Dislocation dynamics in Al–Mg–Zn alloys: A nuclear magnetic resonance and transmission electron microscopic study

Abstract

Pulsed nuclear magnetic resonance (NMR) proved to be a complementary new technique for the study of moving dislocations in Al–Mg–Zn alloys. The NMR technique, in combination with transmission electron microscopy (TEM), has been applied to study dislocation motion in Al–0.6 at. % Mg–1 at. % Zn and Al–2 at. % Mg–2.5 at. % Zn. Spin-lattice relaxation measurements clearly indicate that fluctuations in the nuclear quadrupolar interactions caused by moving dislocations in Al–Mg–Zn are different compared to those in ultra pure Al. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been determined as a function of strain. From the NMR data it is concluded that moving dislocations advance over a number of solute atoms in these alloys as described by Mott-Nabarro's model. At large strains there exists a striking difference between the mean jump distances in Al–0.6 at. % Mg–1 at. % Zn and in Al–1.2 at. % Mg–2.5 at. % Zn. The latter is about five times smaller than the former one. This is consistent with TEM observations that show dislocation cell formation only in Al–0.6 at. % Mg-1 at. % Zn and the macroscopic stress-strain dependences of these alloys.

Effects of solute Mg on grain boundary and dislocation dynamics during nanoindentation of Al–Mg thin films

“…Consequently, the mean jump distance can be predicted by Mott-NabarroÕs model of weakly interacting diffuse forces between Mg solutes and dislocations in Al [33]. A calculation of the effective obstacle spacing assuming that the maximum internal stress around a solute atom has a logarithmic concentration dependence yields a value of 30 nm in Al-2.6wt%Mg [19,34]. This is in fair agreement with our experimental observation of a mean jump distance of the order of 50 nm.…”

Advances in Transmission Electron Microscopy: Self Healing or is Prevention Better than Cure?

“…Due to the single-tilt axis limitation of the indentation stage, the orientation of the slip plane relative to the electron beam is unknown; therefore, the measured jump distance is a projection and a lower bound of the actual jump distance. At the low strains for which jerky-type dislocation motion is observed, solute atoms are the predominant barriers to mobile dislocations, as has been shown in earlier in situ pulsed nuclear magnetic resonance (NMR) experiments [5,[26][27][28]. Consequently, the mean jump distance can be predicted by Mott-Nabarro's model of weakly interacting diffuse forces between Mg solutes and dislocations in Al [27].…”

“…At the low strains for which jerky-type dislocation motion is observed, solute atoms are the predominant barriers to mobile dislocations, as has been shown in earlier in situ pulsed nuclear magnetic resonance (NMR) experiments [5,[26][27][28]. Consequently, the mean jump distance can be predicted by Mott-Nabarro's model of weakly interacting diffuse forces between Mg solutes and dislocations in Al [27]. A calculation of the effective obstacle spacing, assuming that the maximum internal stress around a solute atom has a logarithmic concentration dependence, yields a value of 30 nm in Al-2.6% Mg.…”