the help of deflection of the oxide/metal composite, lateral growth of the oxide and creep of the composite. In specimen extension test, both major surfaces of the specimen are exposed to oxidation. Therefore, residual stresses are relived through elongation of the specimen. The presence of stresses can be deduced with the help of lateral strain developed with oxidation time along with lateral growth of the oxide and creep of the composite.With this assumption the present dissertation stated that strains interaction in both the deflection test and the extension test are lateral growth strain in the oxide scales, creep and elastic strain in both the oxide and the substrate. Thus the purpose of the thesis was to develop an analytical modeling of residual stresses considering the fundamental strains appearing during oxidation period. Then the proposed modeling has been applied to the Al2O3/FeCrAlY composite. NiO/Ni composite and NiO/NigoCr2o to highlight importance of the analytical modeling.In deflection test, creep deflection literatures show that oxide neutral axis lies outside the oxide layer. On such situation, the average stress within oxide is always compressive where as that in metal is tensile to balance the forces in the cross-section of the composite. In our modeling, the similar results were obtained but also found another interesting result i.e. oxide planar stress distribution is determined by the growth stress when the oxide neutral axis coincide outer surface of the oxide layer. This is because lateral growth stress is independent of the position of the oxide neutral axis and depends only on oxidation time and oxide kinetics. This conclusion differs from creep deformation model where the stress at the outer surface of the oxide layer should be zero. Oxide neutral axis approaches outer oxide layer and lies within the oxide layer as oxide thickens. Metal neutral axis always lies within metal substrate as oxide thickens.Irrespective of the test, lateral growth constant influences at the initial oxidation stage and creep constants influences at the later oxidation stage. Increasing lateral growth constant value increases the average oxide stress as well as metal stress. For higher value of lateral growth constant more relaxation of stresses occurs both in the oxide layer and metal substrate. In extension test. Higher creep strength of metal can result zero growth stress during oxidation time i.e. for the Ni/NiO composite. At that time if the lateral expansion of the oxide/metal composite occurs, it is possible only by the lateral expansion of the growing oxide scales.