Shear-driven motion of Mg {101¯2} twin boundaries via disconnection terrace nucleation, growth, and coalescence

Abstract

Twin nucleation and growth are prevalent plastic deformation mechanisms in hexagonal close-packed metals such as Mg. For twin growth to occur, the interfaces that border the twinned domain must migrate and the kinetics associated with this process are yet to be fully explained. Thus, the objective of this study is to characterize the relationship between the kinetics of the ${10\overline{1}2}$ twin boundary in pure Mg in the stress-driven regime, and the nucleation, growth, and coalescence of disconnection terraces that serve as the mechanisms for migration. This problem is addressed via atomistic simulations adopting both two- (2D) and three-dimensional (3D) simulation geometries, and a model for the velocity of the ${10\overline{1}2}$ twin boundary as a function of temperature and shear stress is proposed. This study shows that the kinetics of ${10\overline{1}2}$ twin boundary migration must be addressed using 3D models, as 2D simulations do not properly capture disconnection terrace nucleation and growth processes, demonstrated via differences in activation volumes and energies. Importantly, simulations reveal an autocatalytic terrace nucleation mechanism as playing a role in twin growth, where nucleation of a new terrace is dependent on the growth of existing terraces on the twin plane.