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The combined effect of strain and temperature on the microstructure and detailed internal structure of dislocation boundaries was systematically studied in compressed pure polycrystalline copper and nickel and compared to the microstructure of compressed polycrystalline aluminum. Below 0.5Tm the microstructure of Cu and Ni consists of dislocation cells, however, only in Cu second generation microbands are formed. In Cu and Ni, the dislocations inside the boundaries rearrange themselves from tangles to ordered arrays of parallel dislocations following interplay between strain (requirement for cross slip) and temperature (dislocation mobility and ease of cross slip). The ordered detailed structure is similar to that observed in Al deformed at room temperature and lower strain levels. The amount of strain and temperature applied to Cu and Ni in order to achieve the same detailed structure formed in Al depends on the stacking fault energy (SFE) of the metal- higher strain and temperature as the SFE is lower.
The annihilations of screw dislocation dipoles via cross-slip in Cu were simulated using constant-temperature constant-stress molecular dynamics. The cross-slip mechanism and annihilation process of flexible dislocations in a large dipole configuration was identified as a dynamic variant of the Friedel-Escaig mechanism. The cross-slip rate was found to exhibit exponential dependence on the temperature, from which the activation enthalpy for the cross-slip process was calculated by the Arrhenius relation.
A two-dimensional discrete dislocation dynamics code was developed and used to simulate dislocation patterning. For the present study of annealing, both glide and some climb were allowed in the simulations. In these circumstances patterning takes place even in the absence of external stresses and of dislocation sources. A triangular underlying lattice was assumed, with the three slip systems equally populated initially. Well-defined dislocation walls and cells are observed to form from random initial conditions. The structure coarsens with time, i.e. the typical size of the cells increases as annealing takes place (the smaller cells shrink and disappear from the structure). In the spirit of a bottom-up multiscale approach, it is suggested that a new simulation methodology should be developed, in which the discrete moving objects will be dislocation wall segments rather than individual dislocations.
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