Microstructure And Mechanical Properties Of Slow-Speed Extruded Mg-4Zn-0.5Ca Alloy And SiCp/Mg-4Zn-0.5Ca Composite

In present investigation,Mg-4Zn-0.5Ca alloy and 2vol.% SiCp/Mg-4Zn-0.5Ca composites were fabricated by semi-solid stirring assisted ultrasonic vibration,and followed by slow-speed extrusion.The effects of extrusion temperature and extrusion speed on the microstrues,texture and mechanical properties for the as-extruded alloy and composites had been investigated systematically.Meanwhile,the microstructure evolution of both as-extruded alloy and composites were also investigated in this study.The results of investigation inlustrated that SiCp can not only induce grain refinement for the Mg-4Zn-0.5Ca alloy effectively,but also improve the distribution of Ca₂Mg₆Zn₃ phases in a shape of continual network at grain boundaries,and thus resulting in mechanical properties improvement for the as-cast SiCp/Mg-4Zn-0.5Ca composite.Pronounced grain refinement took place after the alloy and composite suffering slow-speed extrusion,and both the average grain size of DRX grains and its volume fraction are improved with the increase of extrusion temperature and extrusion speed.There are large number of dynamical precipitates MgZn₂ phases in both as-extruded alloy and composite.For the as-extruded alloy,the average size of MgZn₂ phase increase but volume fraction of which decrease with the increase of extrusion temperature and extrusion speed,while the size and volume fraction of MgZn₂ phase increase for the as-extruded composite.Meanwhile,DRX grains were further refined in as-extruded composite,and the DRX volume fraction as well as size of MgZn₂ phase were also much higher than that of as-extruded alloy,both of which indicated that the addition of SiCp can not only facilitate grain refinement,but also improve the DRX volume fraction as well as accelerating the precipitation and growth of MgZn₂ phase.The slow-speed extrusion process of Mg-4Zn-0.5Ca alloy and Si Cp/Mg-4Zn-0.5Ca composite were also the proceeding process of DRX accompanied with dynamically precipitation.For the as-extruded Mg-4Zn-0.5Ca alloy,DRX nucleus were mainly formed at twining boundaries as well as the original grain boundaries through the mechanism of bulgingas.Different from the extruded alloy,for the slow-speed-extruded SiCp/Mg-4Zn-0.5Ca composite,DRX nucleus were preferred formed aroung the SiCp and at original grain boundaries,nucleation at twining boundaries was the additional DRX mechanism.The newly formed DRX grains can facilitate the dynamic precipitation of MgZn₂ phases,leading to the increase of MgZn₂ size and its volume fraction.Meanwhile,the dispersed distributed MgZn₂ phases pining at DRX grain boundaries can hinder the growth of DRX grains in return,and thus resulting in grain refinement.The slow-speed-extruded Mg-4Zn-0.5Ca alloy possessed strongly basal texture,and the intensity of which decreased with the increase of both extrusion temperature and extursion speed.The dynamic precipitates MgZn₂ phases can exert influence on the intensity of basal texture,but the dominated factor were the DRX grian size and its volume fration.The tensile mechanical properties of slow-speed-extruded Mg-4Zn-0.5Ca alloy and SiCp/Mg-4Zn-0.5Ca composite were strongly affected by extrusion temperature and extrusion speed.The yield strength?YS?and ultimate tensile strength?UTS?increased with the decrease of extrusion temperature and extrusion speed for both of as-extruded alloy and composites.The elongation?EL?of the as-extruded alloy decreased with the decrease of extrusion temperature and extrusion speed.As for the as-extruded composite,its EL increased with the decrease of extursion temperature,but decreased firstly and increased subsequently with the decrease of extrusion speed.The optimal mechanical properties were achaived when extruded with the speed of 0.01mm/s at 280 oC for both of Mg-4Zn-0.5Ca alloy and SiCp/Mg-4Zn-0.5Ca composite,but the mechanical properties of extruded composite were superior to the extruded alloy,where YS=324.5MPa,UTS=371.1MPa,EL=7.9% for the extruded alloy,and YS=322.7MPa,UTS=409.1MPa,EL=10.1% for the extruded composite.All of which indicates that the combination of dynamic precipitation of nano-MgZn₂ phases and the addition of micro-SiCp can realize the improvement of strength and ductility for Mg matrix alloy simultaneously.