O.V. Mishin

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.

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.

Orientation correlations in aluminium deformed by ECAE

Abstract Distributions of boundary misorientations measured in an Al–0.13%Mg alloy, processed by equal channel angular extrusion to a von Mises effective strain of 10, have been compared to misorientation distributions generated from a random mix of orientations present in the microstructure. A significant fraction of low angle boundaries is reported and orientation correlation over high angle boundaries is discussed.

Application of high-energy synchrotron radiation for texture studies

A novel experimental technique that employs high-energy synchrotron radiation is used for the investigation of through-thickness texture gradients in two aluminium plates, cold-rolled 40% with either intermediate or small draughts. In these two plates, crystallographic textures are inspected in a large number of layers. Texture maps of pole densities throughout the sample thickness are presented. A texture of the rolling type is developed through the plate thickness after intermediate draught rolling. Pronounced inhomogeneities associated with the shear texture are observed in the sample rolled with small draughts. For selected layers, direct pole figures are compared with those obtained by traditional low-energy X-ray diffraction and by the electron backscattering pattern technique using a scanning electron microscope. A good qualitative agreement between textures measured using the three different techniques is obtained. Experimental aspects and potentials of the new technique are discussed.

Processing and interpretation of EBSD data gathered from plastically deformed metals

Abstract The use of electron backscattered diffraction (EBSD) for the investigation of dislocation boundary microstructures formed during plastic deformation of metals is discussed. A quantitative description is given of the angular resolution limitation in EBSD mapping. The ability of filtering using orientation averaging to improve the angular resolution of the EBSD data is then considered. Orientation filtering is tested using both simulated EBSD maps where a certain orientation noise function and level are assumed and additionally by direct comparison of EBSD and orientation measurements taken in the transmission electron microscope from the same region of a thin foil. Orientation filtering results in an improvement of data quality, but filtering can also introduce artefacts that cannot be distinguished from real microstructural features. The main limitation at present in developing specific recommendations for the use of filtering methods is that more information is required about the nature of the orientation noise at or near dislocation boundaries.

Analysis of through-thickness heterogeneities of microstructure and texture in nickel after accumulative roll bonding

The through-thickness heterogeneities of the microstructure and texture have been investigated in pure nickel processed by six cycles of accumulative roll bonding (ARB) with lubrication applied during each rolling pass. Strong rolling textures are found in the intermediate and center layers of the deformed sample, whereas increased fractions of the shear texture are observed near the surface. Significant differences at different depths are also revealed in the fractions of areas containing either predominantly low angle misorientations or predominantly high angle misorientations. It is found that the fractions of such areas are much more sensitive to the deformation history than the average parameters based on the measurements of the boundary spacing and fractions of high angle boundaries. For the material studied in this work, the area fraction of the low misorientation regions appears to correlate with the summed fraction of the rolling texture components. The observed microstructural and textural variations are discussed and compared with literature data, taking into account the influence of large-draught rolling and lubrication on the distribution of strain imposed during the ARB process.

Characterization and influence of deformation microstructure heterogeneity on recrystallization

The microstructure resulting from plastic deformation of metals typically contains heterogeneity on several length scales. This is also true for samples deformed to large strains, where an important form of heterogeneity is in the variation in microstructural refinement by high angle boundaries. A methodology for quantifying this type of heterogeneity based on the identification of areas classified as low misorientation regions (LMRs) is described, and some parameters for quantification of both the extent and length scale of LMRs are presented. It is then shown how this approach can be used to investigate the early stages of recrystallization in samples deformed to large strains, by direct comparison of electron backscatter diffraction (EBSD) maps of the same area before and after annealing. Methods for estimation of the stored energy of deformation from EBSD data are also surveyed and the problems of each for quantification of the local variation in stored energy are discussed, where it is concluded that a method based on the summation of the contributions from individual boundary segments is considered to be the best suited at present for characterization of the local variation in stored energy on the scale of the dislocation boundary features.

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.

Development of a strong Goss texture during annealing of a heavily rolled Al—0.3% Cu alloy

The evolution of microstructure and texture during isochronal annealing of a heavily cold rolled Al-0.3% Cu alloy has been characterized using electron backscatter diffraction. It is found that the rolling texture of this alloy is dominated by the Brass component and that recrystallization during annealing leads to the formation of a pronounced Goss texture. It is suggested that the development of the strong Goss texture in Al-0.3% Cu is caused by preferred growth of Goss-oriented grains into the Brass-oriented matrix.

Effect of Annealing on Microstructural Development and Grain Orientation in Electrodeposited Ni

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.