Papers by Author: Emmanuel Bouzy

Abstract: Rapid cooling of TiAl-based alloy from α phase (disordered hexagonal, A3) generates  phase (ordered tetragonal, L1o) grains through massive transformation nucleating mostly over the α/α grain boundaries. This current work deals with the identification and the validation of different nucleation mechanisms during  massive transformation in TiAl-based alloys. Special attention has been given to the variant selection criteria for the nucleation of the massive structures along different types of α/α grain boundaries. The  massive domains formed along the grain boundaries were analysed using high resolution electron backscattered diffraction (EBSD). Statistical studies were made on different nucleation sites and different mechanisms are proposed. Two–dimensional studies of the nucleation mechanism suggest that the minimization of the interfacial energy could be the predominant criteria during the grain boundary nucleation. In order to verify this nucleation criterion in three-dimensions, serial sections were made and EBSD maps were taken and analysed in each section. The variant selection observed during the nucleation and the growth of the  massive grains is further discussed after getting a broader view under three-dimensional investigations.

Abstract: The nucleation of new grains during annealing in high purity single crystals of Cu-8%wt. Al alloy with initial (112)[111] orientation has been characterized by detailed transmission electron microscopy (TEM) orientation measurements. The samples were channel-die deformed and then recrystallized to form a substructure composed of a mixture of deformation twins, shear bands (SBs) and the recrystallized grains. Substantial progress in understanding the mechanism of texture transformations at the early stages of recrystallization was possible thanks to orientation mapping using TEM. Microtexture analysis of partly recrystallized samples indicates a simple 25-40o (<111> or <112>) relation, most frequently observed during the early stages of recrystallization between single nuclei and one of the two as-deformed groups of components (twins or matrix). As recrystallization proceeds, recrystallization twinning develops strongly and facilitates rapid growth of the recrystallized phase. The first and higher generations of twins systematically tend to obscure the initial crystallographic relation between the deformed state and the recrystallization nuclei.

Abstract: The orientations of the inherited tetragonal (resp. cubic) variants are calculated from the parent hcp orientation in the case of a strict orientation relation. The numbering of the variants is proposed, as well as the misorientations between them. Conversely, a method for calculating the parent hcp orientation from a sufficient number of inherited variants is proposed. It is based on orientation correlating and orientation averaging, and it is particularly useful when the inherited variants are not exactly related to the parent orientation by a strict orientation relation or when the orientations of the inherited volumes slightly vary at different locations of the variant. The method is illustrated by considering the a to g phase transformation taking place in TiAl-based alloys

Abstract: The local crystallography within shear bands (SB) has been examined in a single crystal of {112}<111> orientation of pure copper deformed at 77K by channel-die compression to strains of about 1. Setting up a system for making high-resolution orientation maps using transmission electron microscopy (TEM) has opened new advantageous circumstances for the analysis of orientation changes within SB. This method with spatial resolution higher than 10nm allows the examination of microstructure images composed of nanoscale subcells forming SB. It has been found that for well-developed shear bands, a crystal lattice rotation about <112> direction tends to dominate and this process is usually accompanied by activation of new slip systems. The present work shows that despite the plane strain deformation mode, the mechanism of lattice rotation within emerging SBs may lead to Goss and Brass texture components.

Abstract: Texture and microstructure were investigated after 1 pass equal channel angular extrusion (ECAE) of a single crystal cube oriented Ni billet. Neutron and X-ray diffraction were used to determine global and local textures, respectively. EBSD and TEM investigations were also carried out in order to study fine scale microstructural features. Substantial variations in texture and microstructure, resulting from different efficiencies in the shearing process, were locally recorded through the billet height. The deformation was heterogeneous on both micro and macro scales.

Abstract: We find that a severely rolled FeCo alloy has anomalous enhancement of the rotated-cube {100}<011> texture component and a decrease of the {111} components after annealing, which is contrast to the recrystallization behaviors reported in traditional BCC metals and alloys. The local texture measurements show that two kinds of grains with obviously different orientations, i.e. {100} and {111}, are heterogeneously distributed in the deformed specimen and the migration of high-angle grain boundaries is observed after annealing in the disordering temperature region.

Abstract: Texture and microstructure were investigated after 1 pass equal channel angular extrusion (ECAE) of a single crystal cube oriented Ni billet. Neutron and X-ray diffraction were used to determine global and local textures, respectively. To investigate the fine scale microstructural features EBSD was used. Substantial variations in texture and microstructure resulting from different efficiencies in the shearing process were locally recorded through the billet height.

Abstract: An original approach based on semi-automatic treatment of Kikuchi patterns collected by Transmission Electron Microscopy is used to analyse the a2 an g variant distributions in a quaternary TiAl-based alloy (Ti-48at.%Al-2%Cr-2%Nb), with two lamellar microstructures of different fineness. Statistic data analysis shows that neither g variant orientations nor interface misorientations are randomly distributed within a given grain. The largest deviation from randomness is observed for the coarser microstructure. We also pointed out the presence of g|g subboundaries between same but slightly misoriented variants, as well as the existence of some semicoherent g|g twins accommodated by dislocation lattices. The analysis of the misorientation distributions of g|g and g|a2|g interfaces suggests that a variant selection occurs during growth and coarsening stages of the a ® a2 + g transformation process.

Abstract: The paper is an account of TEM based automatic orientation mapping summarizing more than two years of using the system. Following a brief introduction of the system elements, some representative applications are described. We focus on the characterization of fine-grain materials, mapping of low symmetry materials (metastable chromium carbide) and semi-automatic analysis of misorientations in a fully lamellar polycrystalline (g+a2) TiAl alloy. Moreover, the current state of the TEM based system is discussed and compared to EBSD systems. In particular, the issues of spatial resolution, accuracy, map acquisition time, reliability are considered.