Kernspinresonanz-Untersuchungen zur Dynamik von Versetzungsbewegungen in Aluminium-Legierungen

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Solution Hardening in Al-Zn Alloys. Mean Jump Distance and Activation Length of Moving Dislocations

Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in Al-Zn alloys. The NMR technique, in combination with strain-rate change experiments and transmission electron microscopy have been applied to study dislocation dynamics in Al-Zn alloys (1-2 at.% Zn). Spin-lattice relaxation measurements clearly indicate that fluctuations in the quadrupolar field caused by moving dislocations in Al-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 measured as a function of strain. Based on the NMR data and data obtained from strain-rate change experiments it could be concluded that moving dislocations advance over a number of solute atoms (order of 10) as described by Mott-Nabarro’s model and interact with forest dislocations as predicted by Friedel’s model. The strain rate change experiments confirm the linear additivity of flow stresses and the additivity of inverse activation length.

Solution Hardening in Aluminium-Magnesium Alloys: A Nuclear Magnetic Resonance and Transmission Electron Microscopic Study

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 27Al 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.

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

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-1.2 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.