Role of temperature on screw dislocation dynamics in Ta, W, and Ta-W alloy

The mechanical properties of body-centered cubic (bcc) materials exhibit a characteristic temperature dependence that have conventionally been associated with the effect of temperature on the stress required for dislocation motion. In this work, we investigate the effect of temperature on screw dislocation dynamics in Ta, W, and Ta-W alloy using a three-dimensional phase-field-dislocation dynamics model combined with Langevin dynamics. The model only uses temperature dependent elastic moduli and generalized stacking fault energy curves from atomistic calculation. For a broad range of temperatures and in all metals, a critical stress associated with a jerky-to-smooth motion transition of a long screw dislocation is predicted. We show that glide at this critical threshold undergoes two temperature-induced transitions. For all three materials, the critical stress for screw dislocation motion declines with increases in temperature in three stages, eventually reaching a plateau where it is insensitive to temperature. These transition temperatures strongly correlate with those corresponding to experimentally measured transitions in yield strength with increases in temperature. At low temperatures, the classic kink pair mechanism prevails and the predicted activation enthalpies for kink pair formation are shown to agree quantitatively with those reported by atomistic and/or experimental studies. Finally, the material deforming under high temperature and stress are subsequently cooled to room temperature and fully unloaded to examine the dislocation line morphologies "post-mortem". It is shown that the initial screw orientation is nearly recovered regardless of the prior deformation temperature and stress. These findings imply that the thermally activated motion of screw dislocations governs the temperature-dependent strength of bcc metals and alloys over a much wider range of temperatures than conventionally thought.