Microstructure evolution and dynamic recrystallization mechanisms of Mg-Al-Ca-Zn-Sn-Mn alloys with different Ca contents during hot extrusion

Enhancing the strength of Mg-Al-Ca series alloys can significantly expand their range of applications. In this study, we examined the mechanical properties of Mg-Al-Ca-Zn-Sn-Mn alloys with different Ca contents. The study revealed that increased Ca content contributes to higher strength, resulting in approximate increases of 100 MPa. As a result, a high-strength Mg-6Al-5Ca-1Zn-1Sn-Mn alloy with a strength exceeding 300 MPa has been successfully developed. The as-extruded Mg-6Al-5Ca-1Zn-1Sn-Mn alloy shows a bimodal microstructure consisting of DRXed grains alongside elongated unDRXed grains. Further exploration was undertaken regarding the microstructure evolution of the experimental alloys, accompanied by an in-depth analysis of the dynamic recrystallization mechanisms. At the early stage, the (1012) extrusion twinning is the primary deformation mechanism, whereas the PSN mechanism dominates the DRX process in the late stage of hot extrusion. A high density of nano-precipitates, combined with the decreased mobility of non-basal slips, greatly hinders the dynamic recrystallization process in the alloy with higher Ca content. Additionally, increased Ca content enhances the activity of non-basal slips and promotes the formation of the RE texture. Finally, a quantitative analysis focused on the strengthening mechanisms was performed, while a qualitative assessment of the plasticity mechanisms was conducted. It was determined that grain boundary strengthening and texture strengthening are the key factors contributing to the high strength, whereas more twins and second-phase particles tends to reduce ductility. This investigation provides important insights that can contribute to the progressive development of high-strength flame-retardant Mg-Al-Ca series alloys.