Is the pinning of ordinary dislocations in γ-TiAl intrinsic or extrinsic in nature? A combined atomistic and kinetic Monte Carlo approach

We address the question of the observed pinning of 12〈11¯0] ordinary screw dislocations in γ-TiAl, which leads to the characteristic trailing of dipoles in the microstructure. While it has been proposed that these may be variously intrinsic or extrinsic in nature, we are able to rule out the former mechanism. We do this by means of very large scale, three-dimensional atomistic simulations using the quantum mechanical bond order potential. We find that the kink-pair formation energy is large – 6 eV – while the single kink migration energy is conversely very small – 0.13 eV. Using these, and other atomistically derived, data, we make kinetic Monte Carlo simulations at realistic time and length scales to simulate dislocation mobility as a function of stress and temperature. In the temperature range of the stress anomaly in γ-TiAl, we determine whether one or several of the pinning and unzipping processes associated with generation of jogs are observed during our simulations. We conclude that the pinning of ordinary dislocations and anomalous mechanical behaviour in γ-TiAl must be attributed to a combination of extrinsic obstacles and extensive cross-slip in a crystal containing impurities.