Microstructure And Properties Of Mg-Ho-Y-Zn-Zr Alloy Strengthened By Stacking Faults

As the lightest structural material with the lowest density in the application of engineering materials,magnesium alloy is called "the green engineering material of the 21 st century" due to its advantages of high specific strength,high specific stiffness,good thermal conductivity and electromagnetic shielding properties and has a broad application prospect in the fields of aerospace,national defense,military industry,automobile and digital 3C.In this thesis,a new Mg-Ho-Y-Zn-Zr alloy was used as experimental material.High performance magnesium alloy was prepared by the combination of homogenization treatment,hot extrusion and hot rolling.Scanning electron microscope(SEM),X-ray diffraction(XRD),electron backscattered diffraction(EBSD)and transmission electron microscope(TEM)were used for characterization.The microstructure evolution process of as-cast,homogenized,extruded and hot-rolled alloys was studied.The formation mechanism of basal stacking faults(SFs)and its influence on the mechanical and thermal properties of the alloys were studied.The mainly research results of this paper are as follows:The microstructure of cast Mg-7.6Ho-4.1Y-1.6Zn-0.4Zr(wt.%)alloy is mainly consist of?-Mg,blocky long period stacking order(LPSO)phase and basal SFs.After homogenization treatment,a lot of blocky 18R-LPSO phase on grain boundary decomposed into lamellar 18 RLPSO phase owned different orientation relationships,and a few remained in grain boundary.The lamellar 18R-LPSO phase could improve the plasticity of the alloy while keeping the strength constant,and reduce the thermal conductivity of the alloy at room temperature.After hot extrusion,the alloy is mainly composed of a large number of dynamic recrystallization grains and a small amount of unrecrystallized deformed grain.The dynamic recrystallized grains form a large amount of randomly distributed basal SFs.After recrustalization,the texture is weakened and the plasticity is beneficial.There are a large number of lamellar 18R-LPSO phase and low angle grain boundaries which gradually become substructure and even high angle grain boundaries in the undynamic recrystallization region,that is,it belongs to the continuous dynamic recrystallization mechanism.The unrecrystallized deformed grains lead to a strong basal texture,which is favorable for the strength of the alloy.Extruded alloy shows good combination properties(YS = 370 MPa,UTS = 402 MPa,EL =17%)and medium thermal conductivity of 52.97 W/(m·K)at room temperature.The lamellar LPSO phase appeared bending,kinking and breaking phenomenon after hot rolling of homogenized alloy plates.Basal SFs started to form at the lamellar LPSO phase fracture position and with the increase of rolling reduction,solute atoms such as Ho,Y and Zn further diffuse,the volume fraction of basal SFs increases.Finally,a sandwich structure with basal SFs sandwiched between two LPSO phase is formed.For extruded alloy plates,the volume fraction of lamellar phase decreases with the increase of the rolling ratio.With rollingr,tatio less than 30%,the longer lamellar phase disappears and breaks into many short fibers LPSO phase,forming a sandwich structure,which is same as homogenized alloy plates.The alloy with rolling ratio of 30 showed the highest mechanical properties(YS = 502 MPa,UTS = 552 MPa,EL =8.5%).The alloy plate with a 50% rolling ratio has the highest thermal conductivity of 60.13 W/(m·K),which prove that the precipitation of basal SFs helps to improve the thermal conductivity of the alloy,while the lamellar LPSO phase reduces the thermal conductivity of the alloy.