  The interaction between carbon and screw dislocations in tungsten is
investigated using ab initio calculations. The presence of carbon atoms in the
vicinity of the dislocation induces a reconstruction, with the dislocation
relaxing to a configuration, the hard core structure, which is unstable in pure
tungsten. The reconstruction corresponds to a strong binding of carbon in the
prismatic sites created by the dislocation which is perfect for high
concentrations of carbon segregated on the dislocation line. However, the
reconstruction is only partial for lower atomic fractions, with the dislocation
tending to fall back in its easy core ground state. This pinning by carbon
atoms of the dislocation in an unstable position is well described by a simple
line tension model. A strong carbon-dislocation attraction is also evidenced at
larger separation distances, when the solute is in the fourth nearest neighbour
octahedral sites of the reconstructed core. The equilibrium concentrations of
carbon in these different segregation sites are modelled with an Ising model
and using a mean-field approximation. This thermodynamic model evidences that
screw dislocations remain fully saturated by carbon atoms and pinned in their
hard core configuration up to about 2500 K.
