Laurent Lahoche

Influence of interface ordering on the thermomechanical properties of oxide-metal scale

Abstract The relation between Ni-ions ordering and the development of growth and residual stresses at the Ni/NiO interface is studied. The crystallographic structure of the 3D Ni/NiO interface is proposed to be two-layered with the space group P4/mmm. Calculated strain—internal elastic stress dependencies reveal thermal growth regimes with a first-order phase transition resulting in the duplex structure of the interface: the ordered lower layer coherent with the substrate, and the upper layer incoherent with the lower one. Numerical modeling of the corresponding growth and residual stresses is performed using two mechanical models. If the new oxide layer in the upper part grows independently of the layers already formed, then the calculated growth stresses after relaxation vary from 0 to −250 MPa with the thickness of the lower layer, and the residual stresses vary from −600 to −850 MPa. When the new oxide layer is initially stressed, the variation of the growth stresses is from 520 to −280 MPa. The residual stresses vary from −80 to −880 MPa in this case.

Influence of epitaxy and ordering on the mechanical behaviour of an oxide layer on a metallic substrate

Abstract We have developed a thermomechanical model which allows to study the influence of the metallic ion ordering in a three-dimensional metal–oxide interface, which is strongly related to the epitaxial relationship. We have shown that a phase transition can lead to a mechanism of low-temperature modification of the interface morphology and duplex structure formation. The model is applied to the case of epitaxial systems such as (001)Ni//(001)NiO and (111)Ni//(001)NiO. According to the calculation, the influence of the epitaxy on the isothermal oxidation growth regimes is stronger for the scale developed on the (111)Ni surface. The numerical calculation of the growth and the residual stresses in the metal–oxide system has been performed taking into account the microstructural duplex morphology of the oxide scale and also the growth deformation due to the interfacial ordering that has been detected by LEED analysis. The variation of the equivalent stress which varies from 0 to 500 MPa during cooling from high oxidation temperature to room temperature is presented together with the stress profiles.

Determination of the Epitaxial Strains of a Thermally Grown Oxide on a Metallic Substrate

A generalization of the Bollmann’s method is used to evaluate the epitaxial strains at the metal/oxide interface. The numerical approach is described and applied to the Ni/NiO system in order to study the result sensitivity to some numerical parameters.

The Effect of Oxide Scale on the Mechanical Behavior of Low Alloyed Steel at High Temperature

The mechanical behavior of oxidized low alloyed steel has been studied at temperatures between 700°C and 900°C. Such a work is carried out to obtain information about the damages which occur in the oxide scales developed on steels during hot rolling process. Actually the mechanical behavior is tested with a four point bending apparatus at high temperature under controlled atmosphere. Some information about mechanical damages have been collected during the mechanical loading thanks to recording of the sample acoustic emission. During this study, two procedures have been used which differ in the thermal scheme used to build the oxide scales. The results obviously show that the procedure has a great influence on the mechanical behavior of the oxidized low alloyed steel. For instance a marked decrease of the stress is observed at constant displacement between the 900°C and 800°C tests of one procedure, as the opposite behavior is observed with the other procedure. Different points are considered to explain such results, as the thermal strains which are only present in one of the two procedures, the phase transformations in the metal and in the oxide.