Stress relaxations during cyclic loading-unloading in precipitation hardened Mg-4.5Zn

Abstract The impact of aging on cyclic loading-unloading responses of extruded Mg-Zn alloys was investigated in compression. In-situ Synchrotron transmission X-ray diffraction and electron backscatter diffraction post-mortem analysis were performed. To aid interpretation additional tests were performed on tensile samples and on samples with lower levels of Zn. The effect of aging is seen to be complex. During initial loading, aging clearly promotes twinning over slip. However, twins in the aged sample are thinner than those in the non-aged material. Upon unloading, additional ‘forward’ twinning occurs in the aged samples and this then is followed by de-twinning. Non-aged samples display de-twinning but no additional forward twinning. With subsequent re-loading, non-aged samples display a re-loading yield effect while the aged samples do not. Aging also leads to a noticeable ratchetting strain. It is proposed that understanding these disparate phenomena requires consideration of the dislocation activity that relaxes the stresses within the highly stressed zones that surround twins and precipitates. Precipitation retards the relaxation of slip events occurring in the highly stressed zone at the twin periphery. At the same time, slip activity in the stressed regions surrounding non-deforming particles is proposed to lead to a creep-type phenomenon that redistributes load and creates the ratchetting effect seen during cyclic loading of aged samples. To model the cyclic response of aged magnesium alloys will require consideration of these complex relaxation effects.