Microstructure And Mechanical Properties Of A Newly Developed Mg-Gd-Dy-Ag-Zn-Zr Alloy

Precipitation hardening is not only one of the main strengthening methods of light Mg alloys but also plays an important role in the development of high strength Mg alloys.A new type of casting Mg-10Gd-4Dy-1.5Ag-1Zn-0.5Zr alloy was prepared.The microstructure,phase composition,mechanical properties and aging hardening behavior of as-cast,pre-cold rolled and as-extruded alloys were studied.In this paper,OM,SEM,EDS,XRD and TEM were used to observe and analyze the alloy.Meanwhile,the peak-aging alloy was mainly characterized to explore the reason for the change of properties.The main results are as follows:The as-cast alloy mainly consists of?-Mg matrix,Mg₅RE phase,high RE phase and SF_S structure.After solution treatment at 500?for 8.5 h,the Mg₅RE phase disappeared,and the SFs structure changed into 14H LPSO phase.The tensile properties of as-cast alloy are weak(YS=204 MPa,UTS=255 MPa,EL=5.7%).After solution treatment,the tensile properties are improved slightly(YS=197 MPa,UTS=270 MPa,EL=9.7%).Although Mg₅RE phase can prevent dislocation slip,it has poor bonding with the matrix,triggering that cracks easily occurred at the interface between Mg₅RE phase and?-Mg matrix,which is unfavorable to the strength of the alloy.The LPSO phase is observed in the T4 state,however,the volume fraction of phase is small.Therefore,the mechanical properties of the alloy are not significantly improved and the comprehensive mechanical properties are still weak.After aging treatment for 108 h,the alloy reached the peak-aging state(T6).The good comprehensive mechanical properties(YS=312 MPa,UTS=356 MPa,EL=4.5%)are achieved.The increase of strength is mainly attributed to the formation of??/??phase during aging treatment.The alloy has the good aging hardening response after 15%pre-cold rolling.The peak value in microhardnesss(141.7 HV)of pre-cold-rolled(T3)alloy was achieved after200°C/22 h.However,the T4 alloy needed 108 h to reach the peak value in microhardness of 135.54 HV.The mechanical properties of T8 alloy are higher than that of T6 alloy(YS=348 MPa,UTS=394 MPa,EL=5.3%).The size of the??/??phase in T8 state is smaller and the density is higher than that in T6 alloy.The introduction of dislocations into the?-Mg matrix through the pre-cold rolling deformation could provide nucleation particles for the precipitate,triggering the improving of the density of precipitate.Moreover,the introduction of dislocations into the?-Mg matrix accelerated the diffusion rate of solute atoms,resulting in the decrease of the time for reaching the peak-aging state in alloy.After the extrusion treatment,1)the phases composition of the alloy are basically unchanged,2)the second phases of the alloy are broken,3)the grains are greatly refined due to dynamic recrystallization,and 4)the SFs structure is found in the recrystallized grains.In addition,there is a small amount of dynamic precipitation formation.Extrusion deformation significantly improve the mechanical properties(YS=341 MPa,UTS=403 MPa,EL=7.9%).A good aging hardening response after extrusion in alloy and the peak-aging state is achieved after 200°C/54 h,and the peak value is 144 HV.Moreover,the mechanical properties of the alloy are further improved(YS=396 MPa,UTS=451 MPa,EL=6.4%).The can be attributed to the formation of nano-(??and??)phases at peak-aging state,but the aging hardening effect is lower than that of T6 alloy due to the dynamic precipitation during extrusion.