Atomistic simulations and stochastic model of twinning and pyramidal slip nucleation at grain boundaries in Mg alloys

V Menon and L Qi, COMPUTATIONAL MATERIALS SCIENCE, 258, 113975 (2025).

DOI: 10.1016/j.commatsci.2025.113975

Enhancing non-basal slip activity in magnesium (Mg) alloys presents a promising approach to improving their mechanical properties, particularly ductility. In this work, we used molecular dynamics (MD) simulations to investigate the nucleation of deformation twinning and pyramidal slip under (c)-axis compression in Mg bicrystals containing pure and solute-segregated symmetric tilt grain boundaries (GBs). Twin nucleation at the GBs is analyzed for different GB geometries and solute concentrations for Mg-Al and Mg-Y alloys. Quantitative trends in the change of critical twin nucleation stress with GB geometry and solute concentration were identified using a mesoscale stochastic twin nucleation model. It is found that increasing Y concentrations at GBs promotes twin nucleation and reduces GB structure-related anisotropy in critical nucleation stresses observed in pure Mg GBs, but the effect of Y becomes insensitive to bulk Y concentrations once a critical concentration is exceeded. Further, a multi-step mechanism is proposed for explaining the effects of Y solutes on promoting pyramidal slip nucleation from the GBs. This mechanism involves: (i) GB dislocation dissociations leading to twin nucleation, (ii) favorable pyramidal I slip formation from twin nuclei, and (iii) full pyramidal I (c + a) slip formation. In contrast, only twin nucleation and growth from GBs are observed in Mg-Al alloys. These findings enhance our understanding of deformation mechanisms in Mg alloys. The proposed defect nucleation mechanisms and the corresponding stochastic nucleation model serve as critical inputs for mesoscale simulations, including crystal plasticity models of deformation of polycrystalline Mg alloys.

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