Seminar Prof. Brandon Runnels (University of Colorado Colorado Springs), 02.03.2023, 10:00

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.