Portevin-Le Chatelier Effect in Aluminium-Magnesium Alloys

Abstract

Tensile tests of aluminium −5.6 wt% magnesium and −8.6 wt% magnesium alloys were made in the temperature range −194° to 200°C to make clear the nature of the Portevin-Le Chatelier effect. The effects of testing temperature, solute concentration and precipitation on repeated yielding in the stress-strain curves were examined in the alloys. Also dislocation structures were observed by a transmission electron microscope.
The behaviour of repeated yielding at the temperatures below −40°C was well explained by Friedel’s relation based on Cottrell’s model. From a comparison of this relation with the present results, it was concluded that the activation energy for migration of a vacancy was 0.27 eV in this phenomenon and the solute concentration on dislocations was the reasonable order of magnitude of 1 when the repeated yielding started.
However, Cottrell’s model was not valid for the repeated yielding at the temperatures above −20°C, because the behaviour of repeated yielding in this temperature range was quite different from Friedel’s relation. Here the repeated yielding should be controlled by the release of dislocations from Cottrell’s locking, because the dislocations have been sufficiently locked by solute atoms in the early stage of deformation.
No changes in dislocation distribution and slip line appearance could be recognized by changing the strain rate and the deformation temperature, although they affected appreciably the appearance of repeated yielding.
Ageing treatment affected largely the repeated yielding. When the alloys were aged for 4∼8 days at 150°C and contained plate-like precipitates, which inhibited most effectively the slip process of dislocations, repeated yielding with a large amplitude appeared at a high level of applied stress.