  Interactions among dislocations and solute atoms are the basis of several
important processes in metals plasticity. In body-centered cubic (bcc) metals
and alloys, low-temperature plastic flow is controlled by screw dislocation
glide, which is known to take place by the nucleation and sideward relaxation
of kink pairs across two consecutive \emph{Peierls} valleys. In alloys,
dislocations and solutes affect each other's kinetics via long-range stress
field coupling and short-range inelastic interactions. It is known that in
certain substitutional bcc alloys a transition from solute softening to solute
hardening is observed at a critical concentration. In this paper, we develop a
kinetic Monte Carlo model of screw dislocation glide and solute diffusion in
substitutional W-Re alloys. We find that dislocation kinetics is governed by
two competing mechanisms. At low solute concentrations, nucleation is enhanced
by the softening of the Peierls stress, which overcomes the elastic repulsion
of Re atoms on kinks. This trend is reversed at higher concentrations,
resulting in a minimum in the flow stress that is concentration and temperature
dependent. This minimum marks the transition from solute softening to
hardening, which is found to be in reasonable agreement with experiments.
