1. Monday, February 15, 2016 4:00p.m. Jacobs Hall Room 298 “A general and predictive model of anisotropic grain boundary energy and morphology for polycrystal-level simulations” DR. BRANDON RUNNELS Assistant Professor of Mechanical and Aerospace Engineering University of Colorado, Colorado Springs ABSTRACT Grain boundaries (GBs) are known to be key players in a variety of mesoscopic processes, such as solidification, recrystallization, grain boundary migration, and severe plastic deformation. Though grain boundary energy (per unit area) is frequently treated as having a constant value, numerous experimental and computational studies indicate that grain boundary energy is strongly dependent on the geometric character of the interface; that is, the 5-dimensional space of the crystallographic orientation relationship and orientation of the interface. Additionally, although grain boundaries are frequently modeled as planar, it has been observed frequently that many interfaces exhibit a tendency to form complex microstructure. Such grain boundaries with complex morphology have been observed to play a crucial role in many micromechanical phenomena such as grain boundary migration, stability, and twinning. In this work, a new model for interface energy and morphology is formulated that is fast, general, and dependent on only three material parameters, and is verified by comparison with more than 40 molecular dynamics datasets for multiple types of symmetric/asymmetric tilt and twist boundaries, as well as experimental measurements, for FCC and BCC materials. In order to account for non-planar interfaces (e.g. with facets) it is necessary to extend the model by means of relaxation. A relaxation algorithm is presented that is able to efficiently compute the optimal facet pattern and corresponding relaxed energy. It is shown that the algorithm, used in conjunction with the grain boundary energy model, is able to predict the experimentally observed faceting patterns as well as thermally activated facet/de-facet transitions. Finally, the grain boundary model is implemented as an interface model in a polycrystal simulation to observe the effects of grain boundaries in conjunction with elastic and plastic deformation. The simulations for basic and realistic test cases are compared with those using an isotropic grain boundary model, and the effect of the grain boundary isotropy on the bulk properties of the sample as well as the microstructural evolution is determined. The results have immediate application towards, e.g., improved grain boundary models in polycrystal simulations, determining the relationship between GB energy and void nucleation, and techniques for optimal GB engineering. BIOGRAPHY Dr. Brandon Runnels is an assistant professor of mechanical and aerospace engineering at the University of Colorado, Colorado Springs. He obtained his BS in Mechanical Engineering from New Mexico Tech in 2011 and his MS from Caltech in 2012. He completed his PhD work at Caltech in the Computational Mechanics group under the advisement of Professor Michael Ortiz, and defended his thesis in June 2015. He recently joined the UCCS faculty in August, 2015. His research interests include: differential geometry applied to continuum mechanics; multiscale modeling of plasticity, phase transformation and grain boundaries; and high performance computing.