Solution hardening in aluminium-magnesium alloys: A nuclear magnetic resonance and transmission electron microscopic study

Abstract

Pulsed nuclear magnetic resonance techniques as well as transmission electron microscopy have been applied to study dislocation motion in aluminium magnesium alloys (0.2–1.6 at.% Mg). The spin lattice relaxation rate in the rotating frame of 27A1 has been been measured at 77 K as a function of strain at constant plastic strain rate ϵ. For finite strain rates, the movement of dislocations induces an additional relaxation rate arising from time fluctuations in the nuclear quadrupole interactions. From the motion-induced part of the relaxation rate the mean free path of mobile dislocations can be calculated. The NMR experiments are combined with transmission electron microscopic investigations to reveal the static structure of defects in the samples. The NMR measurements clearly indicate that fluctuations in the quadrupolar field caused by moving dislocations in AlMg are different compared to those in ultra pure Al. From the NMR data it could be concluded that moving dislocations advance over a number of solute atoms (order of 7) as described by Mott-Nabarro's model. On the other hand, Mott-Nabarro's model does not predict the effective solute spacing as a function of the concentration of solute atoms in accordance with NMR experiments.