Andrew Godfrey

Effect of Temperature on Microstructure and Texture during Compression of AZ31

The hexagonal crystal structure of AZ31 results in a very high mechanical anisotropy and a poor formability of this alloy. In order to address these problems the influence of twinning, slip and dynamic recrystallization (DRX) on the microstructure and texture evolution during compression of AZ31 has been studied over a range of temperatures. Cylindrical samples were tested uniaxially in compression from room temperature to 350oC, with the compression axis parallel to either the normal direction (ND) or the transverse direction (TD) of the hot-rolled sheet from which the samples were cut. The microstructure was characterized using electron backscattered diffraction (EBSD) analysis. Extensive twinning was only observed in the TD samples at all temperatures. The effect of the difference in twinning and slip between the TD and ND samples on the mechanism of dynamic recrystallization has also been investigated. For this, the grains formed during DRX were identified by their appearance in EBSD orientation maps and the orientations of these grains determined.

Modeling of Cube-Texture Evolution during Grain Growth in Ni Thick-Films based on Experimental Observations

. A Monte Carlo Potts model, including anisotropic boundary energies and mobilities, has been used to investigate cube texture evolution. The work is motivated by the post-recrystallization grain growth seen during annealing of highly cold-rolled nickel sheets. It is observed that during such grain growth the cube texture fraction increases from approx. 50% to more than 95%. Electron back-scatter diffraction orientation measurements of fully recrystallized samples have been used to provide starting microstructures for simulations examining the effect of the texture tightness on the development of the cube texture during grain growth. The texture evolution of a single texture component with an initial average size advantage is controlled predominantly by this size advantage once the initial texture spread of this component falls below some value. An additional set of simulations was also carried out to examine grain growth in systems with a quasi-binary texture. The simulations were used to examine cases of grain growth in where one component has a larger average grain size, but where there is still a significant overlap in the grain-size distribution of each component. The initial microstructures were constructed to give different fractions of each texture component, with a given value of both the mean size and the texture spread of each component. The simulations suggest that for such systems there is a well defined range of conditions for which a single texture component can grow to dominate the final texture.

Cube Texture Development in Pure Ni during Annealing in a High Strength Magnetic Field

A 10 Tesla magnetic field was applied during annealing of cold-rolled high purity Ni at 300°C for 2 hours, with samples aligned at various different angles to the magnetic field direction. The effect of the magnetic field annealing on the cube texture evolution and the microstructural development was investigated by use of electron backscatter pattern (EBSP) analysis in the scanning electron microscope. The results show that both the cube texture evolution and the grain size are affected by the application during annealing of a high strength magnetic field, and that the effect varies as a function of the angle between the sample normal and the magnetic field direction. The cube texture volume fraction resulting from 2 hours annealing increases with increasing angle between the field direction and the sample normal direction, though in nearly all cases lower cube volume fractions were obtained compared to annealing without a magnetic field. The average size for all grains and for just cube-oriented grains both decrease with decreasing cube texture fraction, suggesting that the presence of a magnetic field either leads to enhanced recrystallization nucleation, or to a retardation of grain growth after recrystallization.

Direct observation of nucleation in the bulk of an opaque sample

Remarkably little is known about the physical phenomena leading to nucleation of new perfect crystals within deformed metals during annealing, in particular how and where volumes with nearly perfect lattices evolve from structures filled with dislocations, and how local variations at the micrometer length scale affect this nucleation process. We present here the first experimental measurements that relate directly nucleation of recrystallization to the local deformation microstructure in the bulk of a sample of cold rolled aluminum, further deformed locally by a hardness indentation. White beam differential aperture X-ray microscopy is used for the measurements, allowing us to map a selected gauge volume in the bulk of the sample in the deformed state, then anneal the sample and map the exact same gauge volume in the annealed state. It is found that nuclei develop at sites of high stored energy and they have crystallographic orientations from those present in the deformed state. Accordingly we suggest that for each nucleus the embryonic volume arises from a structural element contained within the voxels identified with the same orientation. Possible nucleation mechanisms are discussed and the growth potentials of the nuclei are also analyzed and discussed.

