Abstract: Grain boundary (GB) migration is one of key underlying processes behind mechanisms ranging from recrystallization and solidification to work hardening. GBs have been repeatedly identified as the least understood of all material defects, a well-earned designation resulting from their wide range of behaviors over a very large configurational space. GB migration, in particular, exhibits a strong sensitivity to loading type, with a single boundary able to exhibit a wide range of behaviors under varied boundary conditions. Understanding these behaviors is essential to the analysis and design of many structural materials, and the talk consists of four talks. First, we present work at the atomic scale, using atomistic simulations and optimal transportation theory to provide a systematic way for predicting shuffling mechanisms. Second, we look at boundary migration as a dissipative process within a thermodynamic framework inspired by crystal plasticity. This approach unifies motion by driving force, shear coupling, mode switching, and stagnation as special cases of a general continuum evolution law for boundary motion. The model employs the principle of minimum dissipation potential, and introduces “dissipation energy” as an intrinsic GB property, measured using molecular dynamics. Agreement of dissipation energies across driving forces provides verification for the framework. Third, the mechanistic framework is applied directly to a phase field model for boundary migration. It is shown that, by combining nonconvex boundary energy, elasticity, and the principle of minimum dissipation potential, preferential boundary migration occurs spontaneously through the emergence of horizontally moving steps, which we identify as so-called phase field disconnections. A variety of boundaries are considered, including symmetric and asymmetric tilt boundaries. Fourth, we present “network plasticity,” an extreme multiscale model designed to convey realistic microstructural information to viable continuum length scales.
Bio: Dr. Brandon Runnels is an Associate Professor of Aerospace Engineering at Iowa State University. 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, graduating in June 2016. He moved to the University of Colorado Colorado Springs (UCCS) in August 2015, receiving tenure in August 2022, and then moving to Iowa State University in August 2023. Dr. Runnels is the director of the solid mechanics research group (www.solids.group), whose focus is the development of high-performance computational methods for simulating mechanics and materials in extreme environments. His work has been sponsored by the National Science Foundation, Office of Naval Research, Lawrence Berkeley National Laboratory, and Los Alamos National Laboratory. Dr. Runnels has been awarded the NSF CAREER award and the UCCS Engineering and Applied Science Researcher of the Year award in 2023.