Study Of High Strength Mg-Gd-Y-Zn Alloy And Its Strengthening Mechanisms

Due to the high specific strength and excellent mechanical properties in high temperature,the Mg-Gd-Y-Zn series alloys have a huge potential to apply widely in the aerospace,transportation and national defense and military.While,the research on the Mg-Gd-Y-Zn alloys is not enough and the strengthening mechanisms of the alloys remain to be studied.The effect of heat treatments(solution treatment and aging treatment)and heat extrusion on the microstructure and mechanical properties of the alloys and the strengthening mechanisms of the alloys were analyzed deeply by systematically investigating the microstructure and mechanical properties of Mg-8Gd-4Y-1Zn,Mg-8Gd-4Y-1Zn-0.5Al and Mg-8Gd-4Y-1Zn-0.5Zr(wt.%)alloys at different states(as-cast,as-solution,as-extruded and as-aged).Finally,high strength magnesium alloy sheets were developed successfully and the results of this study could provide theoretical basis to further develop high strength Mg-RE alloys.And the results of this study are listed in the following:(1)The microstructure of the three as-cast alloys consists of Mg matrix,18R-LPSO phase and Mg3(RE,Zn)eutectic phase.In the as-cast Mg-8Gd-4Y-1Zn alloy,the 18R-LPSO phase and eutectic phase distribute as continuous networks along the grain boundaries.The addition of 0.5wt.%Al enhances the formation of LPSO phase,resulting in the larger and more LPSO phase in the Mg-8Gd-4Y-1Zn-0.5Al alloy,and coarsens the eutectic phase distributing at grain boundaries.The addition of 0.5%Zr has little effect on the microstructure of the Mg-8Gd-4Y-1Zn alloy.(2)There are two kinds of LPSO phases with different morphologiesin the as-solution Mg-8Gd-4Y-lZn alloy.One is the needle-like LPSO phase distributed diffusely within the grains.Another is the bulk-like LPSO phase distributed along the grain boundaries.The addition of Al increases the quantity of bulk-like LPSO phase significantly,but decreases the quantity of needle-like LPSO phase slightly.The addition of Zr makes the needle-like LPSO and the bulk-like LPSO phase larger.After heat extrusion,the microstructure of the three alloys consists of fine recrystallized grains,elongated hot-worked grains and elongated LPSO phase.After aging treatment,the solute atoms dissolved in the matrix have precipitated to form β’phase.(3)The mechanical properties of the three as-cast and as-solution alloys are relative poor.For example,the ultimately tensile strength(UTS),yield strength(YS)and elongation(E)of the as-cast Mg-8Gd-4Y-lZn alloy are 196 MPa,179 MPa and 5.0%,respectively.After heat extrusion,the comprehensive mechanical properties of the three alloys are excellent.Such as the UTS,YS and E of the as-extruded Mg-8Gd-4Y-lZn-0.5Al alloy are 364 MPa,283 MPa and 11.5%,respectively.After aging treatment,the strengths of the three alloys improve at the expense of the ductility.Such as the UTS,YS and E of the as-aged Mg-8Gd-4Y-lZn alloy are 461 MPa,395 MPa and 6.0%,respectively.(4)Heat extrusion and aging treatment could increase significantly strength of the alloy.Heat extrusion hardens the alloy by grain refinement through recrystallization and dislocation accumulation.Aging treatment hardens the alloy by formatting the β’ precipitated phase.The major strengthening mechanisms in as-extruded and as-aged alloys are solid solution strengthening and precipitation strengthening of β’phase.