Electric current carried by edge dislocations during the stress relaxation of NaCl single crystal was measured at room temperature.It was found that the current was proportional to the stress relaxation rate if the sample was strained less than about 1.3%.The inverse of current increased linearly with time at the beginning of stress relaxation. This is in accord with the results obtained by Ohring et al. for LiF and KCl single crystals.The fraction of charged dislocations, f, moving in the direction of the observed current is considered to decrease for large strains.
Stress relaxation tests at various strain rates and the repeated stress relaxation tests were carried out at room temperature to obtain the internal and effective stresses as well as the Gupta-Li's stress exponent, m*, in NaCl single crystals.It was found that the constant, a, appearing in the time pharenthesis of the integral form of the Gupta-Li equation, decreased with an increase of the strain rate accompanied by the increase of effective stress.With increasing cycle number of repeated relaxation experiment, it was found that the effective stress decreased monotonously, whereas the internal stress increased.The repeated relaxation curves cannot, in general, be superimposed on the initial stress relaxation curve if the re-strain rate is changed.This indicates that the mobile dislocation density (which is considered to be constant during one cycle of testing period) and the effective stress during the repeated stress relaxation test are not altogether equal to those of the initial stress relaxation test. However, if the mobile dislocation density could be chosen to be the same as that at the initial stress relaxation test, it may be possible to superimpose these two relaxation curves by translating along the time axis to the point where each effective stress agrees.Therefore, if the re-strain rate is too small not to produce the mobil dislocation density comparable with that in the initial test, the superposition is not to be expected. This indicates that the increasing rate of mobile dislocation density as a function of strain depends on strain rate.
The mechanism of divalent impurity aggregation in KCl-KBr mixed crystals has been studied by means of dielectric-loss measurements.When the concentration of KBr is low, the initial aggregation and the aggregation after the first plateau obey the third order kinetics. When the concentration of KBr is high, the kinetics order of the initial aggregation is not clear.However, the aggregation following the plateau obeys the third order kinetics.When the crystal containing KBr in low concentration is annealed at low temperature, another plateau is observed. But if annealed at high temperature, it is not observed.The activation energy for the dipole aggregation decreases with increasing concentration of KBr in the mixed crystals of KCl-KBr.
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