Mechanisms of cyclic softening in precipitation-hardening alloys?A ball model approach and tests at 78 K

Abstract

Tests of random, alternating cuts on a ball-model of θ″ in Al−Cu alloy have been run to check the kinetics at which disordering occurs in an initially ordered precipitate subject to cyclic strain. In addition, fatigue tests at low temperature (78 K) and various microscopical observations have been made on Al−Ag alloy containing GP zones to check whether a structure containing an initially disordered precipitate will cyclically soften or not. It was thus found that the θ″ ball model disorders as a function of accumulated plastic strain consistently with the kinetics of cyclic softening in actual material. Further, Al−Ag alloy was found not to soften at 78 K. Both of these results supoort the disordering hypothesis of cyclic softening over the “dissolution” or “shearing-off” mechanisms, although other systems may be subject to these mechanisms. In spite of the low temperature at which the Al−Ag alloy was tested, small γ′/γ precipitates were found to have formed in the longest lived test; however, this result did not interfere with the validity of the experiment as it did previously at room temperature. The fatigue lives at 78 K were much longer than those previously measured at room temperature although the failure mechanisms were not affected at high strain and only somewhat affected at low strain (there was increased incidence of transgranular crack nucleation and propagation).