Study On Microstructure Of Mg-8Al-1.5Ca-0.2Sr Magnesium Alloy During Hot Deformation

Magnesium alloys have a great potential for wide applications in the automotive industry, aerospace and rail transit because of their high strength-to-gravity ratios. One of the barriers to the applications of magnesium alloys is their low workability, as a result of hexagonal crystal structure. Meanwhile, the creep resistance is bad due to the low melting point of the eutectic phase in Mg-Al alloy.The present Mg-8Al-1.5Ca-0.2Sr magnesium alloy with good heat resistance was developed by laboratory good heat resistance. The flow stress behavior and microstructure evolution of it were investigated in this paper.The Mg-8Al-1.5Ca-0.2Sr magnesium alloy was casted by the bottom-filling foundry way. The microstructure and properties were investigated by optical microscope, scanning electron microscope, X-ray difractometer, differential thermal analysis. The results showed that the Mg17Al12and Al2Ca phases become bone shaped and continuous net-worked, dispersing at the grain boundaries. After uniformity processing at450℃and16h, the grain boundaries become clear, and the second phases significantly decrease and become small clavate.In the present study, hot compression tests of Mg-8Al-1.5Ca-0.2Sr magnesium alloy were performed on Gleeble1500. The flow behavior and influence of the parameters on the stress-strain curves were analyzed. The microstructures of magnesium alloy during compression were observed by using MM-6horizontal optical microscope (OM). The DRX model and flow-stress model were established. The results show that Mg-8Al-1.5Ca-0.2Sr magnesium alloy has typical dynamic recrystallization flow stress curve, and the dynamic recrystallization is the main mechanism for grain refinement and flow softening under the test conditions. This mechanism is influenced by deformation parameters. The recrystallized grain size increases with the increasing of deformation temperature and the decreasing of strain rate. The dislocation climb is actively involved in the plastic deformation and the DRX grain size becomes equiaxzed when the temperature is higher than350℃In the present study, the evolution of the grain structure of Mg-8Al-1.5Ca-0.2Sr during hot extrusion was simulated with the cellular automation method. The Laasraoui-Jonas microstructure model was used to describe the dislocation evolution inside crystallites during dynamic recrystallization. The parameters in the Laasraoui-Jonas model, such as the hardening parameter, recovery parameter and material constants, were determined from the flow stress-strain data obtained from hot compression tests. Good agreements between the predicted and observed grain structures were achieved.