Portevin-Le Chatelier Effect in Al-Mg Alloys

Abstract

Tensile testing of Al-5.6 wt%Mg and Al-8.6 wt%Mg alloys was carried out in the temperature range −194°C to 200°C to make clear the nature of the Portevin-Le Chatelier effect. The temperature dependence of repeated yielding in stress-strain curves, slip line appearence and dislocation distribution were investigated in the alloys.The behaviour of repeated yielding at the temperatures below −40°C was well explained by Friedel’s relation based on Cottrell’s model. From the comparision of this relation with the present results, it was concluded that the activation energy of migration of vacancy was 0.27 eV in this phenomenon and the solute concentration reached about 40% in the dislocation core 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 strain rate and deformation temperature, although they affected appreciably the appearance of repeated yielding.