D. Juul Jensen

Three-dimensional maps of grain boundaries and the stress state of individual grains in polycrystals and powders

A fast and non-destructive method for generating three-dimensional maps of the grain boundaries in undeformed polycrystals is presented. The method relies on tracking of micro-focused high-energy X-rays. It is verified by comparing an electron microscopy map of the orientations on the 2.5 × 2.5 mm surface of an aluminium polycrystal with tracking data produced at the 3DXRD microscope at the European Synchrotron Radiation Facility. The average difference in grain boundary position between the two techniques is 26 µm, comparable with the spatial resolution of the 3DXRD microscope. As another extension of the tracking concept, algorithms for determining the stress state of the individual grains are derived. As a case study, 3DXRD results are presented for the tensile deformation of a copper specimen. The strain tensor for one embedded grain is determined as a function of load. The accuracy on the strain is Δ∊ ≃ 10−4.

Microstructures and boundary populations in materials produced by equal channel angular extrusion

Two aluminum alloys and two copper samples heavily deformed by equal channel angular extrusion (ECAE) were investigated. In these materials, microstructures and misorientation distributions were characterized in the transmission electron microscope. Deformation structures with a mixture of low- and high-angle boundaries were found in all samples. After a similar strain, low-angle boundaries were more frequent in copper than in the Al alloys. Boundary populations and spatial characteristics of different boundary types in ECAE-processed materials are compared with those produced by rolling.

Growth rates and misorientation relationships between growing nuclei/grains and the surrounding deformed matrix during recrystallization

Average growth rates and misorientations between recrystallization nuclei (or grains) and neighbouring deformed matrix material have been studied for partially recrystallized samples by the electron back scattering pattern (EBSP) technique in heavily cold rolled aluminium and copper. It was studied how the annealing time and the crystallographic orientation of nuclei/grains affects the growth rates and distribution of misorientations. The two materials, aluminium and copper, develop a weak and a strong recrystallization cube texture respectively. Information about effects of cube texture strength was therefore also obtained. It was found that grains of cube orientation grow faster than grains of other orientations. A wide distribution of misorientation relationships was observed to exist between the growing grains and the neighbouring deformed matrix, and this distribution was not significantly affected by the annealing time. The faster growth of the cube oriented grains may be ascribed to a larger misorientation between cube grains and deformed matrix than that between other grains and the matrix.

Large strain deformation structures in aluminium crystals with rolling texture orientations

Abstract Four single crystal orientations of high purity aluminium have been deformed in channel die compression up to strains of ∼ 1 to correlate the dislocation substructures, in single and polycrystals with the slip system distribution. Three orientations are close to the stable rolling texture components of fcc metals: (110)[ 1 12], (112)[11 1 ] and (112)[ 17 4] and one is very unstable (121)[ 3 11] . The substructures are characterized on the longitudinal section over a wide range of scales by optical microscopy. TEM and SEM with EBSD. Low energy dislocation matrix structures composed of cells, cell blocks, dense dislocation walls and first generation microbands are observed in all orientations in agreement with the microstructures of rolled polycrystals. The S (213)[ 14 2] and C (112)[11 1 ] orientations also develop narrow bands of localized glide associated with relatively high local misorientations. The S orientation exhibits characteristic S-shaped band structures of first generation microbands sheared on {111} planes whereas the C orientation forms non-crystallographic shear bands. These two orientations can be considered stable in terms of average texture but unstable in terms of microstructure.

Quantitative analysis of grain subdivision in cold rolled aluminium

A procedure is proposed for a statistical characterisation of deformed microstructures from the data collected using Orientation Imaging Microscopy (OIM). This procedure has been applied for a characterisation of 352 grains in commercial purity aluminium cold-rolled to a reduction of 40%. The results demonstrate the different behaviour of grains with different orientations: (i) grains having orientations near the {001} and {025} components develop orientation gradients over distances of 10–20 μm; (ii) grains with orientations close to the {205} component form fragments with relatively large misorientations, and (iii) grains within the β-fibre form fragments with relatively small misorientations. The experimental results are compared to previous observations by transmission electron microscopy (TEM) and good agreement is found. Finally, possible applications of the present observations for advanced texture modelling are discussed.

