MD simulation of asymmetric nucleation and motion of 〈011〉 superdislocations in TiAI

The nucleation and propagation of （011] su- perdislocations in intermetallic TiA1 were investigated using molecular dynamics simulations and static energetics calculation, as part of our systematic effort to understand the twining and dislocation behavior of alloys based on y-TiAl. It was found that compared to ordinary dislocations in disordered crystals, superdislocations in ordered TiA1 lattice behave differently when sheared in the two opposite senses along 01-1] direction. This difference is due to the lower Llo lattice symmetry compared with the face-cen- tered cubic （fcc） lattice that it based on, with different yield stress and strain, and dislocation core dissociation and motion. Superdislocations nucleated in the form of loops dissociated in a planar manner into four Shockley partials separated by three kinds of faults： superlattice intrinsic stacking fault （SISF）, anti-phase domain boundary （APB） and complex stacking fault （CSF）, with partial separations depending on the sense of shearing and dislocation char- acter. During loop expansion, the dislocation core changes both in width and dissociation manner depending on the character of the segment in the loop. The core contains four partials close to edge orientation, gradually changing to three fold near 60°, and finally into twofold dissociation around 30° character. Superdislocations may have multiple critical resolved shear stresses （CRSS） for motion depending on dissociation and shearing sense even for the same slip system, with lower critical stress for the motion when SISF is in leading position.