Search

A general, twisted and tilted, grain boundary in copper has been simulated using the adaptive kinetic Monte Carlo method to study the atomistic structure of the non-crystalline region and the mechanism of annealing events that occur at low temperature. The simulated time interval spanned 67 μs at 135 K. Similar final configurations were obtained starting from different initial structures: (i) by bringing the two grains into contact without any intermediate layer, and (ii) by inserting an amorphous region between the grains. The results obtained were analyzed with a radial distribution function and a common neighbor analysis. Annealing events leading to lowering of the energy typically involved concerted displacement of several atoms—even as many as 10 atoms displaced by more than half an Ångström. Increased local icosahedral ordering is observed in the boundary layer, but local HCP coordination was also observed. In the final low-energy configurations, the thickness of the region separating the crystalline grains corresponds to just one atomic layer, in good agreement with reported experimental observations. The simulated system consists of 1307 atoms and atomic interactions were described using effective medium theory. [ABSTRACT FROM AUTHOR]

Atomic-scale computer simulations have previously identified a deformation mechanism, which becomes important in nanocrystalline metals with grain sizes below 10–15 nm. Instead of proceeding through dislocation activity in the grains, the deformation occurs by slip events in the grain boundaries, leading to a reverse Hall–Petch effect, i.e. a decrease in hardness with decreasing grain size. In this paper, the consequences of this shift in deformation mode are investigated for systems subjected to large strains in a cyclic deformation pattern.In most coarse-grained metals, severe plastic deformation leads to grain refinement. Indeed, severe plastic deformation is often used to generate nanocrystalline metals with grain sizes down to hundred nanometres. The simulations indicate that these processes are suppressed in sufficiently small grains, and instead the sliding events in the grain boundaries dramatically enhance diffusion processes, and lead to grain coarsening as the deformation proceeds. [Copyright &y& Elsevier]

Presents research which conducted computer simulations of the deformation of nanocrystalline copper to show a softening with grain size for the smallest sizes. Properties of nanocrystalline solids; Producing nanocrystalline metals; What the Hall-Petch effect is; Reason for most of the plastic deformation; Limit on strength of nanocrystalline metals.