Atomistic simulations of plasticity in nanocrystalline alloys

Due to their extraordinary mechanical properties, the field of research on nanocrystalline metals and their alloys has been steadily growing since the early synthesis attempts. Especially for the case of alloyed systems, however, a strong link between the macroscopic mechanical properties and the atomistic mechanisms being at the heart of a materials response to an external load is still missing. For addressing this problem, atomistic simulation techniques are used in this work. Several metals and their alloys are studied. The main emphasis is on elucidating generalized structure-property relationships for nanocrystalline microstructures. In the first part, an introduction to the subject and the employed methods is given. Then, the sample preparation technique is explained and the introduction of solute atoms into a nanocrystalline model structure is discussed, where several methods are compared. Characterization of the nanocrystalline alloys with focus on the elemental distribution in the microstructure shows that a high density of grain boundaries can drastically affect the local composition and the phase stability range for a given alloy. Uniaxial deformation simulations of the nanocrystalline alloys allow us to identify the atomic processes, which control the macroscopic mechanical behavior. Modulating the structural feature, which controls the strength i.e. the free volume in the grain boundaries by the introduction of different amounts of segregating solutes helps to develop a new scaling law, which depends not only on the grain size but also on grain boundary energy and the grain boundary relaxation. The variation of the concentration of solutes in different parts of the microstructure, where the composition in the grain interior and the grain boundaries is studied independently, reveals that conventional solid solution hardening is absent in nanocrystalline alloys. This supports the finding that the relaxation state of the grain boundary is controlling the strength of the material. After showing, that dislocation processes in the grain interior do not control the strength of the structures, their role for the ductility of the prepared alloys is tested by introducing intermetallic grains into the microstructure. Thus, the deformation is restricted to processes in the grain boundaries. Here it is demonstrated, that the ductility of the samples is strongly affected by the grain size while the strength controlling parameter is identical to miscible and segregating alloys. The delicate interplay between the different deformation processes mediated by the grain boundary is investigated for the competition between normal grain boundary motion and mesoscopic grain boundary sliding. It is analyzed, how this competition is altered by segregating solutes and under which conditions each mechanism is contributing to plastic deformation in nanocrystalline metals and alloys. The results by conventional molecular dynamics simulations are affected by the very high strain rates. Combining molecular dynamics with Monte Carlo simulations can overcome this limitation and allows to explore, how different deformation mechanisms are influenced by the simulation conditions. The Monte Carlo algorithm accounts for local relaxation by trial exchanges, shortcutting diffusional processes. It is shown how the balance between different contributions to plastic deformation depends on the local relaxation and how conventional molecular dynamics straining simulations overestimate the contribution by dislocation processes. The simulation of thermally activated processes in large systems with molecular dynamics is in general complicated by the limited timescales. Here, designing thermally stable microstructures offers the possibility to study the deformation processes at elevated temperatures without inducing grain growth. Thus, grain boundary creep in nanocrystalline model structures can be studied, if appropriate microstructures are used. Here, the effect of solute atoms is analyzed, where different compositions and different material systems are compared. The results show, how solute atoms affect the creep compliance of a nanocrystalline alloy. The atomic mechanisms governing the mass transport through the grain boundaries are discussed and compared to the processes governing plastic flow in bulk metallic glasses