In-Situ NMR Investigation of Dynamical Behavior of Point and Line Defects During Deformation of NaCl

Abstract

Spin lattice relaxation time in the rotating frame [T1ρ] is investigated in pure NaCl single crystals as a function of temperature in-situ during deformation. Transition from 2- phonon Raman process to atomic diffusion was noted at around 500K in the undeformed material and the activation energy was determined to be that for the diffusion of extrinsic vacancies. Enhanced spin relaxation rates were noted during constant strain-rate deformation at temperatures from ambient to about 750K. These enhancements were identified to arise from dislocation motion at lower temperatures while enhanced diffusion due to excess vacancies at higher temperatures. This excess concentration increased with increased strain-rate and in-situ annealing of deforma-tion induced excess vacancies is noted at high temperatures.

In situ nuclear magnetic resonance study of defect dynamics during deformation of materials

“…impurity levels) 23NaC1 which made it possible to evaluate the temperature and strain-rate dependencies of the strain-induced vacancy concentration. While these results were slightly different from the earlier data [9], they are in agreement with the models on the strain induced vacancy production [9,10]. In addition, experiments were performed on NaF 19 (with no quadrupolar interactions because of spin I = 89 where no dislocation contribution to the spin relaxation is expected.…”

“…The formation process is governed by the non-conservative motion of jogs on screw dislocations and following Detemple et aI. [9] we find…”

“…We have recently reported an investigation of the superimposed effects of deformation on spin-lattice relaxation time (of Z3Na) in the rotating frame, T10, in singlecrystalline pure (~ 1.5p.p.m. impurity) NaC1 at temperatures from ambient to about 900 K at various constant strain-rates [9] and identified strain-enhanced diffusion at high temperatures and the dislocation contribution at low temperatures. This analysis of strain-enhanced diffusion was based on the vacancy production due to the motion of thermally produced dislocation jogs, while mechanical jogs produced due to the dislocation intersections, could also play a major role [10].…”

“…These results on the strain-induced vacancy concentration have important bearing on the understanding of the dynamic strain ageing (DSA) and the development of quantitative models characterizing the DSA [12,13]. In particular, the critical strain, ~, for the onset of serrations is related to the strain-induced vacancy concentration and mobile dislocation density [12,13] Detemple et al [9] clearly revealed the increased vacancy concentration during deformation and made it possible to establish a quantitative relation between the strain-induced vacancy concentration and the applied strain-rate, strain and temperature.…”

“…We use the analyses presented elsewhere [9,10,12] to derive expressions for the excess vacancy concentration during deformation. It is realized that the excess vacancies produced during deformation migrate to sinks such as the jogs on dislocations and thus these strain-induced vacancies follow the rate equation…”