A stress relaxation method is employed to obtain internal and effective stresses in NaCl single crystals at various strain rates before and after a γ‐irradiation. The conditions are found at which dislocation mobility estimated from the plastic strain rate during relaxation coincides with the velocity of individual dislocations measured by a direct method.

The stress relaxation method is used to obtain the internal and effective stresses, and, also, an average dislocation velocity in deformed NaCl polycrystals. The grain size reduction and increasing stress result in the fact that the dragging role of grain boundaries increases and the dislocation interaction gets strong. Owing to this, the effective stress increases and an average dislocation velocity does not coincide with the velocity of individual dislocations as different from the case for NaCl single crystals. An activation volume decrease with deformation caused by point formation during plastic deformation is observed.