Eric Nicholas Hahn

It is generally understood that microstructure plays a significant role in determining the deformation response of materials. During shock compression, grain boundaries serve as dislocation nucleation/pile-up/adsorption sites and grain size can alter the width of the shock front. During tensile release, grain boundaries are often "weak links" where spallation occurs. As such, a current deficit in predictive modeling capability is a quantitative description of these locations and their relative ability to serve as void nucleation sites - a challenging component of such a description is that spallation is inherently stochastic in nature. The inclination of the grain boundary plane with respect to the loading direction is thought to be a critical constituent in the resultant stress and failure at the boundary. Non-equilibrium molecular dynamics simulations are used to statistically quantify the influence of grain boundary inclination on the location of void nucleation and to highlight the emergence of stress hotspots at such boundaries. Boundaries oriented perpendicular to the loading direction are more likely to fail, but grain boundary inclination alone is not a complete predictor - i.e. not all perpendicular boundaries fail during spallation.

Several factors can affect the failure stress of a grain boundary, such as grain boundary structure, energy and excess volume, in addition to its interactions with dislocations. In this paper, we focus on the influence of grain boundary energy and excess volume at the boundary on the failure stress of a grain boundary in tantalum from molecular-dynamics simulations. Flyer plate simulations were carried out for a handful of boundary types with different energies and excess volumes. These boundaries were chosen as model systems to represent various boundaries observed in "real" materials. For a small, but representative, set of boundaries explored, no direct correlation was observed between the void nucleation stress of a boundary and either its energy and excess volume. This result suggests that average properties of grain boundaries alone are not sufficient indicators of the failure strength of a boundary.