A. Godfrey

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.

Scaling of the spacing of deformation induced dislocation boundaries

Abstract Methods for determining the distribution of spacings between near-planar dislocation boundaries are discussed. Subsequently, distributions of spacings for these boundaries have been determined for both single crystal and polycrystal samples. Misorientations across the extended planar dislocation boundaries were also measured for the single crystal samples. The probability density distribution for spacings exhibits a scaling behavior of nearly identical form to that seen for the probability density distribution of misorientation angles across these boundaries. The scaling behavior of the spacing distributions persists over a very wide strain range ( e vM =0.2– e vM =4.5) for different materials and deformation conditions. The scaling behavior of the boundary spacings can be accounted for by simple geometric models allowing for either formation or coalescence of these walls.

Three-Dimensional Orientation Mapping in the Transmission Electron Microscope

Electron microscopy is used to nondestructively map the three-dimensional grain orientations in nanocrystalline aluminum. Over the past decade, efforts have been made to develop nondestructive techniques for three-dimensional (3D) grain-orientation mapping in crystalline materials. 3D x-ray diffraction microscopy and differential-aperture x-ray microscopy can now be used to generate 3D orientation maps with a spatial resolution of 200 nanometers (nm). We describe here a nondestructive technique that enables 3D orientation mapping in the transmission electron microscope of mono- and multiphase nanocrystalline materials with a spatial resolution reaching 1 nm. We demonstrate the technique by an experimental study of a nanocrystalline aluminum sample and use simulations to validate the principles involved.

EBSP investigation of microstructure and texture evolution during equal channel angular pressing of aluminium

Abstract Commercial purity aluminum (99.5% Al) was deformed by equal channel angular pressing (ECAP) using up to 10 passes using route Bc. The evolution of microstructure and texture was characterized using the electron back scattered pattern technique. By use of the stereological parameter length per unit area ( L A ), it was found that on average a sub-micron grain size is developed only after 10 passes. Significant heterogeneity exists however in the sample and in some places a sub-micron grain size is not developed even after 10 passes. The texture strength was found to increase with increasing strain over the strain range investigated. Comparison of microstructural evolution with other deformation modes suggests that ECAP is more similar to cyclic extrusion-compression than to cold-rolling.

On boundary misorientation distribution functions and how to incorporate them into three-dimensional models of microstructural evolution

The fundamental difficulties of incorporating experimentally obtained boundary misorientation distributions (BMDs) into three-dimensional microstructural models are discussed. An algorithm is described which overcomes these difficulties. The boundary misorientations are treated as a statistical ensemble which is evolved toward the desired BMD using a Monte Carlo method. The application of this algorithm to a number of complex arbitrary BMDs shows that the approach is effective for both conserved and non-conserved textures. The algorithm is successfully used to create the BMDs observed in deformation microstructures containing both incidental dislocation boundaries (IDBs) and geometrically necessary boundaries (GNBs). The application of an algorithm to grain boundary engineering is discussed.

Slip pattern, microstructure and local crystallography in an aluminium single crystal of copper orientation {112}〈111〉

The evolution of microstructure and local crystallography has been examined in single crystals of {l_brace}112{r_brace}(111) orientation of pure Al deformed at room temperature by channel die compression to strains of {epsilon} = 0.5 and {epsilon} = 1.5. Despite the channel die deformation mode used, a sample rotation towards the {l_brace}4 4 11{r_brace}(11 11 8) orientation was observed. Macroscopic shear bands were not seen. Instead, bands of localized glide on crystallographic planes were observed, associated both with shearing of the microstructure and with +TD rotated orientations. Such strain localization increased with increasing deformation. The complexity of slip within these regions was confirmed by calculation of dislocation boundary characteristics from local TEM orientation measurements. Dislocation densities calculated using Frank`s formula are consistent with the observed macroscopic slip pattern.

Annealing behavior of aluminium deformed by equal channel angular pressing

Samples of commercial purity aluminum (99.5%) deformed by equal channel angular extrusion (ECAE) to accumulated strains of between 1 and 10 were annealed at different temperatures for a time of 2 h. The microstructural evolutions of both the deformed and the annealed materials were studied by electron back-scattered pattern (EBSP) analysis. At high strains the average cell size is only slowly refined, whilst the average cell boundary misorientation increases more quickly. Examination of the subgrain morphology during annealing suggested that whilst for low strains a recrystallisation occurred in a discontinuous manner, at high strains the samples showed traits of both discontinuous and continuous annealing behavior.

Recrystallisation of channel die deformed single crystals of typical rolling orientations

Abstract The effect of deformation microstructure heterogeneity on recrystallisation behaviour has been studied using channel die deformed single crystals of typical f.c.c. rolling texture components. These orientations exhibit a wide range of both spatial and orientational heterogeneity. The likelihood of recrystallisation nuclei developing from within the deformed microstructure is found to be directly related to the extent of this heterogeneity. The recrystallisation textures comprise both a retained deformation texture component, as well as a component close, but beyond, to one extreme of the deformation texture spread. The growth of recrystallised grains, regardless of orientation, is affected by the degree of heterogeneity of the deformation microstructure. For randomly oriented nucleation, and under equivalent annealing conditions, recrystallisation proceeds two orders of magnitude faster in the lower stored energy homogeneous {110} orientation than in the {123} orientation, with a much more heterogeneous microstructure and higher stored energy, even after recovery.

Influence of grain orientation on twinning during warm compression of wrought Mg–3Al–1Zn

Abstract The deformation of the Mg alloy AZ31 has been investigated in the temperature range from room temperature to 250°C. Microstructural investigation of samples compressed at 150°C showed extensive twinning only in samples where the basal plane normals were initially aligned approximately perpendicular to the compressive stress axis. Despite the large differences in microstructure for compression at this temperature, no significant differences were seen in the ductility. Grain rotations were observed in the non-twinned parts of some twinned grains after a strain of just ϵ=0·1. These rotations limit the extent to which the twinning behaviour can be related to the grain orientation. To interpret the twinning behaviour it is necessary to pay attention to boundaries formed by the intersection of two twin systems within a given grain.

Determination of boundary area and spacing in prismatic structures with applications to dislocation boundaries

Abstract An analysis is presented for the determination of boundary area per unit volume ( S V ) for structures that can be idealized as two-dimensional prismatic foams. An important application for this analysis is in the determination of S V for one class of dislocation boundary observed following deformation to high strains. Assuming a complete uniformly random distribution of inclination angles for these dislocation walls S V = πN L /2, where N L is the number of intercepts per unit length along a line perpendicular to the wall traces, in any plane perpendicular to the prism basal plane. Analyses of the inclination angle distribution from both measurements of projected boundary widths and from spacing measurements in two perpendicular sections show that while the assumption of a complete uniformly random distribution is an approximation, the error introduced by making this assumption is small.