Asymmetry in core structure and mobility of basal dislocations in a Ti3SiC2 MAX phase: An atomistic study with machine-learned force fields

Abstract

MAX phases are a unique class of atomically layered ternary ceramics that deform plastically at room temperature owing to highly mobile basal dislocations (BDs). To understand and control the ductility of these materials, it is crucial to study the core structures and mobilities of BDs. In this study we developed a machine-learning-based spectral neighbor analysis potential (SNAP) to perform atomistic simulations of edge, screw, and mixed BDs in a ${\mathrm{Ti}}_{3}{\mathrm{SiC}}_{2}$ MAX phase. The SNAP was trained on density functional theory (DFT)-calculated data. The SNAP calculations demonstrate that the BD core structure exhibits significant asymmetry that is dependent on the position of the weakly bonded Si layer relative to the $\mathrm{Ti}(4f)--\mathrm{Si}$ slip plane. The dislocation core either splits into Shockley partials or remains compact, depending on whether compressive or tensile stresses act on the Si layer. This asymmetry in the BD core structure agrees well with DFT results. Differential displacement and Nye tensor distribution analyses reveal that undissociated BD cores are centered on Si layers and spread over adjacent parallel basal planes. Additionally, they are three orders of magnitude less mobile than partial BDs. The Peierls stresses of partial edge BDs ($\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}71$ MPa) are closer to those of metals than those of ceramics, suggesting that edge BDs play a key role in the incipient plasticity of ${\mathrm{Ti}}_{3}{\mathrm{SiC}}_{2}$ MAX phases at low temperatures. The findings of this study contribute to a deeper understanding of the complex behavior of BDs at the atomic level and provide theoretical support for elucidating the unique deformation modes of crystals with atomically layered structures owing to the motion of BDs.