Dislocation Mobility in Crystals

Reaction-rate theory is applied to dislocation motion and it is shown that when the activated complex is allowed two degrees of freedom, the activation energy is inversely proportional to the applied stress, so the dislocation velocity, vd is related to the applied shear stress, τ by a relation of the form: v=v0e−D/τ, where v0 is the terminal velocity and D is called the characteristic drag stress. This relation is obeyed by LiF, Ge, NaCl, W, and possibly Fe−3% Si. The observed dependences of dislocation velocities on point defect concentrations and temperature are also predicted. Two classes of crystals are considered. Those in which drag is caused mainly by point defects; and those in which the intrinsic structure (chemical bonds) causes the drag.