Tension-compression asymmetry of pyramidal dislocations in magnesium

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.