Research On Microstructure And Mechanical Properties Of Fine-grained Mg-Al-Mn Alloy

Magnesium alloys are difficult to be processed and formed at room temperature due to insufficient sliding system that can be activated due to the hexagonal close-packed crystal structure(HCP),and the application is severely restricted.Recent studies have shown that the activity of twins in fine-grained magnesium alloys is suppressed,and the deformation is dominated by<a>and<c+a>slip,which can achieve both high strength and plasticity.However,due to the poor plasticity of magnesium alloys,it is very difficult to prepare fine-grained magnesium alloys by traditional low-temperature annealing after large plastic deformation.Studies have shown that dynamic recrystallization occurs during low-temperature extrusion of magnesium alloys,forming fine recrystallized grains or a bimodal structure of recrystallized grains and residual deformed grains,and achieves excellent mechanical properties.In addition,the addition of Mn can significantly improve the extrusion properties of magnesium alloys and refine the grains.In this study,the Mg-0.4Al-xMn(x=0,0.5,1.5)alloy was prepared by the process of low temperature extrusion at 250°C after water-cooled casting.The alloy was studied by mechanical property testing and characterization techniques such as TEM,SEM/EBSD,etc.,The main conclusions are as follows:Based on the Mg-0.4Al alloy,this thesis focuses on the microstructure and mechanical properties of Mg-0.4Al-xMn(x=0,0.5,1.5)alloys,and discusses the interaction of alloy composition-process-structure-performance The main work and conclusions are as follows:(1)The mechanical properties of the extruded Mg-0.4Al-xMn(x=0,0.5,1.5)alloy were tested and analyzed.The results showed that with the increase of Mn content,the strength first increased and then decreased,and the elongation at break increased all the time.Mg-0.4Al-0.5Mn alloy has the highest yield strength of 239MPa,and its elongation at break is 30.1%;Mg-0.4Al-1.5Mn alloy has the highest elongation at break of 52.5%,and its yield strength is 170MPa.(2)Using a scanning electron microscope equipped with a backscattered electron diffractometer to observe and study the microstructure of Mg-0.4Al-xMn(x=0,0.5,1.5)alloy.The results show that almost all of the Mg-0.4Al alloy has a dynamic recrystallization structure,with an average grain size of 4.75?m and some grains grow abnormally with strong texture.After adding Mn element,the alloy recrystallized grains are refined.Mg-0.4Al-0.5Mn and Mg-0.4Al-1.5Mn alloys are both bimodal structures,and part of the alloy has a strong fiber texture elongated in the extrusion direction.In the dynamic recrystallization area,the rest are recrystallized grains with randomly oriented equiaxed fine(?1?m).(3)The transmission electron microscope was used to observe and study the microstructure of Mg-0.4Al alloy and Mg-0.4Al-1.5Mn alloy.The results showed that twins were observed in the samples after tensile fracture of Mg-0.4Al alloy,but no twins were observed in the dynamic recrystallized grains of Mg-0.4Al-1.5Mn alloy after tensile fracture,indicating that the fine-sized grains were indeed suppressed At the same time,uniformly dispersed nanoparticles were observed in the Mg-0.4Al-1.5Mn alloy.According to literature reports,these particles are speculated to be?-Mn and Al₈Mn₅.They are only used in magnesium alloys with Mn added.Occurrence is a key factor in the recrystallization of alloy grain refinement.(4)The internal relationship of Mg-0.4Al-xMn(x=0,0.5,1.5)alloy structure and properties is discussed.It is believed that there are two main reasons for the higher yield strength of Mg-0.4Al-0.5Mn alloy:one is that it is not dynamic again.The proportion of crystalline grains is high(36%).They have a strong fiber texture,which is not conducive to the low critical shear stress of the stretching twins and basal slippage during the stretching process,and they contain high-density dislocations and small dislocations.Angular grain boundaries can hinder the movement of dislocations and increase the strength of the alloy;second,the fine recrystallized grains(1.12?m)can produce fine grain strengthening,and inhibit the activation of internal tensile twins,and promote non-basal surface slip.In addition,the dynamic recrystallization grain size of Mg-0.4Al-1.5Mn alloy is smaller(0.88?m)and accounted for a larger proportion(89%),and the strain rate sensitivity index is as high as 0.15,indicating that grain rotation and slip are involved in deformation.Obtained a very high elongation at break(52.5%).