Effect of stress ratio on high cycle fatigue properties in Mg-6Zn-1Mn alloy

This work investigated the effect of loading mode on high cycle fatigue properties in Mg-6Zn-1Mn (ZM61) alloys. Extruded, T5 treated and double-aged ZM61 alloys were used in this study. High cycle fatigue tests were carried by a servo-hydraulic fatigue testing system under zero-tension load (R = 0) and fully reversed tension-compression load (R = −1), respectively. The results showed that all the three alloys exhibited more outstanding high cycle fatigue strength under zero-tension load than under tension-compression load. By observing microstructures of post-fatigued specimens, a large number of low angle grain boundaries appeared in zero-tension load specimens, but {101̅2} twins were found in tension-compression load specimens. The variations of fatigue properties in zero-tension load and tension-compression load were related to deformation mechanisms under different load conditions. The fatigue deformation mechanism in Mg-6Zn-1Mn alloy under zero-tension load was mainly dislocation slip, while under tension-compression load, the activation of {101̅2} twinning in compression half cycles resulted in the deteriorative fatigue strength.