Deformation-induced dynamic precipitation of 14H-LPSO structure and its effect on dynamic recrystallization in hot-extruded Mg-Y-Zn alloys

Understanding dynamic precipitation in thermo-mechanical processing can shed light on the manufacturing of high-performance Mg-based alloys. This study investigates the dynamic precipitation of long-period stacking order (LPSO) phase with 14H structure and its effect on dynamic recrystallization (DRX) in the hot-extruded Mg98.5Y1Zn0.5, Mg97Y2Zn1 and Mg94Y4Zn2 (at. %) alloys. First, the simultaneous increase of Y and Zn atoms not only promotes the formation of interdendritic block 18R-LPSO phases during solidification but also facilitates the dynamic precipitation of intragranular lamellar and thin 14H-LPSO phases in coarse non-DRX and fine DRX grains, respectively, during hot extrusion. Second, the increased block 18R volume fraction and lamellar 14H number density significantly delays DRX. However, as the deformation strain increases, broken 18R fragments accelerate the DRX particle-stimulated nucleation (PSN) mechanism and fluctuant α-Mg/18R interfaces initiate the discontinuous DRX mechanism. In particular, the deformation kinking and dissolving of lamellar 14H activate the continuous DRX mechanism. Finally, the mechanical strength increases but ductility decreases with increasing the content of Y and Zn atoms for the hot-extruded Mg-Y-Zn alloys. The high strength of the hot-extruded Mg94Y4Zn2 alloy is attributed to fine DRX grain size, strong un-DRX grains, major block 18R and lamellar/thin 14H phases, and deformation kinking, while the high ductility of the hot-extruded Mg98.5Y1Zn0.5 alloy is responsible for large DRX volume fraction, high activity of deformation twins, and easy mobility of basal slip.