Structural materials show novel properties with decreasing microstructural feature size, particularly in the nanocrystalline regime to the amorphous limit of grain size reduction. These materials show remarkable yield strength, radiation hardness, and fatigue resistance. At the amorphous limit, they also show strong corrosion resistance. Combining the two elements together to form a composite, these materials show exceptional yield strength and ductility, to significantly improve toughness that is often compromised in high-strength materials. However, plastic deformation at these length scales is difficult to characterize and novel mechanisms for deformation and failure operate in these system.
Although some progress has been achieved at visualizing deformation of nanocrystalline materials in-situ, the operating mechanisms are still under active debate. Further, classical continuum-based theories break down at the length of the dislocation core and grain boundary. However, this problem is ideally suited to atomistic simulation using molecular dynamics (MD) to characterize the energetics of dislocation-grain boundary interaction. Two limitations of the MD method, however, are the extreme strain rates and uncertainty regarding thermal activation. To bypass both of these, the approach here is not to focus on dislocation nucleation from a grain boundary under applied stress/strain, but rather the energetics of interaction and the potential barrier or trapping strength of athermal dislocation-grain boundary interactions. Studying the interaction between asymmetric tilt boundary and inclined dislocation, a position dependent response and strong resistance to step creation are observed. Also, a metastable higher energy grain boundary undergoes long-range reordering that reduces system energy in excess of the input dislocation line energy.
Nanocrystalline materials are observed to nucleate partial dislocations from grain boundaries, leading to deformation twinning. The energetics of this process is still under discussion. Because continuum theories break down in the region of the dislocation core and grain boundary plane, we must study the interaction energy of these systems using atomistic calculations. Due to the very high strain rates produced in classical MD calculations, the nucleation process is not accessible via this route. Here, we instead focus on a series of energy minimizing atomistic calculations between a low-symmetry boundary and a general dislocation to study the energetics of interaction, the nature of grain boundaries under stress, and what leads to dislocation pile-ups at grain boundaries. We find that boundaries resist dislocations with Burgers vectors normal to the boundary and a strong position dependence within the grain boundary sub-structure.
At the amorphous limit, bulk metallic glasses fail by intensely localized shear bands with very little macroscopic ductility, which limits structural service. It is understood through experiment and thermodynamics that a defect of volume from one to several vacancies plays a critical role in initiating and propagating shear, but not the structure of this defect or how it operates. Here we use atomistic modeling and electron density calculations to study defect distribution and migration in a metallic glass, and evolution under shear. We find a critical stress threshold to activate shear banding, and an enhancement of shear localization in the neighborhood of a dissolving void.