Effect of the Zener-Hollomon parameter on the dynamic recrystallization kinetics of Mg-Zn-Zr-Yb magnesium alloy

The isothermal compression tests for as-homogenized Mg-5.8 Zn-0.5 Zr-1.0 Yb magnesium alloy were performed on a Gleeble-1500 thermo-mechanical simulator at the temperatures of 250, 300, 350 and 400 degrees C and the strain rates of 0.001, 0.01, 0.1 and 1 s(-1). The progress of dynamic recrystallization (DRX) was modeled by the modified Avrami type equation and the DRX kinetics was expressed as a function of the Zener-Hollomon (Z) parameter. The effects of the Z parameter on the microstructure evolution, the DRX fraction, the DRX rate, and the DRX sensitive rate were analyzed. An iso-fraction contour map was proposed to clarify the relations among the Z parameter, DRX fraction and deformation degree. Based on the spacing between the contours, the partial DRX stage was divided into three successive sub-stages and each of them was characterized by the mechanism underlying DRX evolution. Moreover, the ZK60 (Yb-free) alloy was adopted for comparative analysis to elucidate the effect of Yb addition on the DRX kinetics. The results showed that the Z parameter exerted a fundamental influence on the DRX evolution. Considering the robustness and efficiency of DRX, the deformation regime with the lnZ value less than 24 was preferable. Furthermore, the Yb addition can promote the onset of DRX but the progress should be retarded to some extent compared with the Yb-free counterpart. These conclusions were verified by the microstructure observations.