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+opper alloys (Al:xCu with x,, = 1 at.%). The spin-lattice relaxation rate in the rotating frame. 7,' of ".\I has been measured as a function of the plastic strain rate < at 77 K. For fimte strain rates i. the movement of dislocations induces an additional relaxation rate arising Irom time fluctuations in the nuclear quadrupole interaction. From this motion-induce part of the relaxation rate the mean free path L of mobile dislocations can be calculated which is determine 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 agcing process (6' phase, solid solution) and the degree of plastic deformation E are shown. It turns out that in the 0' 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. R&sum&-Nous avons &udiC le mouvement des dislocations dans I'aluminium ultrapur et des alliages d'aluminium et de cuivre (Al:xCu avec x,,, = I at.%), par resonance magnetique nucleaire pulsee et microscopic ilectroniquc par transmission. Nous avons 6galement mesur6 la vitesse de relaxation spin-r&au dans le rep&e toumant, F;,t d' 2' Al en fonction de la vitesse de deformation plastique 1 a 77 K. Pour des vitesses de deformation finies @, le mouvement des dislocations induit une vitesse de relaxation add~tionnelle provenant des fluctuations de l'interaction nucliaire quadripolaire en fonction du temps. Pour cette partie de la vitesse de relaxation induite par le d&placement des dislocations. on peut calculer le libre parcours moyen L des dislocations mobiles, qui est determine par la r&partition des difauts reticulaires agissant comme obstacles d'opposant au dtplacement des dislocations. La RMN. associee a la microscopic eiectronique par transmission, permet de reveler la structure statique des difauts dans les echantillons. Nous presentons les correlations qui existent entre le libre parcours moven L des dislocations libres observe in situ, la structure microscopique des dtfauts provenant du vieiilissement (phase 8'. solution solide) et le degri de deformation plastique e. Dans la phase B et pour de faibles dtformations, nous determinons L a partir de la microstructure: il est egal a la separation moyenne entre les precipites. Dans le cas des fortes deformations. L est d&ermint par la repartition statistique des boucles de dislocations situ&es entre les prCcipit...

Nuclear magnetic resonance at high temperature

“…In technically challenging studies, the motions of dislocations during mechanical strain of ionic crystals and polycrystalline metals has been studied by NMR at both low and high temperature, and as functions of impurity content. 161,[168][169][170][171] In those studies, the sensitivity of Tlp measurements to slow motions was again exploited: relaxation can be greatly enhanced during deformation. The average jump distances and times, as well as the concentrations, of mobile dislocations were all quantified.161…”

Anomalous precipitation hardening in Al-(1 wt%)Cu thin films

“…This combines the desired electrical and microfabrication characteristics of Al, with the required resistance against electro-migration and creep [3][4][5]. Also the main free path of moving and mobile dislocation densities were investigated using a combined pulse-nuclear magnetic and transmission electron microscopy approach in Al-Cu precipitation hardened thin foils [6]. Although this alloy improves device reliability from an electrical point of view, there are important concerns from a mechanical point of view.…”