Fabrication And Plastic Deformation Behavior Of Ultrafine-grained Mg-3Al-1Zn-xAg Alloys

Magnesium?Mg?alloys have a potential application in automobile,aerospace and electronic industries because of low density,high specific strength and superior recyclability.However,Mg alloys exhibit poor ductility in the case of high strength,and the properties of high temperature service and thermal creep resistance are poor,which greatly limits the application of Mg alloys.Thus,the preparation of high strength and high toughness Mg alloys with good heat resistance becomes the research hotspot.The ultrafine grained Mg-3Al-1Zn alloy containing Ag additions were prepared by powder metallurgy,and through the subsequent rolling deformation and aging treatment,developing a high strength and high toughness Mg alloy with good heat resistance in this paper.The basic processes are:firstly,the nanocrystalline Mg alloy powder is prepared by mechanical milling,then compacted by vacuum hot pressing,and extruded and rolled.The microstructure evolution and mechanical properties in the forming processes are studied.The results obtained are as follows:Firstly,the micro structure evolution of Mg-3Al-1Zn-xAg?x=0-2wt.%?powders during mechanical milling was studied.The Effect of milling time and Ag addition on the morphology,element distribution,grain size and microstrain of Mg-3Al-1Zn alloy powders was determined.The results show that Al,Zn and Ag elements are gradually solidified and uniformly distributed in the Mg matrix by prolonging the milling time.The solid solution of Ag element accelerates the grain refinement process of Mg matrix.After 30h milling,the average grain sizes of Mg-3Al-1Zn alloy powders with Owt.%,0.5wt.%,lwt.%and 2wt.%Ag element are 60nm,58nm,53nm and 48nm,respectively.The microstructures of nanocrystalline powders after vacuum hot pressing were studied.It was found that the grain size of Mg matrix grows up after hot pressing,and the microstructure characteristics of mechanical milling disappears and the density reaches more than 90%.The second phase particles?Mg54?Ag,Al,Zn?17 phase?are found in Ag-containing compaction,and the density and size of the second phase particles increase continuously with the increase of Ag content.Zn and Ag also tend to be enriched at the grain boundaries of the Mg grains,and the thickness of Zn+Ag enrichment layer is about 4 nm.The microstructure and mechanical properties of Mg-3Al-1Zn-xAg alloy after extrusion were investigated.The effects of initial microstructure,extrusion temperature and Ag content on the microstructure and mechanical properties of Mg-3Al-1Zn alloy were mainly discussed.It was found that decreasing the grain size of the initial materials and the extrusion temperature are both helpful to the grain refinement of the extruded Mg alloy,which is in accordance with the relationship between the flow stress ?E and the size of dynamic recrystallized grains d_R:?_E=?d_R^-n and obtaining ultrafine-grained Mg-3Al-1Zn alloy with the minimum average grain size of 246nm and the relative density increases to 98%.In addition,with 0.5wt.%and 1 wt.%Ag addition,elemental enrichment layers and Al/Zn/Ag enrichment regions form in the grain boundaries,which inhibit the dynamic recrystallization during the extrusion process and the growth of recrystallized grains.After adding 2wt.%Ag,a large amount of Mg54?Ag,Al,Zn?17 phase precipitates from the Mg matrix,pinning grain boundaries,also results in grain refinement.The as-extruded ultrafine grained Mg alloy exhibits high strength and a uniform yield phonomenon in the tensile test at room temperature.The analysis of the micro structure evolution in the process of tensile test at room temperature was carried out.The results show that the grains grow under tensile stress through "stress induced grain growth”,the average grain size after tesile test is 71.50%larger than that before tensile test,and the grains exhibit a discontinuous grain growth phenomenon.When the stress reaches the upper yield point,the dislocation density reaches to 5.84×10¹⁶m^-2,which motivates "slip induced grain boundary slip".Grain growth and grain boundary slip result in the uniform yield phenomenon.The high strength and high toughness Mg alloy sheets were obtained by multi-pass rolling and aging treatment of ultrafine grained Mg-3Al-1Zn-0.5Ag alloy.The micro structure evolution in process of multi-pass rolling was studied.The results show that the grain growth occurs at the beginning of rolling and then grain refinement is realized again by continuous and discontinuous dynamic recrystallization.It is found that the strength and plasticity improved with the increase of the rolling pass,and the obtained sheet exhibits yield strength of 370.4MPa,ultimate strength of 422.7MPa,and elongation of 9.3%in RD direction.Further aging treatment was conduted on as-rolled sheets.The results show that the strength and plasticity of the material are obviously improved simutaneously,and Mg alloy sheet with the high strength and high toughness is obtained.After aging treatment?140?/10h?,the tensile yield strength increases to 390.5MPa,the ultimate strength increases to 458.0MPa,and the elongation increases to 17.8%.The thermal stability of the as-extruded ultrafine grained Mg alloys was studied.The results show that the grain growth in the annealing process is in accordance with the grain growth kinetics equation:D_n-D₀ⁿ=kt.The grain size growth index n of the ultrafine grained Mg-3Al-1Zn alloy with Owt.%,0.5wt.%,lwt.%and 2wt.%Ag addition is 4,5,7 and 6,respectively,which indicates that the segregation at the grain boundaries and the local enrichment of Ag element reduce the grain boundary diffusion ability,leading to the enhancement of the thermal stability.