Ab-initio simulation of (a/2)⟨110] screw dislocations in γ-TiAl

The equilibrium core structure of an isolated (a/2)⟨110]{111} screw dislocation is calculated using a first-principles pseudopotential plane-wave method within the local-density approximation of the density functional theory. In this work the local dislocation strain field is self-consistently coupled to the long-range elastic field using a flexible-boundary condition method. This ab-initio adaptation of the Green's function boundary condition method makes it possible to simulate the dislocation in a very small periodic cell without compromising the fidelity of the final core configuration. Supercells of 210, 288 and 420 atoms are used to evaluate the local screw and edge displacements of a straight (a/2)⟨110]{111} screw dislocation in γ-TiAl. The predicted dislocation core is nonplanar with significant portions of the dislocation core spread on conjugate {111} glide planes. The nonplanar character of the dislocation core suggests that the dislocation is sessile and would readily glide on either of two {111} slip planes. The dislocation core also produces small but significant edge components that are expected to interact strongly with non-glide (e.g. Escaig) stresses, producing significant non-Schmid behaviour. Preliminary estimates of the lattice frictional stress for a pure (111) shear stress are in the range of 0.01 µ, where µ is the shear modulus.