The matrix-twin transition in a perfect Mg crystal: Ab initio study

By means of ab initio calculations, we studied the matrix - twin transition in a perfect Mg crystal where the {10 (1) over bar2} plane is the twinning plane. The twinning transition is described by the shear of a suitable periodically repeated unit cell and by the atomic shuffling inside the unit cell. We found two hcp - hcp paths that can create the glide and reflection {10 (1) over bar2} twin boundary, respectively. Assuming a perfect hcp crystal, it turned out that the total energy profiles of both hcp - hcp paths are the same. We further pointed out the equivalence between the description of the matrix - twin transition in the frame of an orthorhombic and of a monoclinic unit cell, both containing four atoms. We have tracked the trajectories of the net atomic motion that are energetically very favorable. In addition, the effect of volume relaxation on the total energy profiles was investigated and its influence on the plastic deformation of Mg crystal was discussed.