Papers by Author: Julian H. Driver

Abstract: The objective of this paper is to identify the predominant crystallographic relations between deformed state and recrystallized grains during the early stages of recrystallization of fcc metals with medium and low stacking fault energy. The experimental investigations, based on SEM/EBSD measurements, have focused on the transformations which occur in plane strain compressed single crystals with stable orientations. After annealing the disorientation across the recrystallization front 'defines' the final rotation by angles in the ranges of 25-35^o and 45-55^o around axes mostly grouped near the <122>, <012>, <112> and <111> directions located around the normals of all four {111} slip planes.

Abstract: Two major types of Cube bands/segments have been observed in heavily (90%) cold rolled Al-0.1wt %Mn using the EBSD technique with a FEG SEM: i) intergranular transition bands as thin cube segments aligned along RD between S and Cu oriented grains and ii) as transgranular strain localized bands situated in some particular grains. Their evolution is studied by light annealing at 275°C and 300°C and EBSD observations of exactly the same areas to directly correlate local deformation substructure with recrystallization. Only the intergranular cube transition bands give rapid recrystallization nucleation to cube grains of dimension >10µm. In particular the fastest growing cube grains have a near 40°<111> relation with part of their surroundings.

Abstract: A focused ion beam (FIB) coupled with high resolution electron backscatter diffraction (EBSD) has emerged as a useful tool for generating crystallographic information in reasonably large volumes of microstructure. In principle, data generation is reasonably straightforward whereby the FIB is used as a high precision serial sectioning device for generating consecutive milled surfaces suitable for mapping by EBSD. The successive EBSD maps generated by serial sectioning are combined using various post-processing methods to generate crystallographic volumes of the microstructure. This paper provides an overview of the use of 3D-EBSD in the study of various phenomena associated with thermomechanical processing of both crystalline and semi-crystalline alloys and includes investigations on the crystallographic nature of microbands, void formation at particles, phase redistribution during plastic forming, and nucleation of recrystallization within various regions of the deformation microstructure.

Abstract: Two, high purity, Al-0.1 and 0.3wt%Mn alloys have been cold deformed in plane strain compression to strains of order 1.8 and the kinetics of subsequent recovery by sub-grain coarsening during annealing at 150-300°C measured by high resolution FEG-SEM EBSD. Accurate sub-grain size and misorientation distributions and their evolution with time and temperature have been determined. The average growth rates are then used to estimate the sub-grain boundary mobilities. Growth is analyzed by two well-known growth laws for the average sub-grain size δ (t): i) the standard relation for grain growth: where the exponent n takes values of 2-8 and ii) the relation proposed by Nes for dislocation climb in sub-grain walls: It is shown that the latter relation gives a better fit with the data in terms of the time and temperature dependence of the sub-grain sizes. In particular the activation energies for the logarithmic law are much closer to the values expected for solute-controlled movement of sub-boundaries.

Abstract: The formation of Cube oriented elements in plane strain compressed aluminium has been studied by EBSD for both hot and cold deformations. By following the orientation changes of the same set of 176 grains deformed at 400 °C up to a strain of 1.2 using a split sample, it is shown that about 15% of the grains can break up into several regions of very different orientations, characterized by very large orientation gradients. In particular those grains oriented within about 30° of Cube develop Cube oriented zones in contact with other rolling texture components. Finite element crystal plasticity simulations confirm this mechanism of creation of Cube by plastic deformation. The same type of microstructure can also be observed after heavy cold rolling (strain of 2.3), but at a scale that is much finer by at least an order of magnitude. In this case the micron-sized Cube fragments are located along many grain boundaries or in some particular grains. When the cold deformed sample is annealed, EBSD observations of the same areas reveal that the intergranular Cube fragments are very efficient recrystallization nucleation sites, apparently since they possess mobile high angle boundaries with the local environment.

Abstract: A 2-dimensional vertex dynamics simulation is applied to the annealing behaviour of deformed Aluminium single crystals having different orientations. It is observed in experiments that deformed single crystals of different orientations - typically the common rolling textures like Goss (110)[001], Brass (110)[1-12], Cube (001)[100] – exhibit remarkably different rates of recovery. It is suggested that this difference arises from the deformation microstructures, with sub-grain boundaries of various misorientation values. The sub-grain boundary mobilities and energies, being strong functions of the boundary misorientation, thus affect the recovery rates. This effect is illustrated using vertex dynamics simulation on the same orientations and schematic deformation substructures as above. Good agreement is obtained for the orientation dependency of recovery.

Abstract: The development of orientation spreads within individual grains of a polycrystal submitted to large deformations is analysed by both experiment and simulation. In the experiment, 176 grains on an internal surface of a split sample were followed by detailed EBSD measurements, at successive strains up to 1.2. In parallel, a high-resolution finite element simulation has been carried out on the same polycrystal configuration. For both experiment and simulation, hundreds to thousands of orientation values were obtained in each grain. Most grains showed a “unimodal” rotation, composed of an average rotation and an orientation spread. The experimental and simulated orientation spreads were compared through different statistical metrics. The average lattice disorientations are found to increase rapidly at the beginning of the deformation and to saturate at high strains. The orientation spreads are also analysed in terms of anisotropy along the sample axes. It is shown that the orientation spreads are aligned preferably along TD at the beginning of the deformation, then tend to move to RD in the experiment, and RD or ND in the simulation.

Abstract: The crystallographic alignment of microbands in a Goss oriented single crystal was investigated by two and three dimensional electron back scatter diffraction. The microband boundaries were found to be curved instead of being perfectly flat interfaces, and the overall alignment closely matched a potential slip plane. The bumps and curved were created during subsequent deformation and, thus, deviates the microband boundaries from crystallographic nature.

Abstract: The relations between the active slip systems within the deformed state and the orientations of the first formed nuclei of recrystallized grains have been studied in strongly deformed samples of high purity aluminium single crystal and polycrystalline aggregates. The new grain orientations and misorientations were followed by systematic local orientation measurements using SEM-FEG/EBSD and semi-automatic measurements in TEM. Investigations of the direction of growth indicate the privileged role of {111} planes in the initial stages of recrystallization. The grain boundary migration and ‘consumption’ of the as-deformed areas was always favoured along directions parallel to the traces of the most active {111} slip planes as shown clearly in the stable and structurally homogeneous Goss{110}<001> oriented crystallite. In the case of polycrystalline material the influence of grain boundaries and their triple junctions on the crystallographic aspects of nucleation is discussed.

Abstract: This paper described new characterization methods and data to quantify the influence of solute atoms on grain boundary and sub-grain boundary mobilities in Al-Mn alloys with a view to their integration into recovery and recrystallization modelling. Detailed SEM measurements of grain boundary mobilities during recrystallization have been made by in-situ annealing experiments on cold deformed Al – 0.1 and 0.3wt.% Mn binary alloys. Stored energies are estimated from the sub-grain sizes and misorientations and the boundary velocities directly measured in the temperature range 200-450°C. It is shown that in many cases good agreement with the Cahn, Lücke, Stüwe model for solute drag is obtained, e.g. the activation energies are intermediate between those of boundary and volume solute diffusion. Some particular cases of rapid growth occur in Al-0.1%Mn indicating boundary breakaway from solute clouds. A complementary study of sub-grain boundary mobilities has started on the same alloys; in this case the average mobilities are estimated from FEG-SEM growth data for the average sub-grain size for temperatures in the range 150-300°C. The results are compared with some previous data on Al- Si and show similar rates.