Pyramidal dislocations are important for ductility enhancement of magnesium alloys. In this work, molecular dynamics simulations were employed to study the gliding behavior of pyramidal 〈c + a〉 dislocations under c-axis compressive loading and tensile loading. The Peierls stress of Py-Ⅰ dislocation shows strong tension-compression asymmetry. However, no tension-compression asymmetry is seen on the Py-Ⅱ dislocation and basal dislocation. The tension-compression asymmetry origins from the asymmetry of partial dislocations of Py-Ⅰ dislocation, which leads to the dislocation core contracted under c-axis compressive loading and expanded under tensile loading. By analyzing the forces acting on the partial dislocations, we defined a neutral direction, which deviates from the full dislocation Burgers vector by 70.3°. The neutral direction is dependent on the ratio of lattice stresses of partial dislocations. If the shear stress is applied along the neutral direction, tension-compression asymmetry is eliminated and the dislocation core is un-contracted/un-expanded. The neutral direction of symmetrical dislocations (Py-Ⅱ dislocation and basal dislocation) is just the full dislocation Burgers vector. The tension-compression asymmetry and dislocation core contraction/expansion have an important influence on the dislocation behaviors, such as cross-slip, decomposition, basal-transition and mobility, which can be used to explain the mechanical behaviors of Mg single-crystals compressed along c-axis.

The interactions between a plate-like precipitate and two twin boundaries (TBs) ({1012}, {1121}) in magnesium alloys are studied using molecular dynamics (MD) simulations. The precipitate is not sheared by {1012} TB, but sheared by {1121} TB. Shearing on the (110) plane is the predominant deformation mode in the sheared precipitate. Then, the blocking effects of precipitates with different sizes are studied for {1121} twinning. All the precipitates show a blocking effect on {1121} twinning although they are sheared, while the blocking effects of precipitates with different sizes are different. The blocking effect increases significantly with the increasing precipitate length (in-plane size along TB) and thickness, whereas changes weakly as the precipitate width changes. Based on the revealed interaction mechanisms, a critical twin shear is calculated theoretically by the Eshelby solutions to determine which TB is able to shear the precipitate. In addition, an analytical hardening model of sheared precipitates is proposed by analyzing the force equilibrium during TB-precipitate interactions. This model indicates that the blocking effect depends solely on the area fraction of the precipitate cross-section, and shows good agreement with the current MD simulations. Finally, the blocking effects of plate-like precipitates on the {1012} twinning (non-sheared precipitate), {1121} twinning (sheared precipitate) and basal dislocations (non-sheared precipitate) are compared together. Results show that the blocking effect on {1121} twinning is stronger than that on {1012} twinning, while the effect on basal dislocations is weakest. The precipitate-TB interaction mechanisms and precipitation hardening models revealed in this work are of great significance for improving the mechanical property of magnesium alloys by designing microstructure.