Papers by Author: O.V. Mishin

Abstract: Deformation structures and annealing behaviour have been analysed in the centre layer of two AA1050 samples cold-rolled to von Mises strains of 3.6 and 6.4. During annealing at 270-300°C structural coarsening and discontinuous recrystallization occurred in both samples. In the coarsened microstructure, the fraction of high angle boundaries was slightly lower than that in the as-rolled conditions. Recrystallization textures of both samples contained significant fractions of the rolling texture components. The fraction of the retained rolling texture was however greater in the strain-6.4 sample. The {001}<310> and {110}<566> components were also pronounced in this sample. The size of recrystallized grains having orientations of the rolling texture was considerably smaller than the size of grains having other crystallographic orientations. This may be attributed to orientation pinning that hinders growth of grains with orientations of the rolling texture.

Abstract: 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.

Abstract: 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.

Abstract: Thick (up to 5 mm) Ni electrodeposits were produced by the pulsed electrodeposition (PED) technique. The PED-Ni was investigated in planar and cross-sections using high resolution scanning electron microscopy. Grain size and local texture were studied by electron backscatter diffraction. Thermal stability and grain growth behaviour were investigated using in-situ annealing in the transmission electron microscope. It is observed that columnar grains are present in the material and that the orientation of grains is not uniform. Textures and in-situ annealing behaviour are compared to previous data on nanocrystalline PED-Ni and Ni-Fe, where a subgrain coalescence model adopted from recrystallization is used to describe the occurrence of abnormal grain growth upon annealing and where twinning was found to be responsible for the texture development.

Abstract: The microstructural evolution during annealing of a commercial Al-Mn alloy cold rolled to a high strain was investigated using EBSD and Gallium Enhanced Microscopy. The precipitation of manganese, coarsening of precipitates and tensile properties were monitored at different stages. It was found that during recovery the subgrains grow until they reach the limiting subgrain size when the driving force has been reduced to the same level as the Zener drag from the dispersoids. New grains are nucleated at constituent particles and a few are able to grow. The softening during recovery and the onset of discontinuous recrystallisation are analyzed and discussed in terms of recent theories of recovery and recrystallisation.

Abstract: 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.

Abstract: The softening behaviour during annealing was investigated in cold and hot rolled AA3103 alloys after different heat treatments. It was found that the evolution of boundary spacing determined using gallium enhanced microscopy gives a very good representation of the softening behaviour. The results show that cold rolled Al-Mn alloys soften by continuous growth of the subgrain structure, “continuous recrystallisation”, provided the pre-treatment of the ingots has been made to avoid too high a density of dispersoids and the cold rolling reduction has been very large. The very high strain creates a microstructure with a large fraction of high angle boundaries that are mostly parallel to the sheet surface. A recently developed subgrain growth model which takes the effect of solute drag into account, gives a good description of the softening kinetics. The solute drag is controlled by bulk diffusion of Mn. The simultaneous precipitation of Mn from the solid solution takes place by grain boundary diffusion of the Mn atoms mainly to pre-existing particles. The solute concentration decreases as the inverse of the boundary spacing, which is due to the grain growth mainly in the thickness (normal) direction.