Papers by Author: Thierry Baudin

Abstract: The results of studies carried out on AA1200 aluminum alloy deformed by Accumulative Roll Bonding (ARB) are presented in this paper. The commercial purity material was deformed up to 10 cycles (equivalent plastic strain of 8) at room temperature. The deformed microstructures and the crystallographic textures were characterized by transmission (TEM) and scanning (SEM) electron microscopes and high resolution orientation mapping. It was found that increased deformation leads to a strong increase of quantity of high angle (>15°) grain boundaries and strong grain refinement (up to 200-300 nm). The microstructure observations and TEM and SEM local orientation measurements allowed identifying fine and strongly disoriented planar dislocation structure of nanolayers described by strong texture components close to two nearly complementary positions of {112} orientation.

Abstract: The Armco iron is one of the purest commercial iron with very low levels of carbon, oxygen and nitrogen. In order to improve the mechanical properties, it is worth applying severe plastic deformation to obtain ultrafine-grained bulk materials, with grain size lower than 1 μm. In this study, samples of Armco iron were subjected to a technique of severe plastic deformation named Accumulative Roll Bonding (ARB). This method consists in rolling to 50% two sheets pack of which the stacked surfaces were initially cleaned. Then, the rolled strip is sectioned in two halves, cleaned and stacked again and the procedure of roll-bonding repeated. Practically, the process can be repeated without limits. The important parameter of ARB is the number of cycles and then the consequent number of layers of the final sample. By means of the Electron Backscattered Diffraction (EBSD) technique, the evolution of both microstructure and texture as regard to the number of ARB cycles was studied. The analysis of mean grains size and high angle grain boundaries (HAGB) fraction as a function of the number of cycles showed an early formation of a subgrained structure with low angle boundaries and then the evolution of the microstructure towards an ultrafine-grained structure with an increase of HAGB.

Abstract: This investigation uses electron backscatter diffraction (EBSD) to study the development of microtexture with increasing deformation in an AlMgSi alloy having an initial grain size of about 150 µm subjected to high pressure torsion (HPT) up to a total of 5 turns. An homogeneous microstructure was achieved throughout the disc sample at high strains with the formation of ultra-fine grains. Observations based on orientation distribution function (ODF) calculation reveals the presence of the torsion texture components often reported in the literature for f.c.c. materials. In particular, the C {001}<110> component was found to be dominant. Furthermore, no significant change in the texture sharpness was observed by increasing the strain.

Abstract: The Armco iron is one of the purest commercial iron with very low levels of carbon, oxygen and nitrogen. In order to improve the mechanical properties, it is worth applying severe plastic deformation to obtain ultrafine-grained bulk materials, with grain size <1µm. In this study, samples of Armco iron were subjected to a technique of severe plastic deformation named Accumulative Roll Bonding (ARB). The important parameter of ARB is the number of cycles and then the von Mises equivalent strain. By means of the Electron BackScattered Diffraction (EBSD) technique, the texture evolution with the number of cycles was studied. The microhardness was also measured in function of the equivalent strain. Finally, the mean grain size and the fraction of high angle grain boundaries were determined as a function of the number of cycles.

Abstract: This paper describes nucleation and grain coarsening at very early stages of recrystallization in AA3104 aluminum alloy containing complex structure of second phase particles. Measurements of individual sub-cell orientations in ECAP-processed and slightly annealed structures were possible by using TEM-based orientation mapping in combination with recrystallization carried out in the microscope. The results were compared with those obtained by SEM/EBSD system. The investigation shows that the 40^o<111>-type relation occurs rarely, and the rotation axes of misorientations between deformed and recrystallized areas were strongly scattered. Grouping of the misorientation axes near the <012>, <221>, <112> and <110> crystallographic directions was noticeable.

