Papers by Author: Brigitte Bacroix

Abstract: The goal work of this work is to describe the qualitative and quantitative behaviour of titanium T40 during tensile test. Material characteristics were determined using EBSD and X-ray techniques. Textures, twin boundary fractions, residual stresses and coherent domain size were determined. It was found that deformation mechanisms and microstructure characteristics are different in the samples stretched along rolling and transverse directions. For example the average grain size, as determined from EBSD measurements, is higher in the sample stretched along rolling direction. Also smaller coherent domains form and residual stress is more easily relaxed in this sample. A strong appearance of tensile twins was observed in the samples deformed along transverse direction. In the present paper a complex study of material characteristics and deformation mechanisms is presented. A special emphasis is done on residual stress characteristics determined in the samples stretched in two perpendicular directions.

Abstract: To get a quantitative estimate of residual stresses in polycrystals from XRD measurements, a micromechanical modeling is required, except in particular cases. The most widely used method is only valid for homogeneous and isotropic samples. We present here the possibility to determine residual stresses by coupling measurements with the portable INEL^TM Xsolo equipement with a self-consistent polycrystalline model. This methodology may take into account texture and intergranular stresses induced by thermomechanical treatments. One example obtained for titanium subjected to tensile loading illustrates the methodology.

Abstract: Asymmetric rolling can be used in order to modify material properties and to decrease forces and torques applied during deformation. This geometry of deformation is relatively easy to implement on existing industrial rolling mills and it can provide large volumes of a material. The study of microstructure, crystallographic texture and residual stress in asymmetrically rolled titanium (grade 2) is presented in this work. The above characteristics were examined using EBSD technique and X-ray diffraction. The rolling asymmetry was realized using two identical rolls, driven by independent motors, rotating with different angular velocities w₁ and w₂. It was found that asymmetric rolling leads to microstructure refinement, texture homogenization and lowering of residual stress.

Abstract: The goal of this work was to study the asymmetric rolling process using the Finite Element Method (FEM) coupled with the deformation model of polycrystalline material. The Leffers-Wierzbanowski (LW) model was selected to be implemented into FEM. This implementation enables a study of heterogeneous plastic deformation process, like asymmetric rolling, taking into account its crystallographic nature. The asymmetric rolling was realized using two identical rolls, driven by independent motors, rotating with different angular velocities. This enabled to obtain a controlled range of rolling asymmetry. Our aim was to examine the properties of asymmetrically rolled commercially pure titanium (Grade 2).

Abstract: Asymmetric rolling is a promising forming technique offering numerous possibilities of material properties modification and the improvement of technological process parameters. This geometry of deformation is relatively easy to implement on existing industrial rolling mills. Moreover, it can provide large volume of a material with modified properties. The study of microstructure, crystallographic texture and residual stress in asymmetrically rolled titanium (grade 2) is presented in this work. The above characteristics were examined using EBSD technique and X-ray diffraction. The rolling asymmetry was realized using two identical rolls, driven by independent motors, rotating with different angular velocities ω₁ and ω₂. This ensured a wide range of rolling asymmetry: A=ω₁/ω₂. It was found that a strong shear stress induced in the asymmetrically rolled material allowed to obtain a microstructure refinement, texture homogenization and lowering of residual stress.

Abstract: EBSD investigation of texture and microstructure evolution during a complete thermomechanical treatment of commercially pure titanium (HCP-Ti) is presented. Titanium was cold rolled to reach various degrees of thickness reduction: 20%, 40% and 60%. Next, annealing in air atmosphere was conducted at different conditions to achieve the recrystallized state. EBSD topological maps were measured on RD-TD and RD-ND surface of each sample. Strong heterogeneity of deformed titanium microstructures is described with focus on the important role of twinning mechanisms. Texture evolution in investigated titanium appears to be limited, especially in recrystallized state. However some subtle mechanisms are discussed.

Abstract: The analysis of deformed and recrystallized zirconium, used in nuclear industry is presented. The main purpose of the present work is to describe and analyze changes in texture, microstructure and misorientation profile, which are observed during a complete thermomechanical treatment. Zirconium samples were channel compressed till various degrees of deformation. The samples were then annealed. In both deformed and recrystallized states, topological maps were measured using the well known EBSD (Electron Backscatter Diffraction) technique. The obtained data were necessary for further analysis which consisted of several approaches: quantification of textures, identification of principal components, volume fractions, misorientations, grain size and IQ histograms. For the misorientation analysis, several parameters have also been tested (ie. correlated misorientation profile, grain average misorientation, kernel, etc.). Based on this complete set of data a scenario is proposed to explain the observed microstructural evolutions.

Abstract: The classical definition of lattice rotation leads in some cases to different textures than the definition based on the preservation of orientations of selected sample directions and/or planes. For example, if classical {111} slip is taken into account for f.c.c. materials, the former approach enables to predict both copper and brass types of rolling texture, while classical approach predicts only the first one. The analysis of rolling texture formation is done for two types of lattice rotation in function of grain-matrix interaction parameter used in a deformation model. Predicted textures and correlation factors estimating the similarity of predicted and experimental textures are presented.

Abstract: The effect of the metallurgical state of austenite (undeformed vs. deformed vs. deformed + recrystallised) on the properties of the austenite to bainite transformation were investigated thanks to thermal (Gleeble simulations) and thermomechanical (hot torsion) treatments. No obvious influence of the state of austenite was found, using electron backscatter diffraction, on the resulting microtexture. Advantages and drawbacks of using misorientation angle histograms vs. axis-angle pair distribution are discussed regarding investigations of local variant selection. For an austenite grain size higher than about 50 µm, a strong effect of the transformation temperature was evidenced, bainite formed at lower temperature (530°C) exhibiting a microtexture close to that of lath martensite in the same steel.

Abstract: A micromechanical model was developed to account for the particular microtexture of upper bainite in low alloy steels, i.e. the non-random spatial distribution of variants within a given former austenite grain. A self-consistent scheme and an Eshelby approach, considering both transformation shape strain and viscoplastic strain as eigenstrains, was applied to estimate coupling between parent austenite and two or more bainite variants without any applied stress. Model predictions concerning self-accommodation between variants are sensitive to the plane of the first “lattice invariant shear” in the crystallographic model used to determine the shape strain. No obvious effect of the constitutive equations of phases and of the other model parameters was found.