Structure and kinetics of three-dimensional defects on the [formula omitted] twin boundary in magnesium: Atomistic and phase-field simulations

Abstract

Twinning is an important plastic deformation mechanism in Mg. While computational and experimental efforts have studied the two-dimensional equilibrium structure of boundaries and facets that surround twin domains, little consideration has been given to three-dimensional nonequilibrium structures. Furthermore, the relationship between the structure of nonequilibrium facets and the kinetics of twinning is also not well understood. Thus, the objective of this work is to characterize the structure of three-dimensional defects on the {101¯2} twin boundary and to study the kinetics associated with motion of the facets that bound these three-dimensional defects. Atomistic simulations show that the three-dimensional defects are bounded by nonequilibrium facets with prismatic/basal and twist pyramidal/pyramidal interfaces. The three-dimensional defects are surrounded on the {101¯2} twin boundary by disconnection loops. The kinetics of the twin domain facets at finite temperature are analyzed by both molecular dynamics and a newly proposed anisotropic phase-field model. The latter allows the deconvolution of the competing role of interface energy, mobility and internal stress state. Molecular dynamics simulations show that inclined facets control the annihilation process; this behavior is captured in the phase-field model using mobility for the coherent twin boundary that is significantly lower than that of crater inclined facets. Furthermore, molecular dynamics simulation results are best matched via the introduction of facet orientation dependent excess energies.