Disconnection-mediated twin embryo growth in Mg

TL;DR

This study uses molecular dynamics simulations and a phenomenological model to elucidate the atomic processes governing twin embryo growth in magnesium, highlighting disconnection propagation as the rate-limiting step and correlating disconnection velocity with shear stress.

Contribution

It introduces a quantitative model linking disconnection dynamics and twin tip propagation in Mg, validated by simulations and experimental data.

Findings

Disconnection propagation controls twin boundary motion.

A phenomenological model accurately fits simulation data.

Linear relationship between disconnection velocity ratio and shear stress.

Abstract

While deformation twinning in hexagonal close-packed metals has been widely studied due to its substantial impact on mechanical properties, an understanding of the detailed atomic processes associated with twin embryo growth is still lacking. Conducting molecular dynamics simulations on Mg, we show that the propagation of twinning disconnections emitted by basal-prismatic interfaces controls the twin boundary motion and is the rate-limiting mechanism during the initial growth of the twin embryo. The time needed for disconnection propagation is related to the distance between the twin tips, with widely spaced twin tips requiring more time for a unit twin boundary migration event to be completed. Thus, a phenomenological model, which unifies the two processes of disconnection and twin tip propagation, is proposed here to provide a quantitative analysis of twin embryo growth. The model fits the simulation data well, with two key parameters (twin tip velocity and twinning disconnection velocity) being extracted. In addition, a linear relationship between the ratio of twinning disconnection velocity to twin tip velocity and the applied shear stress is observed. Using an example of twin growth in a nanoscale single crystal from the recent literature, we find that our molecular dynamics simulations and analytical model are in good agreement with experimental data.