Effect of Ca addition on plastic flow in nanocrystalline magnesium by atomistic simulation

Abstract

Atomistic simulations are used to study the effect of calcium (Ca) addition on plastic flow of nanocrystalline hexagonal close-packed (hcp) magnesium (Mg) under uniaxial tensile deformation. The nanostructure materials are constructed using the Voronoi tessellation method, and the plastic deformation behavior is studied by molecular dynamics simulation. A second nearest neighbor modified embedded atom method (2NN-MEAM) is applied to represent interatomic potential energy. Calcium atoms with different concentration of 0.0–1.3 wt% are randomly distributed in the magnesium nanocrystal with mean grain size of 8.5 nm as substitution atoms. Surprisingly, deformation twinning is not observed in the samples unlike the easy occurrence of twinning in Mg nanocrystal with coarse-grained structure. Structural analysis demonstrated that plastic deformation occurred in the studied Mg-Ca alloys via slip of partial dislocations, nucleation and growth of disordered atom segments (DAS) inside grains. The continuous accumulation of stacking faults induces hcp to fcc (face centered cubic) phase transformation. MD simulation results show that the tensile stress required for partial dislocation nucleation decreases with increasing Ca concentration. The presence of Ca atoms as solid solution in the structure of nanocrystalline Mg-Ca alloy influences the densities of stacking faults and DAS which promotes nucleation of partial dislocations and DAS.