Materials Science Forum Vol. 753

Abstract: An Al-1%Si alloy cold rolled to a von Misses stain of 4.5 was isothermally annealed at 210°C. A random recrystallization texture was obtained, which was attributed to the effects of particles of different sizes on the nucleation and growth of grains during recrystallization.

Abstract: Friction stir processing was carried out on the Al-Mg-Mn alloy to achieve ultrafine grained microstructure. The evolution of microstructure and micro-texture was studied in different regions of the deformed sample, namely nugget zone, thermo-mechanically affected zone (TMAZ) and base metal. The average grain sizes of the nugget zone, TMAZ and base metal are 1.5 μm ± 0.5 μm, 15 μm ± 8 μm, and 80μm ± 10 μm, respectively. The TMAZ exhibits excessive deformation banding structure and sub-grain formation. The orientation gradient within the sub-grain is dependent on grain size, orientation, and distance from nugget zone. The microstructure was partitioned based on the grain orientation spread and grain size values to separate the recrystallized fraction from the deformed region in order to understand the micromechanism of grain refinement. The texture of both deformed and recrystallized regions are qualitatively similar in nature. Microstructure and texture analysis suggest that the restoration processes are different in different regions of the processed sample. The transition region between nugget zone and TMAZ exhibits large elongated grains surrounded by fine equiaxed grains of different orientation which indicate the process of discontinuous dynamic recrystallization. Within the nugget zone, similar texture between deformed and recrystallized grain fraction suggests that the restoration mechanism is a continuous process.

Abstract: The coupling between local texture and microstructure is analyzed during restoration processes in aluminum cold-rolled to high and ultrahigh strains. The deformed microstructure is composed of lamellae with orientations of rolling texture components that occupy different volume fractions and vary in the spatial distribution. The individual lamellae are separated by low and high angle boundaries and significant local differences are produced in the deformed microstructure both in terms of the stored energy and boundary mobility. These differences influence recovery and recrystallization processes, resulting in significant local variations in mechanisms and kinetics of these processes. The observations suggest that the characterization of the local texture and microstructure both in the deformed state and after annealing is important in order to underpin the analysis of recovery and recrystallization on the sample scale.

Abstract: The effects of net driving force for migration of high angle grain boundaries were emphasized beside many other factors which could influence the process of texture formation during recrystallization annealing of 95% cold rolled pure aluminum sheets. The net driving force consists basically of stored energy. However, it could be reduced by recovery, boundary drag, solute drag and Zener drag in different extents, in which only boundary drag is mis-orientation dependent. It was indicated that both oriented nucleation and oriented growth have obvious influence on recrystallization texture, and how far they influence the texture depends also on the level of net driving force when the grain growth starts during annealing. Oriented growth, which is induced by the differences in boundary drag of differently oriented grains, and the corresponding texture formation, could be observed easily when the recrystallization proceeds under relative higher solute drag and Zener drag in commercial purity aluminum. The oriented nucleation process prevails during recrystallization of sufficiently recovered high purity aluminum with very low solute drag and Zener drag, after which strong cube texture forms. In this case the oriented growth indicates limited effect. Both the oriented growth and oriented nucleation will fail if high purity deformation matrix without clear solute drag and Zener drag has not experienced an obvious recovery before recrystallization grain growth, since extremely high net driving force leads to very small critical nucleus size and multiplicity of growing grains, which results in randomization of recrystallization texture.

Abstract: The hot rolled AA6082 aluminium alloy with aluminide dispersoids is deformed up to large strains to obtain a fine grained microstructure. Friction stir spot welding (FSSW) is carried out on rolled plates by means of a device provided by MTS System Corporation. FEM simulations determine that the material can flow up to local strains between 10 and 50 when the material reaches temperatures between 300-500°C. With this information, hot torsion tests at constant temperatures are carried out in a Gleeble ® 3800 machine for different strain rates. In both cases, in situ water quenching is applied to freeze the microstructure and avoid any static recrystallization effect after hot deformation. Light optical microscopy is used to identify the evolution of the grains as a function of the local deformation parameters determined by FEM simulations. The microstructure development by FSSW as well as by torsion is then further characterized by means of EBSD. At small strains the material deforms mainly by dynamic recovery with small low angle grain boundary formation and boundary dragging by fine aluminides and Mg2Si. At large strains grain refinement by continuous dynamic recrystallization takes place heterogeneously as a function of the original crystallographic orientation and precipitation state of each grain.

Abstract: Early stages of recrystallization were observed for the technical purity aluminium alloy (AA1050). The samples were severely deformed by equal channel angular pressing and then annealed to obtain the state of partial recrystallization. The nucleation of new grains and the changes in ‘density’ of low- and high- angle boundaries were analysed using scanning electron microscopy equipped with high resolution EBSD system. After deformation in six passes along route A the alloy contained a structure of flat grains. On annealing, the structure coarsened and got transformed into nearly equiaxed grains by both discontinuous and continuous recrystallization. A particular role in the rise of nuclei and the structure spheroidization is attributed to migration of low-angle boundaries.

Abstract: Hydrogenation and hot forging (upsetting) of cast Ti-6Al-4V has been performed at 550°C-1000°C at a strain rate of 10s-1. Post-deformation annealing studies were performed to establish minimum recrystallisation temperatures for optimizing grain refinement. Hydrogenation at approximately 20at.% hydrogen is shown to reduce the peak flow stress by up to 35% and lower the deformation temperature and recrystallisation limits to 550°C and 800°C respectively without compromising ductility during hot forging. Decomposition of the recrystallised β-phase and subsequent dehydrogenation provides substantial grain refinement of the cast structure.

Abstract: Recrystallization of rolled Zr single crystals is considered in comparison with analogous recrystallization processes in rolled coarse-grained iodide Zr and polycrystalline plates of commercially pure Zr. Diffractometric X-ray methods were used by texture and X-ray line profile measurements. The treatment of obtained data included construction of correlation diagrams, connecting as-rolled and recrystallized conditions of samples. A number of recrystallization mechanisms, operating in rolled α-Zr under annealing, were revealed on the basis of found regularities of texture changes.

Abstract: X-ray diffraction (XRD) studies of ECAP (equal-channel angular pressing) materials were performed after annealing and by in-situ measurements in XRD high-temperature chamber for samples prepared by different number of passes and number of revolutions, respectively. Main attention was given to Cu and Cu-Zr samples. Significant dependence on number of passes was found for ECAP samples. In-situ measurements were focused not only on temperature dependence but also on time evolution of the diffraction line profiles. Evaluation in terms of dislocation densities, correlation and crystallite size and its distribution was performed by our own software MSTRUCT developed for total powder diffraction pattern fitting. Abnormal growth of some grains with annealing is well-known for copper and leads to the creation of bimodal microstructure. Therefore a special care must be given to the evaluation and a model of two Cu components (larger and smaller crystallites) was fitted to the data if an indication of some crystallite growth appears either in the XRD line profile shape or in two-dimensional diffraction patterns.

Abstract: Recrystallization kinetics in copper cold-rolled to 90% reduction with and without significant widening was investigated by electron backscatter diffraction. It was found that the recrystallization process was slightly retarded and the development of cube recrystallization texture was largely inhibited in the widening sample. Cube grains were observed to have a growth advantage by a factor of 2 in the non-widening sample, while this growth advantage was not observed in the widening sample. The recrystallization kinetics and the development of cube texture in the two samples are discussed.