  We address the question of the observed pinning of 1/2 <110] ordinary screw
dislocations in gamma-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: 6eV, while the single kink migration
energy is conversely very small: 0.13eV. 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 gamma-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 gamma-TiAl must be attributed to a combination of extrinsic
obstacles and extensive cross-slip in a crystal containing impurities.
