Investigation of dislocation dynamics by nuclear magnetic resonance

Abstract

Inspired by recent experiments of Sleeswyk, Kanert et al. investigating the effect of plastic deformation on the simultaneously measured nuclear spin-lattice relaxation time $T_{1\rho }$ in the rotating frame, the influence of moving dislocations on $T_{1\rho }$ is studied theoretically. As illustrated recently by Wolf, this necessitates the calculation of quadrupolar "lattice" correlation functions associated with the relative motion of dislocations and nuclear spins. Assuming discrete and random dislocation jumps during (i) plastic deformation, (ii) internal friction, and (iii) fatique experiments, these correlation functions are determined. Their general form allows prediction of $T_{1\rho }$ in both the strong-collision (Rowland-Fradin) and the quadrupolar weak-collision region. It is demonstrated how, from low-field $T_{1\rho }$ experiments performed during either one of the three types of deformation modes, dislocation parameters such as the mobile and immobile dislocation density, the mean time between successive jumps of a dislocation, and the distribution of loop lengths and jump widths may be extracted. As a declared goal, this article aims at the understanding of the properties of the $T_{1\rho }$ minimum (i.e., of its position, depth, and width) in terms of the underlying microscopic mechanism of dislocation motion, an information which, in the past, could not be gained from purely mechanical (e.g., internal friction) experiments. Consequently, a variety of new combined NMR-deformation experiments is proposed to investigate the microscopic dynamics of dislocation motion.