UC Irvine

Faceted grain boundaries, where grain boundary area is increased in the name of producing low-energy segments, have unique energetic properties and defect structures that have yielded important insights into grain boundary structure-property relationships. However, less information exists about their dynamic behavior. What is known shows that faceted boundaries may impact microstructure evolution in unexpected ways. Recent research showing a variety of faceted Σ3 boundaries in Ni to exhibit new migration trends, motivating a deeper study of other faceted boundary systems. The faceted Σ11 tilt boundaries represent a promising but as yet unexplored set of interfaces with have highly asymmetric, unique geometries, which suggests that they may have similarly unique migration mechanisms and segregation tendencies. The first part of this dissertation is dedicated to exploring this possibility in pure face-centered cubic materials. Molecular dynamics studies across a range of different faceted Σ11 <110> tilt boundaries in Cu and Ni are performed. It is revealed that these boundaries’ mobilities are strongly dependent on the direction of the applied driving force, a phenomenon we name directionally-anisotropic mobility. This effect generally becomes smaller, but does not disappear completely, as temperature is increased. In contrast, the same faceted bicrystals in Al demonstrate similar mobilities in either direction, illustrating that directionally-anisotropic mobility is a material-dependent phenomenon. An atomistic migration mechanism related to stacking fault energy is identified as an important mediator of a rate-limiting process. The second part of this dissertation expands the study of faceted Σ11 boundaries to alloy systems. Facet-specific segregation trends are systematically studied through changes in temperature and composition. In the Cu-Ag system, site-specific segregation is found to be related to excess volume and a local tension-compression field discontinuity introduced by the emission of Shockley partial dislocations near facet junctions. Additional mobility simulations using a high-temperature, dilute variant reveals that solute atoms can induce directionally-anisotropic mobility. Finally, similar studies conducted in multi-principal element alloys reveal strong intrinsic segregation tendencies, even up to 90% of the melting temperature of the alloy. A spatial analysis of the elemental composition near facets reveals regions of enrichment beyond what is typically considered a part of the grain boundary structure.