Deformation behaviour of nanocrystalline alloys studied by hybrid MD/MC simulations

Karsten Albe
Technishche Universtitat Darmstadt

Nanocrystalline metals show peculiarities in their deformation behavior due to the large density of grain boundaries, which act as sources and sinks for dislocations and allow for grain boundary sliding as well as rotation. Although molecular dynamics simulations have significantly contributed to an understanding of atomistic mechanisms in these materials, in the past, simulation results have been questioned since the virtual strain rates exceed those in experiments by several orders of magnitudes and lead to high local stresses. Moreover, the influence of solutes on the deformation behavior of nc-metals has hardly been considered in simulation studies. In this contribution I will address these issues and show how a coupled MD/MC scheme that allows to equilibrate the atomic distribution in alloys can be used to mimic alloying and also strain rate effects, thus providing new insights into the interplay of atomic scale processes. The influence of segregating solutes on the deformation mechanisms is studied for binary nanoalloys and different cases of solute distributions are compared. We find that the competition between mesoscopic grain boundary sliding and coupled grain boundary motion is controlled by the concentration and distribution of segregating solutes. By analyzing the microstructural evolution and dislocation activity we make a connection between the atomistic solute distribution and the mechanisms of deformation, explaining the observed stress-strain behavior. The detailed analysis of the normal GB motion reveals a stick-slip behavior and a coupling factor which is consistent with results from bicrystal simulations. Using a novel method to quantify the amount of crystal slip strain from atomistic data, it is demonstrated how plastic deformation carried by dislocation changes as a function of the local relaxation. The results indicate that conventional molecular dynamics simulations overestimate the contributions of dislocation slip to the overall plastic deformation of nanocrystalline samples.

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