Evolving dislocation cores at Twin Boundaries: Theory of CRSS Elevation

Abstract

Superior mechanical response of twinnable materials fundamentally arises from an elevation of Critical Resolved Shear Stresses (CRSS) due to Dislocation-Twin Boundary (D-TB) reactions. These reactions exhibit rich variety with several possible outcomes and exhibit complex dependence on microstructural properties, causing state-of-the-art models to adopt a case-by-case simulation of each reaction relying on empirical potentials or twin-interaction parameters. We develop an analytical “Evolving Dislocation Core” (EDC) model devoid of empiricism, capable of predicting the CRSS-elevation for any reaction, given the microstructural properties (elastic constants, twin crystallography, etc.). The approach is fundamentally rooted in energy-minimization within a fully-anisotropic framework revealing the evolution of dislocation cores with progression of the reaction. The core-structure of complex dislocations (e.g. stair-rod) in the reaction is proposed, for the first time in literature, as a non-planar composite of disregistries distributed on slip and twin planes. The model is applied to multiple slip-incorporation reactions in several Face-Centered-Cubic (FCC) materials (Pb, Ag, Cu, Ni-Co alloys and Ni-Ti alloys and high-entropy alloy FeNiCoCrMn). The predicted CRSS-elevations show agreement with atomistic simulations (Ni) and experiment (FeNiCoCrMn). The model further establishes a strong correlation of the elevation with unstable stacking/twinning fault energy and the magnitude of the sessile dislocation's Burgers vector, while revealing poor correlation with the stable intrinsic stacking fault energy which is a common benchmark. Thus the analytical EDC model developed in this study advances understanding of slip-twin interactions on multiple fronts while serving as an effective predictive model for CRSS-elevation instrumental in materials design.