Abstract: The goal of the present study inspired by previous works on high purity aluminiun was to manufacture aluminium sheets of commercial purity, grade 1050, with a strong cube texture. In this preliminary work on AA1050, sheets which cube volume fraction reaches 65% have been manufactured. Parameters controlling cube orientation development are mainly the solute dragging due to impurities in solid solution and the stored deformation energy. Besides the 85% cold rolling (CR), two extra annealings and a slight cold rolling are introduced in the processing route to increase the cube volume fraction. The cube orientation, whose substructure is equiaxed, is important for its recovery. It develops thanks to the difference of stored energy relative to that of its first neighbors; the slight cold rolling enhances growth of these cube grains.

Abstract: A Ni-5.7%Cr-25.2%W (wt%) alloy was deformed by cold rolling in different reduction conditions (50%, 70%, and 90%) and then annealed under hydrogen atmosphere. Microstructure and texture evolutions were analyzed using Electron BackScattered Diffraction (EBSD). Orientation Distribution Functions (ODFs) and stored energy were calculated from neutron diffraction measurements. A strengthening of the α-fiber texture was observed after 90% cold rolling and a homogenous microstructure was obtained after annealing at 900°C.

Abstract: Abstract. Anisotropy of physical and mechanical properties of textured polycrystalline materials strongly depends on microstructural characteristics, such as subgrain sizes, lattice deformations, etc. Generalized Pole Figures (GPF) are an attempt to estimate the anisotropy of these properties; so, the energy stored during plastic deformation is a key parameter in primary recrystallization. In this work, the technique to measure GPF (measurements and software) was implemented for X-Ray diffraction and applied to study of property anisotropy of a Fe50%Ni alloy. GPF’s of texture, crystallite size, stored energy and diffraction peak shift, (among others) have been characterized. The Full Width at Half Maximum (FWHM) of obtained instrumental functions shows that defocusing is significant for polar angle higher than 50°. The mixing parameter of the pseudo-Voigt function using in fitting, presents important dispersions.

Abstract: An experimental study of compression tests at high temperature and different engineering strains was carried out on INCONEL 718 above the delta phase solvus. The objective is to investigate the mechanical behaviour in relation with the microstructure evolution. After deformation the samples were quenched with helium gas to avoid metadynamic recrystallization (MDRX). The quench efficiency is discussed by microstructural and hardness comparison. During forging process and without MDRX, there is generally a competition between deformation and dynamic recrystallization state (DRX) i.e. a dependence on dislocation density increase and dislocation annihilation, respectively. To investigate this competition, samples are characterized at different scales by EBSD method to determine local texture and grain size and by TEM to understand the dislocation evolution and determine the nucleation mechanism. In parallel, a numerical model using a three-dimensional finite element model of crystalline plasticity (CristalECP) has been developed in ABAQUS™ finite element code and coupled with a Recrystallization Cellular Automaton (CA_ReX). Results of forging process simulations are compared to those of experimental studies presented before and then discussed in terms of evolution.

Abstract: The processing of bulk metals through the application of severe plastic deformation provides the opportunity for introducing significant grain refinement into bulk solids. In the present investigation, an aluminum alloy (Al-6061) was processed by high-pressure torsion (HPT) at room temperature under an applied pressure of 6.0 GPa up to a total of 5 turns. Detailed measurements after processing revealed the occurrence of continuous grain refinement and material strengthening with increasing imposed strain. The average grain size of the alloy was reduced from ~150 m to a grain size in the range of ~500 nm through processing by HPT. Although there was a difference in the average grain size of samples processed to different levels of imposed strain, careful inspection showed that the structures became similar after annealing at 250°C for 5 min. This suggests that the additional grain refinement introduced at large amounts of deformations is less stable at high temperatures. The results of this investigation, including the distributions of the grain sizes after annealing, are consistent with the predictions of a model based on the occurrence of continuous recrystallization in aluminum alloys having fine grain structures, large fractions of high-angle grain boundaries and where there is a large amount of deformation.