Materials Science Forum Vols. 715-716

Abstract: In a previous work [ we introduced the geometry of a granular system that allowed the study of the effect of a finite mobility of the quadruple and triple junctions on grain boundary migration. One of the most important conclusions of this work was that the triple junctions drag more effectively the motion of the grain boundaries than the quadruple junctions. Nevertheless, this conclusion was drawn without consideration of the grain size. For this reason, this conclusion might be contradictory with our understanding of the grain boundary junctions because while the effect of the triple lines is inverse linear with the grain size that of the quadruple junctions is proportional to the inverse square of the grain size and thus, quadruple junctions are expected to drag more effectively, at least, for very small grain sizes. In the present investigation, we studied comprehensively the effect of grain size on the evolution of the granular system under the assumption of a finite mobility of the boundary junctions. For this purpose, several network model simulations were carried out for different grain sizes ranging from nanoto micrometers using a fully periodic grain arrangement. The results seem to corroborate that the triple junctions drag more effectively the motion of the grain boundaries, however, for very low junction mobility and grain sizes the effect appears to be indistinguishable. It was also observed that for very low quadruple junction mobility the geometry of the granular system undergoes a severe transformation which results in the unfulfillment of the equation derived in [.

Abstract: The paper introduces first investigations on how low angle grain boundaries can influence the recrystallisation behaviour of crystalline metallic materials. For this purpose a three-dimensional cellular automaton model was used. The approach in this study is to allow even low angle grain boundaries to move during recrystallisation. The effect of this non-zero mobility of low angle grain boundaries will be analysed for the recrystallisation of deformed Al single crystals with Cube orientation. It will be shown that low angle grain boundaries indeed influence the kinetics as well as the texture evolution of metallic materials during recrystallisation.

Abstract: Abstract.are preferentially eliminated, thus leading to interface texture development and a higher pop-ulation of low energy grain boundaries. However, when stress is introduced as an additionaldriving force, the dynamics of grain growth change. To model these effects, a three dimensionalanisotropic multi-level set model was modified in order to account for the effect of stress field ongrain growth. For this mesoscale study, grain boundaries were treated as dislocation structuresand their associated net Burgers vectors were calculated using the misorientation informationand boundary inclinations. Using these net Burgers vectors and their associated densities, ad-ditional forces due to stress field were calculated via the Peach-Koehler equation. Qualitativecomparisons of 5 parameter grain boundary character distribution will be carried out in orderto analyze the differences in texture evolution during grain growth.

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 theoretical description of grain growth was based for many years on the so-calledspherical model. The spherical model represents a polyhedral grain with N faces and a volume, V ,by a sphere with an equal volume having the equivalent grain radius, R. That model leads to severalinteresting results concerning normal and abnormal grain growth as well as grain size distribution.Nevertheless, representation of grains by spheres entails a fundamental limitation: namely, all topo-logical information of the polyhedral grain is forsaken. The rich variety of grain shapes occurringin three-dimensional polycrystalline networks, however, makes their energetic and kinetic analysesextremely difficult. To simplify analyses of isotropic polycrystals, average N-hedra and generalizedN-hedra ANHs or GNHs .N D 3; 4; 5;1/ were created as a set of regular polyhedra, consisting ofN identical faces that act as topological proxies for analyzing irregular grains containing N mixedfaces. The adoption of ANH/GNH as representations of polyhedral grains led to further progress inour understanding of grain growth, particularly those aspects related to topological behavior. This pa-per summarizes some recent advances of representing polyhedral grains by ANHs/GNHs rather thanby spheres.

Abstract: The grain structure and texture evolution during annealing an Al-0.13%Mg submicron grained alloy, deformed by plane strain compression (PSC) at cryogenic temperatures, has been investigated. On annealing the grain structure coarsened and transformed from lamellar to equiaxed. But, remarkably, the fraction of low angle boundaries (LABs) increased, from less than ~ 25% to ~50% above 300 °C, leading to instability and discontinuous coarsening at higher temperatures. The surprisingly large increase in LAB fraction on annealing is shown to be related to orientation impingement originating from the strong texture present after PSC in liquid nitrogen.

Abstract: Recent results of experimental research into stress induced grain boundary migration in aluminum bicrystals are briefly reviewed. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for any <100> tilt boundary in the entire misorientation range (0-90°). Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.

Abstract: A new approach to dynamic recrystallization (DRX) is introduced. It is based on the assumption that the critical conditions for DRX and the arrest of DRX grain boundaries are related to the development of mobile subboundaries. The theoretical predictions are compared to experimental results during incipient and steady-state DRX. The grain size sensitivity of the DRX grains establishes the desired link between deformation and DRX microstructure.

Abstract: The results of recent experimental, theoretical and computer simulation studies of the thermodynamics and kinetics of grain boundaries and grain boundary junctions are presented and discussed.

Abstract: Metals deformed to high and ultrahigh strains are characterized by a nanoscale microstructure, a large fraction of high angle boundaries and a high dislocation density. Another characteristic of such a microstructure is a large stored energy that combines elastic energy due to dislocations and boundary energy. Parameters of the deformed microstructure significantly affect annealing processes such as recovery and recrystallization. For example, the recovery rate can be significantly increased after high strain deformation and restoration may occur as either discontinuous recrystallization or structural coarsening. A characterization and analysis of deformed and annealed microstructures presented in this work covers Al, Ni, Cu and Fe heavily deformed by rolling, accumulative roll bonding (ARB), equal channel angular extrusion (ECAE) and high pressure torsion (HPT). The important effect of recovery on subsequent restoration processes is discussed along with the effect of heterogeneities both on the local scale and on the sample scale.