Strain hardening behavior and strengthening mechanism in Mg-rich Al–Mg binary alloys subjected to aging treatment

Abstract

The aging hardening response and tensile strain hardening characteristics of cast Al–Mg alloys with high Mg contents (5–13 wt%) were investigated. Aging hardening became significant when the Mg content was greater than 10 wt%. As the Mg concentration increased in the solid solutionized matrix, the strain hardening rate increased. During aging heat treatments at 423 and 453 K, the strain hardening rate increased as the aging time increased and it increased more rapidly as the Mg content in the alloys increased. The effects of the Mg matrix concentration and the precipitates on the strain hardening characteristics were interpreted using the Kocks-Mecking model. For the solid solutionized alloys, as the Mg matrix concentration increased, the dislocation storage rate (k1) increased, whereas the recovery rate (k2) decreased. For the aged alloys, both k1 and k2 increased with increasing Mg content and aging time. The contributions of the Mg matrix concentration and precipitates to the k1 and k2 values in the aged alloys could be separated. The effect of the precipitates to the k1 and k2 values were more significant than the effect of Mg matrix concentration in the aged alloys. The increase in Mg matrix concentration promoted a transition in the Portevin–Le Chatelier (PLC) effect from type B to type C. After the aging treatment, the type B + C serrations occurred in the Al-10 wt%. Mg alloy due to significant decrease of Mg concentration in the matrix. The critical strain for the onset of PLC increased with increasing Mg matrix concentration and aging time. These results were explained in terms of the concentration of plastic-deformation-induced vacancies involving in the diffusion process of solutess, which may decrease as the Mg matrix concentration and the amount of precipitates increase. After aging, the contribution of solid solution strengthening to yield strength decreased, whereas the contribution of particle strengthening to yield strength greatly increased. The particle strengthening effect accounted for 56.8% of the yield strength in Al-13 wt% Mg alloy.