Mechanical Properties And Electron Microscopy Investigations Of Mg-Co-Y And Mg-Co-Y-Sn Alloys

As the lightest structural alloys,magnesium alloys have great potential for applications in the automobile and aerospace industries.Long-period stacking-ordered（LPSO）phases in magnesium（Mg）–transition metal（TM）–rare earth metal（RE）alloys have attracted significant research interest because of their particular crystallographic features and good mechanical properties.The LPSO phase plays a significant role in improving the mechanical properties of magnesium alloys.Recently,it was reported that new types of LPSO structures were found in Mg-Co-Y alloys.In this study,the mechanical properties of Mg-x at%Co-6.0 at%Y（x=0.0,1.0,2.0,4.0）alloys and Mg-1.0 at%Co-6.0 at%Y-x at%Sn（x=0.0,0.5,1.0,2.0,4.0）alloys were examined,and their microstructures were studied by using an optical microscope（OM）,X-ray diffractometer（XRD）,scanning electron microscope（SEM）,transmission electron microscope（TEM）to explain the changes in their mechanical properties.As-cast Mg-x at%Co-6 at%Y（x=0.0,1.0,2.0,4.0）alloys were prepared,and then some as-cast ingots were hot extruded.Tensile tests were performed on the as-cast and as-extruded Mg-x at%Co-6.0 at%Y（x=0.0,1.0,2.0,4.0）series alloys.The ultimate tensile strength（UTS）increased from 179 MPa to 246 MPa with the addition of 1.0 at%Co in the as-cast alloys.With the addition of 2.0 at%Co,the UTS decreased to 218 MPa.With the addition of 4.0 at%Co,the UTS slightly increased to 220 MPa.The UTS increased from 306 MPa to 369 MPa with the addition of 1.0 at%Co in the as-extruded alloys.With the addition of 2.0 at%Co,the UTS decreased to 339 MPa.With the addition of 4.0 at%Co,the UTS decreased to 327 MPa.The Mg-1 at%Co-6 at%Y alloy had the peak UTS values for both the as-cast and as-extruded forms.It was found that the as-cast Mg-6 at%Y alloy consisted ofα-Mg,Y andβ-Mg₂₄Y₅ phases,whereas the alloys with Co additions consisted of five main phases,i.e.,α-Mg,β-Mg₂₄Y₅,Mg YCo₄,Mg₃（Co,Y）and LPSO phases.According to statistics,the average grain sizes of as-cast Mg-x at%Co-6.0 at%Y（x=0.0,1.0,2.0,4.0）were～36.0μm,～13.5μm,～9.9μm,and～14.1μm,respectively.Therefore,it was concluded that element Co had obvious effect on grain refinement of the Mg-Y alloys.The increase in the UTS was resulted from the strengthening from grain refinement and the formation of LPSO phases.According to statistics,the volume fraction values of LPSO phases in the as-cast Mg-x at%Co-6.0 at%Y（x=1.0,2.0,4.0）alloys were 29.2%,34.1%and 13.4%,respectively.It was noted that effects of strengthening of the LPSO phase was partially declined by the presence of Mg YCo₄ particles in the as-cast Mg-4at%Co-6 at%Y alloy.Therefore,the decline of the UTS value was assumed to the increase in the volume fraction of the Mg YCo₄ phase.Grain refinement due to extrusion was achieved in Mg-x at%Co-6 at%Y（x=0.0,1.0,2.0,4.0）alloys.Therefore,the as-extruded alloys exhibited apparently better UTSs than the as-cast alloys.According to statistics,the volume fraction values of LPSO phases in the as-extruded Mg-x at%Co-6.0 at%Y（x=1.0,2.0,4.0）alloys were 24.8%,20.3%and 15.6%,respectively.Hence,the UTS reach their highest valued with the addition of 1 at%Co in the as-extruded alloys.The increase in the UTS was resulted from the formation of aligned LPSO phases,which acted as strong reinforcing component in extruded alloys due to the very small Schmid factor for the basal slip in aligned LPSO phases.Alloy ingots with the nominal compositions of Mg-1.0 at%Co-6.0 at%Y-x at%Sn（x=0.0,0.5,1.0,2.0,4.0）were prepared for improve the ductility of Mg-Co-Y alloys.The ingots were treated with solid solution treatment at 773K for 16h followed by air cooling,they were then hot extruded.Tensile tests were performed on the as-extruded Mg-1.0 at%Co-6.0 at%Y-x at%Sn（x=0.0,0.5,1.0,2.0,4.0）series alloys.With the addition of 0.5 at%Sn,the UTS increased from 381 MPa to 391 MPa,the yield strength（YS）decreased from 377 MPa to 364MPa,and the elongation increased from 0.5%to 0.8%.With the addition of 1.0 at%Sn,the UTS decreased to 356 MPa,the YS decreased to 323 MPa,and the elongation increased to4.5%.With the addition of 2.0 at%Sn,the UTS decreased to 229 MPa,the YS decreased to 174MPa,and the elongation increased to 19.7%.With the addition of 4.0 at%Sn,the UTS decreased to 147 MPa,the YS decreased to 144 MPa,and the elongation decreased to 1.7%.The UST of the as-extruded Mg-1.0 at%Co-6.0 at%Y-x at%Sn（x=0.0,0.5,1.0,2.0,4.0）alloys reached the peak value with the addition of 0.5 at%Sn.The as-extruded alloy without Sn addition had the best YS.The elongation reached the peak value with the addition of 2.0 at%Sn.Microhardness tests were performed on the as-extruded Mg-1.0 at%Co-6.0 at%Y-x at%Sn（x=0.0,0.5,1.0,2.0）alloys.With the addition of 0.5 at%Sn,the microhardness decreased from 144.7 Hv to 130.4 Hv.With the addition of 1.0 at%Sn,the microhardness decreased to127.7 Hv.With the addition of 2.0 at%Sn,the microhardness decreased to 71.5 Hv.It was found that the as-extruded Mg-1.0 at%Co-6.0 at%Y alloy mainly consisted ofα-Mg,β-Mg₂₄Y₅ and Mg YCo₄ phases,whereas the alloys with 1.0 and 2.0 at%Sn consisted of four main phases,i.e.,α-Mg,β-Mg₂₄Y₅,Mg YCo₄,and Sn₃Y₅ phases.And the volume fraction of the Sn₃Y₅ phase increased with increasing Sn content in the as-extruded Mg-1.0 at%Co-6.0at%Y-x at%Sn（x=1.0,2.0）alloys.SEM image of the fracture surface of the as-extruded Mg-1.0 at%Co-6.0 at%Y-2.0 at%Sn alloy revealed that there were dimples around Sn₃Y₅ phases,which resulted in decreases in both UTS and YS of the investigated alloys.