Wolfgang Pantleon

Grain orientation and dislocation patterns

Dislocation patterns have been characterized by transmission electron microscopy and Kikuchi line analysis in pure, polycrystalline aluminium deformed in tension at room temperature in the strain range 0.05–0.34. The angle strain relationship of the dislocation boundaries, their scaling behaviour and the occurrence of similitude show that two dislocation patterns coexist in all grains, albeit, with very different characteristics, dependent on the grain orientation. An analysis of the hardening behaviour of the grains in the polycrystal and a comparison with single crystal behaviour show a similar strong correlation, pointing to the slip pattern as a dominating factor both behind the microstructural evolution and the hardening. The division of the stereographic triangle representing all possible crystallographic orientations at the tensile axis based on microstructural characterization and hardening behaviour, correlates with a division based on slip pattern characteristics.

Revealing deformation microstructures

A variety of features broadly classed as deformation microstructure elements are created during the plastic deformation of polycrystalline metals. While virtually all elements of deformation microstructure are composed of dislocations, describing the creation and evolution of larger-scale elements in terms of interactions between individual dislocations is a goal that has not yet been achieved. A hierarchical approach is thus favored in which structure creation and evolution are described at a range of length scales, from the nanometer to millimeter scale.

3-Dimensional Characterization of Polycrystalline Bulk Materials Using High-Energy Synchrotron Radiation

The implementation of 3-Dimensional X-Ray Diffraction (3DXRD) Microscopy at the Advanced Photon Source is described. The technique enables the non-destructive structural characterization of polycrystalline bulk materials and is therefore suitable for in situ studies during thermo-mechanical processing. High energy synchrotron radiation and area detectors are employed. First, a forward modeling approach for the reconstruction of grain boundaries from high resolution diffraction images is described. Second, a high resolution reciprocal space mapping technique of individual grains is presented.

Characterisation of orientation distributions of individual grains within deformed metals

Abstract An experimental method is presented for determining the orientation distribution function (ODF) for individual grains within deformed polycrystals. The non-destructive method is based on diffraction with monochromatic hard X-rays. First the detected diffraction spots are sorted with respect to their grain of origin. Next, for each grain the ODF is reconstructed by a variant of ART – an iterative algebraic algorithm. First results are reported for tensile deformation of aluminium to 1%. The potential of the method is discussed.

Quantitative characterization of the orientation spread within individual grains in copper after tensile deformation

Abstract By means of electron backscatter diffraction, orientations are determined on a regular grid on a polished section of a copper specimen after tensile deformation to 25 %. Individual grains separated by boundaries with disorientation angles above 7 are identified and the microtexture in the form of the orientation distribution function of each individual grain is analyzed. Extent and shape of the disorientation distribution in orientation space are quantified by the tensor of the second-order central moments, its principal values and directions. The latter characterize the main rotation axes of the orientation spread and the former the extent of the distribution in orientation space along these principal axes allowing a classification of the disorientation distribution into archetypical shapes. The disorientation distributions developed in grains with tensile axes close to [001] or [011] are more anisotropic than expected from their eight possibly activated slip systems indicating a mutual suppression of possibly activated slip systems.

Supercube grains leading to a strong cube texture and a broad grain size distribution after recrystallization

This work revisits the classical subject of recrystallization of cold-rolled copper. Two characterization techniques are combined: three-dimensional X-ray diffraction using synchrotron X-rays, which is used to measure the growth kinetics of individual grains in situ, and electron backscatter diffraction, which is used for statistical analysis of the microstructural evolution. As the most striking result, the strong cube texture after recrystallization is found to be related to a few super large cube grains, which were named supercube grains. These few supercube grains become large due to higher growth rates. However, most other cube grains do not grow preferentially. Because of the few supercube grains, the grain size distribution after recrystallization is broad. Reasons for the higher growth rates of supercube grains are discussed, and are related to the local deformed microstructure.

Disorientations after Severe Plastic Deformation and their Effect on Work-Hardening

Plastic deformation creates orientation differences in grains of originally uniform orientation. These disorientations are caused by a local excess of dislocations having the same sign of the Burgers vector. Their increase with increasing plastic strain is modeled by dislocation dynamics taking into account different storage mechanisms. The predicted average disorientation angles across different types of boundaries are in close agreement with experimental data for small and moderate plastic strains. At large plastic strains after severe plastic deformation, saturation of the measured average disorientation angle is observed. This saturation is explained as an immediate consequence of the restriction of experimentally measured disorientation angles to angles below a certain maximum value imposed by crystalline symmetry. Taking into account the restrictions from crystalline symmetry for modeled disorientation angles does not only lead to an excellent agreement with experimental findings on Ni after high pressure torsion, but also rationalizes the work-hardening behavior at large plastic strains as well as a saturation of the flow stress.

The effect of strain path change on subgrain volume fraction determined from in situ X-ray measurements

The evolution of dislocation structures in individual bulk grains in copper during strain path changes is studied with a new in situ synchrotron technique which combines high angular resolution with fast three-dimensional reciprocal space mapping. Deformed copper contains regions with vanishing dislocation density called subgrains bounded by dislocation rich walls. With the new technique reciprocal space maps, consisting of sharp peaks arising from the subgrains superimposed on a cloud of lower intensity arising from the dislocation walls, are obtained, which allows properties such as subgrain volume fraction to be quantified. The studied strain path changes are tension-tension sequences. Polycrystalline copper sheets are pre-deformed in tension to 5% strain, and tensile samples are cut with varying angles between the first and second loading axis. The second tensile deformation up to additional 5% strain is performed in situ while mapping a selected X-ray reflection from one particular bulk grain with high angular resolution. The reciprocal space maps are analyzed with a recently developed fitting method, and a correlation is found between the evolution of the subgrain volume fraction and the degree of strain path change the sample is subjected to.

Effects of Initial Parameters on the Development of Cube Texture during Recrystallization of Copper

A series of oxygen free high conductivity copper samples with different initial grain sizes, cold rolling conditions and storage times as well as slightly different impurity contents was used to investigate the effects of these initial parameters on the development of cube texture during recrystallization. For rolling reductions of 90% and 95%, cube textures with volume fractions between 3% and 50% were observed. Higher rolling reduction led to a stronger cube texture. Cube texture development is very sensitive to the initial grain size before rolling. In general, fine grained material gives a strong cube texture after recrystallization, and the requirement on fineness of the grain size may vary for materials with different purity. Large sample widening during rolling can largely inhibit the development of cube texture after recrystallization. Neither storage time, nor the slight change in impurity content had large effects in the present investigation.

Effect of dynamic plastic deformation on microstructure and annealing behaviour of modified 9Cr−1Mo steel

Abstract The effect of dynamic plastic deformation on the microstructure of a modified 9Cr−1Mo steel has been investigated in comparison with the effect of quasi-static compression. It is found that the boundary spacing after dynamic plastic deformation is smaller and the hardness is higher than those after quasi-static compression. The microstructure after dynamic plastic deformation is however less stable than the microstructure after quasi-static compression. Annealing at 675 and 700°C leads to structural coarsening and recrystallisation in each sample, but with recrystallisation occurring faster in the sample annealed after dynamic plastic deformation. The lower thermal stability of the microstructure produced by dynamic plastic deformation is attributed to a higher driving force for recrystallisation in the dynamically deformed material.