Papers by Keyword: Grain Boundary Energy

Abstract: Relative grain boundary energy as a function of misorientation angle was measured in a cube-oriented, 120 µm-thick Al foil and in a <111> fiber-textured, 1.7 µm-thick Al film using a multiscale analysis of the grain boundary dihedral angles. For the Al foil, the energies of low-angle boundaries increased with misorientation angle, in good agreement with the Read-Shockley model. For the Al film, two energy minima were observed for high-angle boundaries. Grain growth was studied in 25 and 100 nm-thick films that were annealed at 400 °C for a series of times in the range of 0.5 to 10 h. For the 100 nm-thick film, grains approximately doubled their size (equivalent circular diameter) before grain growth stagnated. The steady-state distributions of reduced grain area for two-dimensional, Monte Carlo Potts and partial differential equation based simulations showed excellent agreement with each other, even when anisotropic boundary energies were used. However, the simulated distributions had fewer small grains than the experimental distributions.

Abstract: A newly developed model based on vertex concept is presented in this paper. Contrary to its standard version, which is strictly deterministic, some concepts of Monte-Carlo type method were introduced. It makes the model more flexible and allows to introduce some parameters appearing in vertex movement equations, which are not easy to express in analytical form. Initial microstructure in the model is characterized by topology, crystallographic orientations and stored energy values of the grains. The boundary energies and mobilities are anisotropic in general. Nucleation mechanism of a given type is selected at the beginning of calculations. Deformation texture, stored energy distribution and initial microstructure are input parameters of the model. The aim of the calculations is to predict the texture and microstructure modifications during recrystallization. The model was also applied to the study of the kinetics of grain growth and recrystallization. The preliminary tests of the model are presented.

Abstract: A mesoscale, variational simulation of grain growth in two-dimensions has been used to explore the effects of grain boundary properties on the grain boundary character distribution. Anisotropy in the grain boundary energy has a stronger influence on the grain boundary character distribution than anisotropy in the grain boundary mobility. As grain growth proceeds from an initially random distribution, the grain boundary character distribution reaches a steady state that depends on the grain boundary energy. If the energy depends only on the lattice misorientation, then the population and energy are related by the Boltzmann distribution. When the energy depends on both lattice misorientation and boundary orientation, the steady state grain boundary character distribution is more complex and depends on both the energy and changes in the gradient of the energy with respect to orientation.

Abstract: In the present study, grain boundary energy and atomic structure of <110> symmetric tilt boundaries in copper were evaluated by molecular dynamics (MD) simulation. From the simulations, the grain boundary energy of <110> symmetric tilt boundaries depended on misorientation angle and there were large energy cusps at the misorientation angles which corresponded to (111) S 3 and (113) S 11 symmetric tilt boundaries. It was found that the atomic structure of each <110> symmetric tilt boundary was described by the combination of three kinds of structural units which consisted of (331) S 19, (111) S 3 and (113) S 11 symmetric tilt boundaries and two single crystal units which consisted of (110) S 1and (001) S 1 single crystals. From the the analysis of the excess free volume in each grain boundary, it was found that the energy of structural units depended on the excess free volume of the units and that the misorientation dependence of grain boundary energy agreed with that of the free volume in grain boundaries.

Abstract: The grain boundary character distribution in an Fe-1%Si steel has been measured as a function of lattice misorientation and boundary plane orientation. There is a weak texture in the space of grain boundary planes that favors the {110} orientation. At specific misorientations, the anisotropy is larger. For example, when the lattice misorientation is 60° around [111], symmetric tilt boundaries comprised of two {110} planes on either side of the interface dominate the population. The results are consistent with observations suggesting that in a range of crystalline materials, the low energy, low index surface planes are found to dominate the distribution of internal interfaces.

Abstract: Anisotropy in grain boundary “thermo-kinetics” is central to our understanding of microstructural evolution during grain growth and recrystallization. This paper focusses on role of atomic-scale computer simulation techniques, in particular molecular dynamics (MD), in extracting fundamental grain boundary properties and elucidating the atomic-scale mechanisms that determine these properties. A brief overview of recent strides made in extraction of grain boundary mobility and energy is presented, with emphasis on plastic strain induced boundary motion (p-SIBM) during recrystallization and curvature driven boundary motion (CDBM) during grain growth. Simulations aimed at misorientation dependence of the grain boundary properties during p-SIBM and CDBM show that boundary mobility and energy exhibit extrema at high symmetry misorientations and boundary mobility is comparatively more anisotropic during CDBM. This suggests that boundary mobility is dependent on the driving force. Qualitative observations of the atomic-scale mechanisms in play during boundary motion corroborate the simulation data. p-SIBM is dominated by motion of dislocation-interaction induced stepped structure of the grain boundaries, while correlated shuffling of group of atoms preceded by rearrangement of grain boundary free volume due to single atomic-hops across the grain boundary is frequently observed during CDBM. Comparison of the simulation results with high-purity experimental data extracted in Al indicates that while there is excellent agreement in misorientation dependent anisotropic properties, there are significant differences in values of boundary mobility and migration activation enthalpy. This strongly suggests that minute concentration of impurities retard grain boundary kinetics via impurity drag. Finally, the paper briefly discusses current and future challenges facing the computer simulation community in studying grain boundary systems in real materials where extrinsic effects (vacancy, impurity, segregation and particle effects) significantly alter the microscopic structure-mechanism relations and play a decisive role in determining the boundary properties.

Abstract: The recrystallization process in polycrystalline material was studied using the newly developed two–dimensional model based on the vertex concept. In the model presented below the microstructure of polycrystalline material is represented by two-dimensional network of grains. The initial microstructure is characterized by topology, crystal orientations and stored energy values of the grains. The boundary energies and mobilities are anisotropic in general. Additional driving forces in recrystallization, are exerted on vertices and are derived from the stored energy gradients between adjacent grains. The nucleation mechanism of a given type is selected at the start of the calculations. Two different nucleation types were tested. Deformation texture, stored energy distribution and initial microstructure are input parameters of the model. The goal of the calculations is the prediction of texture and microstructure modification during recrystallization. A comparison of predicted and experimental characteristics enables the verification of the model assumptions.