  We employ density-functional-theory calculations to analyze the interactions
between oxygen interstitial atoms and <a>-type screw dislocations (<a> =
a<11-20>/3 ) in alpha-titanium, based on investigations of generalized stacking
fault (GSF) energies, dislocation core structures and the strain field of an
oxygen interstitial. Compared with the substitutional atoms such as Al, there
is a large repulsive interaction between oxygen interstitials at the octahedral
site and an <a>-type screw dislocation core, since the volume for the
interstitial atom between Ti lattice sites is largely reduced in the core.
Differential displacement maps and distributions of interstitial volume
surrounding the dislocation core show that the interaction with an oxygen
interstitial is very short ranged (~ 5 Angstrom) and directional, mainly
located on the prismatic plane where the majority of lattice displacements in
the <a> screw dislocation core occurs. At long distances the interactions
mediated by the strain from the interstitial oxygen and the dislocation vanish.
We show that the large repulsive interaction at short range can push an oxygen
atom from its original interstitial site to a new interstitial site on the
basal plane in the dislocation core. The strong repulsive interaction and the
mechanical shuffle of oxygen interstitials can be expected to have a large
impact on dislocation dynamics, and can be used to explain the strong
strengthening effects of oxygen impurities in alpha-titanium observed
experimentally.
