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Abstract

Two-dimensional dislocation dynamics (2D-DD) simulations are employed to study the different mechanisms of plastic deformation of ultra-fine grained (ufg) metals at different temperatures. Besides conventional plastic deformation by dislocation glide within the grains, we also consider grain boundary mediated deformation originating in the absorption of dislocation into grain boundaries. To accomplish this, we treat absorbed dislocations by splitting up their Burgers vectors into two components parallel and orthogonal to the grain boundary, respectively. The first grain boundary dislocation, with the Burgers vector parallel to the grain boundary, is moving conservatively and thus causing grain boundary sliding; the latter grain boundary dislocation is climbing under production or annihilation of vacancies. The motion of this dislocation thus requires calculating the local concentration of vacancies within the grain boundary network. This is accomplished by solving the diffusion equation and coupling it to the dislocation motion via source and sink terms. The local vacancy concentration in turn exerts an osmotic force on the climb dislocations. The “material” is modeled as an elastic continuum that contains a defect microstructure consisting of a preexisting dislocation population, dislocation sources, and grain boundaries. The mechanical response of such a material is tested by uniaxial loading. In particular, the temperature and strain rate dependence of the grain boundary hardening and softening are studied. It is found that the strain rate sensitivity increases approximately linearly with temperature in ufg material whereas it practically remains constant at a low value for coarse grained materials. These numerical findings agree very well – in a qualitative sense – with experimental results known from the literature. Thus we conclude from our model results, that the absorption of dislocations in the grain boundary, which is controlled by grain boundary diffusion, is the rate-limiting factor for the plastic deformation of ufg metals.