Local Atomic Configuration of Dislocation-Accumulated Grain Boundary and Energetics of Gradual Transition from Low Angle to High Angle Grain Boundary in Pure Aluminum by First-Principles Calculations
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Dislocation-accumulated grain boundaries were systematically investigated in terms of local atomic coordinates in the vicinity of grain boundary and energetics on grain boundary evolution by first-principles calculations. Detailed numerical analyses of energy and local atomic configuration at a grain boundary with fixed misorientation angle identified the most stable configurations both for the dislocation-distinctive model and the coincident-site-lattice model with kite-shaped structural units on grain boundary planes. The energy profiles of structural optimization using both initial models indicate that the distinctive dislocations at a grain boundary can be readily converted into kite-shaped structural units without noticeable energy barrier, though they look quite different, and reverse conversion may also be realized under external stress, enabling grain boundaries functioning as dislocation sources and sinks. Systematic calculations for grain boundary with misorientation angles ranging from 5.7° to 53.1° revealed that the interaction energy between dislocation is blunted within a dislocation core region. Furthermore, the energy needed to increase the misorientation angle during severe plastic deformation is quantitatively evaluated.Keywords:
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Dislocation activities in polycrystalline grains depend on their grain boundary (GB) structures such as misorientation angle between two adjacent grains. In order tensile loading are computed based on molecular dynamics (MD) running on 20 cores parallel computers. The computation time reduction as increasing the number of processors is evaluated. As the results, at the GBs with small misorientation angle, dislocations can penetrate easily through them, because the GB function as an obstacle against dislocation movement is less active. On the other hand, at GBs with large angle, penetration is not often observed. According to the strength of each polycrystals, inverse Hall-Petch effect is observed and it corresponds with the former experimental results of this grain size scale. Results also indicates that dislocation activities still remains even in the small grains when inverse Hall-Petch effect is observed.
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Grain growth process of polycrystalline metals which comprises of grain boundary migration and rotation of crystal orientation has been clarified by employing Phase Field Method. The strain energy stored due to the plastic deformation of polycrystal is evaluated by the crystal plasticity theory and approximately introduced to the free energy. The effects of such factors as strain energy, misorientation between grains, and curvature of grain boundary on the grain growth process have been clarified. The large misorientation between grains causes dominant migration of grain boundary, while the small misorientation causes dominant rotation of crystal orientation. The larger the magnitude of strain energy yields the faster grain growth. The changing rate of the area of grain boundary is preserved during the grain growth process. The rate of grain boundary migration depends on misorientation, and is proportional to the curvature of grain boundary.
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The fracture strength in molybdenum depends markedly on the grain bounday character. In order to clarify the relationship between the strength and the grain boundary structure, bicrystals with 〈110〉 symmetric tilt boundaries were prepared and observed by means of transmission electron microscopy. The observations were conducted using the bicrystals with near Σ1 and Σ3 boundaries, which are higher in fracture strength, and with boundaries of misorientation angle around 30°, which are lower in fracture strength. The results obtained are as follows.(1) Σ1 low angle and near Σ3 coincidence boundaries have a good coherency. The grain-bounday dislocations are observed and they can be described by a boundary dislocation model. (2) The boundaries of misorientation angle around 30° are poor in coherency comparing with Σ1 and Σ3 boundaries. However, the grain boundary structure can also be described by the grain-boundary dislocation model. (3) It is considered that the grain boundary structure in molybdenum with a high covalency in bonding is not greatly different from that in normal metals.
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Dislocation-accumulated grain boundaries were systematically investigated in terms of local atomic coordinates in the vicinity of grain boundary and energetics on grain boundary evolution by first-principles calculations. Detailed numerical analyses of energy and local atomic configuration at a grain boundary with fixed misorientation angle identified the most stable configurations both for the dislocation-distinctive model and the coincident-site-lattice model with kite-shaped structural units on grain boundary planes. The energy profiles of structural optimization using both initial models indicate that the distinctive dislocations at a grain boundary can be readily converted into kite-shaped structural units without noticeable energy barrier, though they look quite different, and reverse conversion may also be realized under external stress, enabling grain boundaries functioning as dislocation sources and sinks. Systematic calculations for grain boundary with misorientation angles ranging from 5.7° to 53.1° revealed that the interaction energy between dislocation is blunted within a dislocation core region. Furthermore, the energy needed to increase the misorientation angle during severe plastic deformation is quantitatively evaluated.
