Study On Heterogeneous Structure Control And Mechanical Properties Of AZ91 Extruded Magnesium Alloys

As a lightweight material,magnesium alloy has huge development potential and wide application prospects.However,magnesium alloy has a close-packed hexagonal crystal structure,which has few slip systems that can be started when deformed at room temperature and has poor room temperature plasticity.In addition,although magnesium alloy has high specific strength,its real strength is relatively low.These factors restrict its application and development.In recent years,as a new research field,heterostructure materials have provided a new idea for the realization of strong-plastic combination of various alloys,including magnesium alloys,and there have been certain research results.However,in existing reports,the preparation process of heterogeneous materials is usually complicated and special.Based on these,this paper adopts traditional extrusion technology to control the microstructure of AZ91 by controlling the extrusion parameters to prepare AZ91 magnesium alloy with heterostructure.Using metallurgical microscope,scanning electron microscope,electron backscatter diffraction and universal testing machine and other analysis and testing methods,the effects of different extrusion ratios and extrusion temperatures on the microstructure and mechanical properties of AZ91 alloy are discussed,and the strengthening and toughening mechanism of heterogeneous structure AZ91 is revealed.The main content and conclusions are as follows:(1)After being extruded at 260°C-340°C,the grain size of AZ91 alloy was significantly refined from 60μm to below 15μm,and the average grain size increased with the increase of extrusion temperature.The yield strength,tensile strength and elongation of the alloy have been greatly improved.The yield strength decreases with the increase of the grain size,and the change rule of tensile strength and elongation is not obvious.However,the two heterogeneous structures of E260 and E300(the number is the extrusion temperature)have a good combination of strength and ductility.Among them,E260 is a heterogeneous structure formed by small recrystallized grains surrounding large unrecrystallized grains.The yield strength,tensile strength,and elongation are 271 MPa,380 MPa,and 8.4%,respectively,and the yield strength is the largest among extruded alloys.E300 is a band-like heterogeneous structure composed of larger recrystallized grains and smaller recrystallized grains.The yield strength,tensile strength,and elongation are 240 MPa,392 MPa,and 11.2%,respectively.The tensile strength is the largest among extruded alloys.(2)The extrusion ratios of 7,11,17,and 26(respectively denoted as AE7,AE11,AE17,AE26)are carried out at 300°C to control the completely recrystallized heterogeneous structure.The results show that the larger the extrusion ratio,the larger the average grain size.AE7 is a uniform fine-grained structure,AE26 is a uniform coarse-grained structure,and AE11 and AE17 are heterogeneous structures.In addition,as the extrusion ratio increases,the yield strength and tensile strength of the alloy gradually decrease,and the overall elongation tends to increase.Among them,AE11 with a heterogeneous structure has the best combination of strength and ductility.The small average grain size and Schmid factor play a certain role in improving its strength.Its largest heterogeneous deformation inducing(HDI)force makes it produce the largest HDI strengthening and hardening during the stretching process,which improves both its strength and plasticity.(3)The extrusion ratios of 7,11,and 17(respectively denoted as AE7,AE11,and AE17)were carried out at 260°C to control the partially recrystallized heterogeneous structure.The results show that as the extrusion ratio increases,the degree of recrystallization gradually increases.Dynamic recrystallization nucleation from AE7 to AE11 is dominant,and the number of recrystallized fine crystals increases without significant changes in size.Dynamic recrystallization nucleation and growth occurs at the same time from AE11 to AE17,the number of recrystallized fine crystals increases and the size increases.AE17 has a large degree of recrystallization and a relatively uniform structure.Part of the recrystallized heterogeneous structure is formed in AE7 and AE11.Among them,AE7 has the best strong and tough combination.Its small average grain size and the low Schmid factor caused by the hard orientation of unrecrystallized coarse grains,and the largest HDI strengthening and hardening are the reasons for its high strength and plasticity.