Cyclic deformation and anelastic behavior of ZEK100 magnesium alloy: Effect of strain ratio

Wrought magnesium alloys with low rare-earth (RE) contents are being considered for lightweight automotive applications. The purpose of this study was to identify the effect of strain ratio on cyclic deformation behavior of a rolled ZEK100-O Mg alloy with 0.2 wt% neodymium. The microstructure in its annealing condition consisted of equiaxed grains which were oriented with most c-axes perpendicular to the rolling direction. This alloy exhibited a superior combination of tensile strength and ductility due to its weak basal texture and decreased stacking fault energy. Significant plastic deformation occurred in the tensile phase of the first cycle at strain ratios of Rε=0 and 0.5. The asymmetry of initial hysteresis loops tended to diminish towards the mid-life cycles. With increasing strain ratio, fatigue life first increased, reached its maximum at a strain ratio of Rε=−1, and then decreased. This was attributed to a combined effect of mid-life stress amplitude, plastic strain amplitude and mean stress, where the stress amplitude increased and plastic strain amplitude decreased with increasing strain ratio. The closer the strain ratio to Rε=−1 was, the lower the absolute value of mean stress was. The mean stress relaxation occurred mainly in the initial stage and for the strain ratios more remotely from Rε=−1. The anelastic behavior of this alloy largely remained arising from the twinning and detwinning, with the strain ratio identified as an influential parameter via sensitivity analyses. The anelastic strain amplitude, along with three newly-defined parameters (eccentricity, angle deviation, and relative slope change) all decreased with increasing strain ratio, reflecting more symmetric hysteresis loops.