Characterisation of dynamic strain ageing in two magnesium alloys

The internal stresses in AZ91 and WE54 magnesium alloys are investigated. The specimens are deformed at a constant initial strain rate. At given strains (stresses), stress relaxation (SR) experiments are performed. A post-relaxation effect, i.e. an increase of the flow stress at the beginning of plastic deformation after stress relaxation in comparison with that at the beginning of stress relaxation, is observed in some cases. This post-relaxation effect is sensitive to strain and relaxation time. Internal stresses are estimated at various strains from stress relaxation tests and under creep conditions. Possible changes in the internal stresses and in the mobile dislocation density are considered to be responsible for the post-relaxation effect.