Papers by Author: Etienne Patoor

Abstract: The Kossel microdiffraction in a scanning electron microscope allows for local stress determination. This technique has been applied to monitor stress evolution within grains of austenite in the course of martensitic transformation in a shape memory alloy. Kossel diffraction patterns were recorded during in situ tensile straining of Cu-Al-Be alloy. These innovative measurements show large stress heterogeneities between grains, with the stress ratio exceeding two. As martensite variants are stress-induced, shear stress components appear in individual grains of austenite.

Abstract: The thermomechanical behavior of Shape Memory Alloys (SMAs) is described by many micromechanical and phenomenological models. The first ones have material parameters whose physical meaning is based on the crystallography of the phase transformation related to the studied alloy. In contrast, phenomenological models often have material parameters whose physical meaning is not obvious and that makes them difficult to identify, some of which are based on mathematical considerations. In this paper, we propose to use the formulation of the phenomenological model of Chemisky et al., and to consider the particular case of a superelastic SMA. In this case, the constitutive equation should be easily expressed analytically through the strain tensor as a function of applied load direction and material parameters. The behavior is then characterized by a complete and proportional loading. This analytical model contains 8 material parameters, 2 related to the elasticity and 6 to the phase transformation. Based on several isothermal tensile tests at various temperatures, material parameters of this model are identified using the Levenberg-Marquardt algorithm and an analytical calculation of the sensitivity matrix. Their physical meaning and their influence on the thermomechanical behavior of the alloy studied are highlighted and discussed.

Abstract: In this study, the internal stress evolution of the ferrite phase of 16MND5-A508 has been determined using X-Ray Diffraction (XRD). The results of in situ tests combined with XRD analyses and performed at different temperatures (-150°C and 22°C) exhibit a difference of about 200MPa between the macroscopic stress and the ferrite one. The stress state in the cementite is determined by a mixture law; it reaches very high values up to 9000MPa. These results highlight the need to analyze the stress directly in the cementite phase by using appropriate tools, since its volume fraction does not allow it using XRD.

Abstract: A Kossel microdiffraction experimental set up is under development inside a Scanning Electron Microscope (SEM) in order to determine the crystallographic orientation as well as the inter- and intragranular strains and stresses. An area of about one cubic micrometer can be analysed using the microscope probe, which enables to study different kinds of elements such as a grain boundary, a crack, a microelectronic component, etc. The diffraction pattern is recorded by a high resolution Charge-Coupled Device (CCD) camera. The crystallographic orientation, the lattice parameters and the elastic strain tensor of the probed area are deduced from the pattern indexation using a homemade software. The purpose of this paper is to report some results achieved up to now to estimate the reliability of the Kossel microdiffraction technique.