Flow stress anisotropy in aluminium

The plastic anisotropy of cold-rolled high purity aluminum (99.996%) and commercially pure aluminum (99.6%) has been investigated. Sample parameters were the initial grain size and the degree of plastic strain (ϵ < 3.00). Flow stresses (0.2% offset) were measured at room temperature by uniaxial tension as a function of the angle between the tensile axis and the rolling direction. Textures were determined by neutron diffraction, and Taylor M-factors were calculated. The microstructures were studied by TEM. It was found that the flow stress varies significantly with orientation both at low and high strains. It is shown that for most experimental conditions, texture effects alone cannot explain the observed anisotropy, and microstructural anisotropy effects have to be taken into account. In those cases, a correlation between the microstructural anisotropy and the development of microbands is discussed.

Applications of high-energy synchrotron radiation for structural studies of polycrystalline materials.

The large penetration power of high-energy X-rays (>60 keV) raises interesting prospects for new types of structural characterizations of polycrystalline materials. It becomes possible in a non-destructive manner to perform local studies, within the bulk of the material, of the fundamental materials physics properties: grain orientations, strain, dislocation densities etc. In favourable cases these properties may be mapped in three dimensions with a spatial resolution that matches the dimensions of the individual grains. Imbedded volumes and interfaces become accessible. Moreover, the high energies allow better in-situ studies of samples in complicated environments (industrial process optimization). General techniques for research in this energy range have been developed using broad-band angle-dispersive methods, on-line two-dimensional detectors and conical slits. Characterizations have been made at the level of the individual grains and grain boundaries as well as on ensembles of grains. The spatial resolution is presently of the order of 10-100 micom. Four examples of applications are presented along with an outlook.

Microstructural path and temperature dependence of recrystallization in commercial aluminum

Abstract The isothermal recrystallization of 90% cold-rolled commercial purity aluminum alloy AA1050 was studied by means of quantitative microscopy at four temperatures from 245°C to 280°C. The microstructural properties, V v , the volume fraction recrystallized, S v , the interfacial area density separating recrystallized grains from deformed grains and 〈 λ 〉, the mean recrystallized grain free length, were measured stereologically as a function of time. The kinetics, microstructural path, grain boundary migration rates and temperature dependence of recrystallization were quantified experimentally. Based on analysis of all data and microstructural path modelling, recrystallization was determined to be growth (boundary migration rate) controlled; all nucleation occurred in time periods short compared to the earliest annealing times. The activation energy for grain boundary migration was calculated to be 172–183 kJ/mole suggesting that a solute-limited grain boundary migration rate mechanism was operative in the alloy. The recrystallization microstructural path was found to be isokinetic, i.e. identical at all the annealing temperatures studied. Two stages of recrystallization kinetics were observed; an early transient-like stage characterized by decreasing growth rates and a later stage in which the kinetics approached Avrami behavior and the growth rates were approximately constant. The transient-like behavior is attributed to the steep, deformation-induced stored energy gradients surrrounding precipitate particles where the recrystallized grains are nucleated.

Slip pattern, microstructure and local crystallography in an aluminium single crystal of brass orientation {110}(112)

Abstract The evolution of microstructure and local crystallography has been examined in pure Al single crystals of {110}〈112〉 orientation (brass or B-orientation) deformed by channel die compression at room temperature to logarithmic strains of ϵ =0.5 and ϵ =1.5. A homogeneous cell block deformation microstructure is seen at the low strain. At higher strain the cell blocks are less distinct and some evidence of strain localisation is observed. A shallow texture gradient is seen, corresponding to a lattice rotation about [101] and consistent with slip occuring on just the two most highly stressed slip systems. However an analysis using the Frank formula of the dislocation content for some of the observed boundaries suggests that a significant amount of slip takes place on systems not predicted by a plasticity analysis. This unpredicted slip stabilises the dislocation walls on planes other than those predicted using the Frank formula.

The deformation behaviour of grain boundary regions in polycrystalline aluminium

Abstract The deformation pattern at grain boundaries and at triple junctions in polycrystalline high purity aluminium (99.999%) has been studied by electron back scattering pattern (EBSP) observations. Specimens of two different grain sizes rolled to give 5% and 30% reductions have been examined by these different EBSP scans: (i) scans across grain boundaries, (ii) scans along grain boundaries and (iii) two-dimensional scans near triple junctions. These scans are carried out in small steps (1–5 μm) over long distances (up to 50 μm). The EBSP measurements show that the level of perturbations increases with strain and that enhanced zones of perturbations are observed at grain boundaries and especially near triple junctions. In specimens deformed by 30%, such zones of large perturbation are observed at most of the grain boundaries, and in the specimens deformed by 5%, at some triple junctions. The EBSP measurements are compared to previous microstructural observations by transmission electron microscopy and ...