| Magnesium alloys exhibit low density,high specific strength and stiffness,good damping properties,and have a broad application prospect in the automotive industry,aerospace,3C products and other fields.However,the low strength,poor plasticity and so on restrict their wide applications seriously.In recent years,long-period stacking ordered(LPSO)structure,which can improve the strength and plasticity of magnesium alloy effectively,has been found in the Mg-RE-Zn series alloys(RE/Zn>l).The Mg-8Gd-4Y-lZn-xAl(x=0,0.5,1.0,1.5,2.0wt%)alloy samples were prepared by the conventional gravity casting,homogenization,hot extrusion and aging treatment in the present thesis,respectively.The microstructures and phase composition of the alloys with different conditions were investigated by OM,SEM,EDS,XRD and DSC.Meanwhile,the corresponding microhardness,tensile mechanical properties,modulus of elasticity,compression properties and impact properties were tested.The influences of the Al content on the microstructures,LPSO structure and mechanical properties of the alloys were analyzed systematically,and then it can provide the theoretical basis for designing the magnesium-rare earth alloys with high strength and toughness.The main results are as follows:(1)The as-cast microstructure of Mg-8Gd-4Y-lZn alloy consists of ?-Mg matrix,18R structure,island-shape Mg3(RE,Zn)eutectic phase and a small amount of Mg-RE particle phase.After adding Al,the irregular polygon-like and strip-like Al2(Gd,Y)new phase appears and increases with the increase of Al content.With increasing the Al content,the as-cast comprehensive tensile mechanical properties and compression properties first increase,and then decrease.The properties reach the best when WAl=0.5wt%,where the tensile strength,elongation and compressive strength are 225 MPa,9.0%and 434 MPa at room temperature,which increase by 14%,29%and 17%compared with the corresponding values for Mg-8Gd-4Y-lZn alloy,respectively.(2)After the alloys were homogenization annealed at 520 ? for 14 h and cooled in the furnace,the Mg3(RE,Zn)eutectic phase dissolve into the a-Mg matrix.The microstructure consists of the supersaturated a-Mg matrix,a small amount of Mg-RE particle phase and thin layer 14H structure distributing within the grain densely for Mg-8Gd-4Y-lZn alloy.After adding the metal Al,the irregular polygon-like Al2(Gd,Y)new phase appears in microstructure,and meanwhile the block 14H structure appears along the grain boundary.With the increase of A1 content,the comprehensive tensile mechanical properties and compression properties of the homogenized alloys first increase and then decrease,and reach the best when WAl=0.5wt%.(3)After hot extrusion,there exist many extrusion flow lines in the microstructures.Recrystallization occurs in all alloy bars.The 14H structure is bent,twisted,broken and kinked,and the grain size decreases with the increase of the extrusion ratio.The grain is equiax crystal and the average grain size is about 10 ?m for the alloy rods extruded with the extrusion ratio of 40.The microstructure of extruded alloys consists of ?-Mg matrix,14H structure,a small amount of Mg-RE particle phase and the irregular polygon-like Al2(Gd,Y)phase.With the increase of the extrusion ratio,the comprehensive mechanical properties of extruded alloys gradually increase.With increasing the A1 content,the comprehensive tensile mechanical and compression properties of extruded alloys(?,=40)first increase,and then decrease.The properties reach the best when wAl=0.5%,where the tensile strength,yield strength,elongation,compressive strength and modulus of elasticity are 376 MP?329 MPa?9.0%523 MPa and 45 GPa at room temperature,respectively.(4)The aging effect is the best for the extruded bars at 200 ?.The microhardness of extruded Mg-8Gd-4Y-lZn alloy at 200? for 96 h is 121.6 HV.And its tensile strength,yield strength and elongation are 426 MPa,366 MPa and 6.0%at room temperature,respectively.And the former two are 11%and 13%higher than those before aging treatment,respectively.The microhardness of extruded Mg-8Gd-4Y-1Zn-0.5Al alloy at 200? for 108 h is 114.2 HV.And its tensile strength,yield strength and elongation are 368 MPa,294 MPa and 9.0%at room temperature,respectively. |
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