Internal stresses and dislocation mobility in NaCl single crystals

Abstract

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.

Effect of the Polycrystalline Structure on the Relaxation Processes and Dislocation Motion in NaCl Crystals

“…change of ui) and, secondly, the conditions of overcoming the obstacles by dislocations do not change due, foi instance, to their increasing density during plastic deformation. Then, these dependences will be parallel to one another and shifted by the value of A lg em, here @(a*) == v(a), which has been observed previously in NaCl crystals [3]. From Fig.…”

“…2 it is seen that no such parallelism between curves is observed in polycrystals, which implies the invalidity of one or both of the abovementioned conditions. Experiments on the repeated loading and relaxation analogous to those in [3] showed that the stresses where plastic flow occurs as well as the form of relaxation curves are held unchanged under reloading. Thus, it is believed that no hardening of polycrystals occurs in the process of relaxation.…”

“…The stress relaxation technique can be used to determine internal and effective stresses, bi and O* and such measurements have been repeatedly carried out in NaCl single crystals [l to 31. With this method we were able to determine the dependence V(G*) [3] which really proved coincident with the dependence of the velocity of individual dislocations on stress, ~( 6 ) .…”

“…NaCl single crystals of approximately the same chemical composition as studied earlier [3] were used to obtain the polycrystalline samples. As-grown crystals 60 nini in diameter and 120mm long were polished subsequently using water, 50% ethyl alcohol solution in ammonia, and pure ethyl alcohol with washing in butyl alcohol to finish, until a cylindrical form was obtained.…”

“…Apart from this, for the polycrystals with d = 30 and 15 pm a t u 2 24 MPa, the courses of the curves a(&) and 0 005 010 015 020 &(%I -L (%I -Fig. 1. a) Stresses o (1, 3, 5, 7)and internal stresses ui (2, 4, 6, 8) versus plastic strain E for polycrystals with grain sizes 5 ym (1, 2), 15 pm (3, 4), 30 ym(5, 6) and for pure NaCl single crystals[3] (7,8). b) The same as a) but for a larger scale of the initial microplastic stage (9).…”