Experimental study on the stress range of class I behaviour in the creep of AlMg alloys

Abstract

The influence of the solute concentration C on the stress dependence of the creep rates ϵ ̇ S in AlMg alloys (0.5–3.3 mol.% Mg) has been examined at 600 K over the stress range between 2.5 and 85 MPa. The stress exponent n varies with stress as predicted by Yavari and Langdon: n is 3.4 in the intermediate stress region, whereas it is 4.8 in the high and low stress regions. The upper transition stress is almost linearly proportional to C. In the high stress region the solute atmosphere can, at least partly, break away and some segments of dislocations glide in a free-flight-like manner, although the transition stresses are only about one-seventeenth of the maximum stresses predicted from the Cottrell model of the solute atmosphere dragging. The lower transition stress is almost independent of concentration. This independence results from the inverse proportionality of ϵ ̇ S to C in the low stress range as well as in the intermediate stress range where n = 3.4. The necessity of thorough examinations of the creep behaviour of solid solution alloys in the low stress range is suggested.