Research On Microstructure,Properties And Dynamic Recrystallization Behavior Of The Mg-5Zn-1Mn-xSn Magnesium Alloys

The high zinc content Mg-Zn-Mn alloys have excellent mechanical properties,but their plastic processing performance and formability are poor.So it is of great significance to explore the method to improve the plastic processing performance and formability for its application.In recent years,Research on Sn alloying of Mg-Zn alloys has received extensive attention.However,there were few reports on the effect of a minor Sn addition on the microstructure,properties and dynamic recrystallization?DRX?mechanism of Mg-Zn alloys under high strain rate deformation.In this study,the effects of a minor Sn addition on the microstructure and mechanical properties of Mg-5Zn-1Mn?ZM51?alloy sheets rolled by high strain rate rolling?HSRR?processing were investigated.The effects of a minor Sn addition on the nucleation and growth mechanism of DRX under high strain rate deformation were studied by hot compression tests.The obtained results and conclusions can provide theoretical basis for regulating the microstructure and mechanical properties of fine-grained magnesium alloys sheet.The main conclusions of the thesis are as follows:?1?During high strain rate rolling,high density dislocation induced a large number of finely dispersed distributed dynamic precipitates.When Sn content was lower than 0.9mass%,a disc-shaped MgZn₂ phase and a rod-shaped?-Mn phase mainly exist in the as-rolled sheets.But Mg₂Sn precipitation with irregular shape appeared in the sheet containing 1.2mass%Sn or more.Since the higher volume fraction of precipitation has a stronger pinning effect on the subgrain boundary and the grain boundary,the DRX grain size gradually decreases with the increase of the Sn content.Theaddition of Sn element reduced the driving force required for the nucleation of twins in the alloys.?2?The ZM51-0.9Sn alloy sheets exhibited the best mechanical properties.The ultimate tensile strength?UTS?,yield strength?YS?and elongation?El?were 358MPa,262MPa and 20.4%respectively,which can be attributed to the highest DRX volume fraction?95%?,the fine DRX grain size?1.22?m?and residual dislocations in the matrix.?3?With increasing of Sn content,the critical strain for the onset of DRX firstly decreased and then increased,while the DRX volume fraction showed the opposite trend,and the optimum Sn content for DRX was 0.9mass%.The dominating DRX mechanism in both Sn-containing alloys and non-Sn alloy was continuous DRX with twin nucleation.The high density dislocations inside twins gradually evolved into cellular structures and sub-grains by dynamic recovery,and then the sub-grains turn into the DRX nuclei.?4?The essence of the influence of Sn addition on the DRX process is to regulate the DRX nucleation rate and growth rate by affecting the twin density,stacking fault energy?SFE?and the volume fraction of the precipitated phase.The addition of Sn increased the nucleation site of DRX by increasing the twin density,and promoted the growth of the DRX core by reducing the stacking fault energy of the magnesium alloy.The volume fraction of the dislocation-induced precipitation phase increased with the increase of Sn content,which leads to high pinning effect to hinder the formation of cellular structure by recovery and inhibits the rotation and growth of the subgrain.