Jean-Lou Lebrun

The determination of stresses during oxidation of Ni: in situ measurements by XRD at high temperature

Abstract In order to characterize the respective importance of growth stresses, thermal stresses and stress relaxation developed in oxide scales, two high temperature chambers for X-ray diffraction were designed. Stresses were determined in the Ni/NiO system, in both the oxide and the substrate, with the sin 2 ψ technique, during all oxidation steps. At room temperature after oxidation, NiO is subjected to compressive stresses whose level depends on the substrate thickness and on the oxidation time and temperature. In the substrate, compressive stresses are mainly due to internal oxidation. During oxidation at 900°C, the oxide scale is subjected to slight tensile stresses which can be due to partially anionic diffusion, internal oxidation or temperature gradients. During heating-cooling sequences, the stresses in the scale decrease with increasing temperature and become negligible when the oxidation temperature is reached. The reversibility of the stress-temperature curve indicates that no stress relaxation occurs. The stresses found at room temperature are only due to thermal stresses and fit well with the theoretical calculation of thermal stresses in the NiO scale, based on newly-determined thermal expansion coefficients of Ni and NiO. These results show that stresses found at room temperature are mainly generated during cooling and that the role of the Pilling and Bedworth ratio is negligible.

A study of the influence of the metallurgical state on shear band and white layer generation in 100Cr6 steel: application to machining

The aim of this paper is to better understand the material behaviour involved in machining operations. During machining, the workpiece experiences large strains, high strain rate, high temperatures, complex loading histories, and recovery. To reproduce these loadings and to understand the behaviour of 100Cr6 bearing steel, quasi‐static and dynamics mechanical shearing tests were carried out. These tests made it possible to reproduce the primary shear zone observed on the chips after cutting using specimens with special geometries “hat‐shaped specimens”. The geometry of these specimens results in a localised shearing zone when loaded in compression. Two metallurgical states of the material were investigated (with and without carbides). For each state, three material hardnesses are used (46, 51 and 55HRc). The tests parameters investigated were the strain rate and temperature. For all tests, the microstructures of the shear zones were examined. Results show that the presence of carbides has the tendency to ...

Composition and structure of NiTiCu shape memory thin films

Preliminary results on NiTiCu shape memory alloy thin-film deposition on silicon are reported. Characteristics of these films (composition, density and electrical resistivity) were correlated with working gas pressure and rf power during sputtering. The films were then annealed in order to crystallize them. The properties of these films were examined as functions of annealing and measurement temperatures in order to evidence shape memory behaviour.

Thermal and thermo-mechanical simulation of laser assisted machining

Laser Assisted Machining (LAM) improves the machinability of materials by locally heating the workpiece just prior to cutting. The heat input is provided by a high power laser focused several millimeters in front of the cutting tool. Experimental investigations have confirmed that the cutting force can be decreased, by as much as 40%, for various materials (tool steel, titanium alloys and nickel alloys). The laser heat input is essentially superficial and results in non‐uniform temperature profiles within the depth of the workpiece. The temperature field in the cutting zone is therefore influenced by many parameters. In order to understand the effect of the laser on chip formation and on the temperature fields in the different deformation zones, thermo‐mechanical simulation were undertaken. A thermo‐mechanical model for chip formation with and without the laser was also undertaken for different cutting parameters. Experimental tests for the orthogonal cutting of 42CrMo4 steel were used to validate the sim...