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To obtain a fundamental understanding of the effect of structure and geometry of grain boundary on the diffusion kinetics in nanocrystalline materials, the influence of grain boundary misorientation on the effective diffusion coefficient (apparent diffusivity) in nanocrystalline aluminum was investigated using molecular dynamics simulations. Nine series of [001] symmetric tilt grain boundaries, including high and low symmetric boundary planes, were studied. The apparent diffusivity in the samples was calculated in the temperature range from 423 K to 823 K by monitoring the mean square displacement of atoms as a function of simulation time. A temperature dependence of the effective diffusion coefficient according to the Arrhenius law was obtained for all samples. It is found that the apparent diffusivity is anisotropic and it is a strong function of grain boundary misorientation at low and high temperatures. At all temperatures, Σ29 [001]/(520) symmetric tilt grain boundary with misorientation angle of 43.68° exhibits the highest effective diffusion coefficient among the investigated grain boundaries. The simulation results show that the activation energy and pre-exponential factor are affected significantly by the grain boundary misorientation angle. Moreover, the results indicated that the misorientation dependence of activation energy for diffusion exhibits two local maxima, which correspond to two symmetric tilt grain boundaries. Additional calculation of misorientation dependence of the pre-exponential factor shows two local minima at the same symmetric tilt grain boundaries. The misorientation dependence of the effective diffusion coefficient was explained on the basis of grain boundary energy and the crystallographic structure of grain boundary.
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Abstract The role of grain boundary misorientation angles on the dislocation–grain boundary interactions was incorporated into a micro hardening scheme. The current formulation is applicable to both coarse‐ and ultrafine‐grained alloys, and evidences the experimentally observed dominant role of the misorientation angles on the deformation response of the latter.
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Abstract It has been suggested that the experimentally observed orientation dependence of the mobility of grain boundaries in f.c.c. metals may be related to the dependence of the rate of self diffusion in grain boundaries on the disorientation across the boundary. Later, this relative orientation effect on the rate of boundary diffusion and self diffusion was experimentally observed. It was shown by Hoffman and Turnbull that in bicrystals of silver misoricnted around (100) by 9° to 28°, self diffusion along the boundary (parallel to the common (100)) may be described in terms of a coefficient of self diffusion in individual grain boundary edge dislocation pipes, orders of magnitude larger than the coefficient of lattice self diffusion. It is significant that the coefficient of self diffusion in grain boundary dislocation pipes was found to be independent of the misorientation (i.e., of the density of edge dislocations in the boundary) at least up to 28°, suggesting that even a boundary of such a great misorientation may be considered as a network of dislocations, as far as self diffusion is concerned. In recent experiments the relative mobilities of boundaries in various orientations between a deformed (99.98% pure) aluminum single crystal and recrystallized grains growing in it in fairly well defined, lattice orientation relationships were compared. The matrix crystal was rolled to 80% R.A. on a (110) plane in a [112] direction, after which the strip still retained its initial orientation and the texture was very sharp. Recrystallized grains quite accurately oriented so as to have highest overall boundary mobility, i.e., corresponding to 40° rotations around the two 111 axes of the matrix grain lying in the rolling plane, were produced in large numbers by random nucleation on one side of the strip (rubbing one side with sandpaper and annealing). The re crystallized grains, that were at first growing in very large numbers and quite randomly but only in the thin surface layer highly deformed by abrasion (nucleation side), on annealing for 600 sec at 350°C grew across the whole thickness (0,010″) of the rolled single crystal. As a result of very selective growth, the recrystallized grains reaching the other side of the strip (growth side) showed a very sharp texture consisting of four components with the orientations described.
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