Dislocation dynamics in aluminium and in aluminium-copper 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 ultrapure aluminium and aluminium-copper alloys ( Al: xCu with x max = 1 at.%). The spin-lattice relaxation rate in the rotating frame. T 1 ρ −1of 27Al has been measured as a function of the plastic strain rate ϵϵ at 77 K. For finite strain rates ϵϵ, dislocations induces an additional relaxation rate arising from time fluctuations in the nuclear quadrupole interaction. From this motion-induced part of the relaxation rate the mean free path L of mobile dislocations can be calculated which is determined by the distribution of lattice defects acting as obstacles for moving dislocations. The NMR experiments are combined with transmission electron microscopic investigations to reveal the static structure of defects in the samples. Correlations between the in situ observed mean free path L of mobile dislocations and between the microscopic defect structure arising from the ageing process (θ′ phase, solid solution) and the degree of plastic deformation ϵ are shown. It turns out that in the θ′ phase at small strains L is determined by the microstructure and is equal to the mean separation between the precipitates. For large strains L is determined by the statistical distribution of the dislocation loops lying between the precipitates. On the other hand, in ultrapure aluminium L is determined by the dislocation cell structure.