Study On Microstructure And Mechanical Properties Of Mg-Gd-Y-Nd(-Zn)Alloys

The microstructure of as-cast,as-homogenized,as-pre-precipitated,as-extruded and aged Mg-7Gd-5Y-1Nd-xZn-0.5Zr(x=0,1,2 wt.%)alloys were investigated using scanning electron microscopic analysis techniques,transmission electron microscopy analysis;and the mechanical properties of three as-extruded and aged alloys were studied.In the as-cast alloys,the second-phases of OZn alloy is mainly MgsRE phase;the second-phases of 1Zn and 2Zn alloys are mainly composed of(Mg,Zn)3RE,Mgs(RE,Zn)phases and long period stacking ordered(LPSO)structures.The LPSO structure in the as-cast alloy is 18R-LPSO structure,and LPSO structure building blocks in the α-Mg matrix.Besides,the square RE phase and Zr-rich particles were observed in three as-cast alloys.After homogenized treatment at 515℃/48 h,the MgsRE phase in OZn alloy completely dissolved;Mg5(RE,Zn)and(Mg,Zn)3RE phases of 1Zn and 2Zn alloys were fully dissolved,while the LPSO structure was not dissolved completely.During homogenized treatment,the 18R-LPSO structure transformed into 14H-LPSO,the LPSO structure building blocks were dissolved.In 1Zn alloy and 2Zn alloy,Zr-rich particles formed Zn-Zr clusters combined with Zn,which can be divided into parallelogram ZnZr,bar ZnZr2 and cubic Zn2Zr3 phases according to the different morphologies.After 515℃/48 h,cooled in furnace to 480℃,and maintained 8 h,a large number of lamellar 14H-LPSO structures were precipitated along(0001)α in 1Zn and 2Zn alloys.During the extrusion,the extrusion ratio is 20:1,the extrusion rate is 1 mm/s,the mold temperature is 480℃,and the extrusion barrel temperature is 450℃.For three extruded alloys,the 2Zn alloy obtained the optimal mechanical properties,with the tensile strength,yield strength and elongation of 365 MPa,276 MPa,and 17.5%,respectively.The mechanical properties could be attributed to grain refinement,second phase strengthening,and weak<0001>//ED and<1010>//ED mixed textures.The peak aging of the three extruded alloys were 200℃/32 h,200℃/36 h,and 200℃/44 h respectively;precipitation is P’ phase.The tensile strength,yield strength,and elongation of 2Zn alloy are 426 MPa and 329 MPa and 8%respectively.The 2Zn extruded alloy maintains good thermal stability aged at 200℃,but the elongation decreases with the aging time.The growth of β’ phase has been hindered by LPSO structures along<0001>α direction.