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

Abstract

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.

Acknowledgements

This work was supported by the US Air Force Office of Scientific Research under contract F33615-01-C-5214, and by a grant of computer time from the US Department of Defense High Performance Computing Modernization Program at the Aeronautical Systems Center Major Shared Research Center. This work was performed at the US Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright–Patterson Air Force Base.

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Additional information

Notes on contributors

S. I. Rao

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