Papers by Keyword: High Angle Boundary

Abstract: The structure of disconnections in symmetrical low- and high-angle [0001] tilt boundaries in an hcp metal are studied using atomic-scale simulation. Applied engineering strains cause such defects to move conservatively along the boundaries, producing coupled shear and migration. The Peierls stresses causing such motion are found to decrease precipitously through the transition from low- to high-angle boundaries. The reason underlying this behaviour is discussed.

Abstract: Severe plastic predeformation of crystalline materials leads not only to formation of a steady-state dislocation structure including low-angle boundaries, but also brings the high-angle boundary structure into a steady state. When the steady-state flow stress is high enough, the material becomes ultrafine-grained or even nanocrystalline. The change from coarse-grained to ultrafine-grained is accompanied by a distinct change in the steady-state deformation resistance that is measured after predeformation. This is explained by two opposing effects of high-angle boundaries, namely enhanced dislocation storage and accelerated dislocation recovery. The first one causes net hardening at high temperature-normalized strain rate Z (Zener–Hollomon), the second one net softening at low Z. This means that the rate-sensitivity of the flow stress is enhanced, which causes the paradoxon of enhanced strength at enhanced ductility. Tests with abrupt large changes of deformation conditions bring the strain associated with dynamic recovery into the focus. The results of such tests indicate that the boundaries, low-angle as well as high-angle ones, migrate under concentrated stress during deformation and thereby contribute to straining and recovery. The corresponding system of differential equations needed to model structure evolution and deformation kinetics on a semi-empirical basis is briefly discussed.

Abstract: A new recrystallization phase-field method is proposed, in which the three stages of recrystallization phenomena, i.e., recovery, nucleation and nucleus growth are sequentially taken into account in a computation. From the information of subgrain patterns and crystal orientations in a polycrystal that are obtained by a dislocation-crystal plasticity FE analysis based on a reaction-diffusion model, subgrain groups surrounded by high angle boundary are found out. Next, subgrains in the group are coalesced into a nucleus by rotation of crystal orientation and migration of subgrain boundaries through a phase-field simulation. Then a computation of nucleus growth is performed also using the phase-field method on account of an autonomic incubation period of nucleation, in which stored dislocation energy assumes a role of driving force. It is shown that the present method can numerically reproduce the three stages of recrystallization as a sequence of computational procedure.

Abstract: Microstructural change and soft ening behavior during annealing were investigated for deformed ferrite and lath martensite in an ultralow carbon 1.5mass%Mn-0.0018mass%B steel, and then the difference in recrystallization behavior between the materials was discussed in terms of the nucleation site of recrystallized grains. The ferritic and martensitic materials were obtained by furnace-cooling and water-quenching, respectively, after solution treatment. The ferritic material was cold-rolled at a reduction of 80% to give the same dislocatio n density as of the martensitic material. The deformed ferritic material contains a large number of geometrically necessary boundaries with large misorientations, while the martensitic material does only contain original grain boundaries such as prior austenite grain boundaries, packet boundaries and block boundaries. The recrystallization during annealing is markedly retarded in the martensitic material compared with the deformed ferritic material. As a result, the time for completing the recrystallization was roughly a hundred times longer in the martensitic material than in the deformed ferritic material. This is due to the difference in nucleation site of recrystallized grains, that is, the geometrically necessary (GN) boundaries introduced by the deformation for the ferritic material, and only the original grain boundaries for the martensitic material.