The influence of multiscale heterogeneity on recrystallization in nickel processed by accumulative roll bonding

Microscopic and sample-scale heterogeneities have been characterized in nickel processed by accumulative roll bonding (ARB) to a von Mises strain of 4.8, and their influence on recrystallization have been analyzed. The microscopic deformation heterogeneities in this material are mostly associated with regions near the bonding interface, which are more refined and thus possess a higher stored energy than other regions. These regions also contain characteristic particle deformation zones around fragments of the steel wire brush used to prepare the surface for bonding. The sample-scale heterogeneities are seen as variations in the distribution of different texture components and in the fractions of high misorientation regions between the subsurface, intermediate, and central layers. Each of these heterogeneities affects the progress of recrystallization. Regions near bonding interfaces and particle deformation zones are found to act as preferential nucleation sites. Preferential nucleation is also observed at shear bands and within cube-oriented lamellae. On the sample scale, recrystallization proceeds faster in the intermediate layer than in the central and subsurface layers.

EBSD-Based Techniques for Characterization of Microstructural Restoration Processes during Annealing of Metals Deformed to Large Plastic Strains

Some methods for quantitative characterization of the microstructures deformed to large plastic strains both before and after annealing are discussed and illustrated using examples of samples after equal channel angular extrusion and cold-rolling. It is emphasized that the microstructures in such deformed samples exhibit a heterogeneity in the microstructural refinement by high angle boundaries. Based on this, a new parameter describing the fraction of regions containing predominantly low angle boundaries is introduced. This parameter has some advantages over the simpler high angle boundary fraction parameter, in particular with regard to data collected from electron-backscatter diffraction investigations, where boundaries with very low misorientation angles cannot be reliably detected. It is shown how this parameter can be related to the recrystallization behavior. Another parameter, based on mode of the distribution of dislocation cell sizes is outlined, and it is demonstrated how this parameter can be used to investigate the uniformity, or otherwise, of the restoration processes occurring during annealing of metals deformed to large plastic strains.

Nanostructured Aluminium - Recovery and Recrystallization

The isochronal annealing behavior of nanostructured commercial purity aluminium (AA1100 and AA1200) following either cold – rolling or accumulative roll bonding up to an ultra high strain of εvM = 6.2 (99.5% reduction in thickness) has been studied via hardness testing and by a microstructural investigation. A large effect of rolling strain is observed on the recovery at temperatures below approx. 200 °C. At higher temperatures an assessment of the changes in hardness and microstructure leads to a characterization of the annealing process as one of conventional (discontinuous) recrystallization.

Characterization of boundary misorientations in a superplastic al-alloy hot-deformed by ECAE

Microstructure and boundary populations were investigated in a superplastic Al-5.5%Mg- 2.2%Li-0.12%Zr alloy hot-deformed by equal channel angular extrusion. The microstructure was found to be inhomogeneous and revealed deformation structures being subdivided by both low- and high-angle boundaries.

Structure and strength of sub-100 nm lamellar structures in cold-drawn pearlitic steel wire

ABSTRACT Pearlitic steel wire, with a representative sub-100u2009nm lamellar structure, is the strongest mass-produced steel with an excellent combination of formability and strength. This overview summarises investigations of cold-drawn pearlitic steel wire in the last decades, covering the microstructural evolution and strengthening mechanisms. Based on quantitative structural parameters, this overview covers a quantitative and extensive analysis of structure–strength relationships. By focusing on the structure, challenges and future strategy are outlined to further improve the mechanical behaviour and performance of pearlitic steel wire to widen its use in society. This is part of a thematic issue on Pearlitic Steel Wires.

EBSD Analysis of Deformed and Partially Recrystallized Microstructures in ECAE-Processed Copper

The deformed microstructure and recrystallization behavior of copper samples processed using equal channel angular extrusion (ECAE) have been investigated. The heavily deformed microstructure was found to be non-uniform through the sample thickness and to vary in a manner consistent with the non-uniform distribution of strain imposed by processing. The through-thickness heterogeneity of the deformed microstructure resulted in a different extent of recrystallization in different layers during annealing. Recrystallized grains were also observed in samples that were not annealed, but stored at room temperature, which indicates that the deformed microstructure of ECAE-processed pure copper is unstable even at room temperature. In each sample, recrystallization was found to initiate in regions containing predominantly large misorientations.