Grain Refinement and Second Phases Evolution of Mg-13Gd-4Y-2Zn-0.4Zr Sheet During Processing by Rotary Forward Extrusion

The grain refinement and second phases evolution of Mg-13Gd-4Y-2Zn-0.4Zr sheet during the rotary forward extrusion (RFE) process were investigated. The microstructures were observed by using optical microscopy, scanning electron microscopy, electron backscatter diffraction, X-ray diffractometer, and transmission electron microscope. The result shows, the modified cellular automata model fully considers the influence of long-period stacking ordered phase on the dislocation density, which can predict the grain size evolution of Mg-Gd-Y-Zn-Zr alloy during the RFE process, and the relative error can be controlled within 10%. With the increase of RFE revolutions, the average grain size firstly decreases quickly (RFE1–RFE25), and then increased gradually (RFE25–RFE100). The decreased grain size is related to the enhanced dynamic recrystallization process as the c-axis of most grains is rotated continuously to a transverse direction and more random texture types are formed. The increase in grain size is linked with the solid solution of the second phase, and the limitation on grain growth due to the pinning effect of the second phase almost disappears. In the early stage (RFE1–RFE25), the contribution percentage of grain refinement and solid solution strengthening to microhardness improvement is 180.6% and 61.7%, respectively. In the subsequent stage (RFE25–RFE100), the dislocation density strengthening and solid solution strengthening accounts for 60% and 440% of the increase in microhardness, respectively.