Microstructure,Mechanical Properties,and Texture Evolution Of Mg-Zn-(Cu,Y)-Zr Alloy Fabricated By Hot Extrusion-Shearing Process

In recent years,the wrought magnesium alloys fabricated by severe plastic deformation are desirable candidates as the lightest structural materials in aerospace and automotive industries due to their excellent mechanical properties.Herein,a novel extrusion-shearing deformation process combining conventional extrusion and equal channel angular pressing?ECAP?was developed.The OM,SEM,EDS,XRD,EBSD and TEM were utilized to investigate the effect of Cu element,Zn/Y mass ratios,solution treatment and extrusion-shearing deformation process on the microstructure,grain size,phase constitution,fracture surfaces and mechanical properties of the as-cast Mg-6Zn-xCu-0.6Zr?x=0,0.5,1.0,1.5?and Mg-Zn-Y-0.6Zr?Zn=2,3,4;Y=3,2,1?alloys.The effect of microstructure,grains preferred orientation and variation of texture on the strengthening mechanism of the alloys during the extrusion-shearing deformation process was also investigated.Moreover,the microstructure evolution of the alloy at die corner would account for the potential DRX mechanism.The results showed that the as-cast Mg-Zn-Cu-Zr alloys were mainly composed of?-Mg matrix,MgZn₂ and MgZnCu phases.With Cu content increasing,the volume fraction of MgZnCu phase increased while that of MgZn₂ phase decreased.The extrusion streamline was constituted of seriously broken MgZn₂ phase in the primary extrusion-shearing alloys,while the mass of MgZnCu phase were still dispersed at the?-Mg matrix in the form of bulk-shaped.The volume fraction of bimodal grain structure of primary extrusion-shearing alloys in the forming area decreased obviously as compared with that of in the extrusion area.However,the grains of extrusion-shearing alloys in the forming area grow up to some extent upon the existence of temperature rise.But not in extrusion-shearing ZK60+0.5Cu alloy,in which,the grains were further refined and corresponding the average grain sizes in the extrusion and forming areas were 2.99?m and 1.63?m,respectively.Besides,the extrusion-shearing ZK60+1.0Cu alloy in the forming area possessed the optimal mechanical properties,ultimate tensile strength?UTS?,tensile yield strength?TYS?and tensile elongation???were 337MPa,225MPa and 22%,respectively.The bimodal grain structure completely disappeared and the fine and uniform equiaxed DRX grains emerged in the secondary extrusion-shearing Mg-Zn-Cu-Zr alloys,the strength of basal texture decreased remarkably,especially for the ZK60 alloy.The average grain size decreased from 21.02?m of primary extrusion-shearing ZK60 in the forming area to 4.68?m of secondary extrusion-shearing alloy in the forming area,corresponding the strength of basal texture decreased from 30.3 to 6.51.Meanwhile,the strength and plasticity of secondary extrusion-shearing alloys have been significantly enhanced as compared with primary extrusion-shearing alloys,corresponding the UTS,TYS and?of the ZK60+1.0Cu alloy were 425MPa,382MPa and 26%,respectively.Composition of the main phase was converted from the?-Mg,I?Mg₃Zn₆Y?,and W?Mg₃Zn₃Y₂?phases to?-Mg and W phases as the Zn/Y mass ratio of the as-cast Mg-Zn-Y-Zr alloys was decreased from 4 to 0.67 and the long-period stacking ordered?LPSO?phase did not emerge in the as-cast ZWK230 alloy.For the as-homogenized ZWK410 and ZWK320alloys,needle-shaped YZn₅ and semi-nanometer Mg₇Zn₃ phases occurred within grains.Moreover,sub-grains gradually evolved into DRX grains caused by the dispersion distribution of the W phase of primary extrusion-shearing ZWK230 alloy.Compared to the extrusion area,alloys in the forming area exhibited lower strength anisotropy owing to an decreased the basal texture of the alloy.A notably weakened basal texture with RE texture component was also achieved by adding more of the Y element.The primary extrusion-shearing ZWK230 alloy possessed the excellent tensile yield strength-ductility combination,were 262 MPa and 24%,respectively,which was ascribed to grain boundary strengthening and texture strengthening effect.