Study On Microstructure Evolution And Mechanical Properties Of Mg-Zn-Nd System Eutectic Alloys

A large amount of researches found that the Mg-Zn-Nd alloy not only has good eutectic forming ability,but also has high strength and good plastic deformation ability.The studies also found that strength of the eutectic alloy is greatly improved without loss of plasticity when the eutectic structure is gradually refined.Therefore,the microstructure transformation and mechanical properties of Mg-Zn-Nd eutectic alloy and Mg-Zn-Nd-Ca eutectic alloy were investigated in this paper.The smelted eutectic alloy is maintained at 400℃ for 1000 h for equilibrium treatment to obtain an as-cast eutectic alloy.Then the alloy is remelted and quenched at 680℃ for a short time and under different cooling media.Studies have shown that eutectic alloys were treated by equilibrium treatment and quenching of 680℃,the eutectic transformation L→W+Mg-Nd occurs at about 530℃.A(W+Mg-Nd)eutectic structure is formed,which is fibrous;The eutectic transformation L→α-Mg+W occurs at about 510℃,and(α-Mg+W)is formed around the primary W,and the average component content is 88.57Mg-8.86Ca-2.57Nd;the eutectic transformation L→αMg+T3 occurs at about 500℃,a lamellar eutectic structure of(α-Mg+T3)is formed around the primary T3,which has a small eutectic cluster size,and the average component content is 77.10Mg-10.96Zn-11.94Nd;The eutectic transformation L→αMg+T2 occurs at about 480℃,and a fibrous eutectic structure of(α-Mg+T2)is formed inside the primary T2.The sheet is thin and short,and eutectic clusters size are small.Moreover,the average component content is 71.95Mg-21.59Zn-6.46Nd.The microstructure of eutectic alloy significantly affects the mechanical properties:As the holding time increases,the lamellar eutectic structure transfer to the globular eutectic structure,and the plasticity of globular eutectic structure is much better than lamellar eutectic structure.The plasticity of alloy is related to the primary T3 phase and the number and size of the T3 phase in the(α-Mg+T3)eutectic structure.The smaller proportion and unit area of primary T3 phase,the larger proportion of T3 phase in the lamellar eutectic structure,in these circumstances,the alloy has better plasticity;The strength of the alloy is related to the size of the ply spacing and the amount of eutectic structure.The smaller of the ply spacing and the more eutectic in the alloys structure,and the alloy has higher strength.A eutectic alloy that have a bimodal structure((α-Mg+T2)eutectic structure and(α-Mg+T3)eutectic structure)exhibits a combination of higher strength and better plasticity.Holding for 5 minutes at about 680℃,then cooled by ice water,the highest compression of 5#alloy was 8.3%.Holding for 5 minutes at about 680℃,then cooled by ice water,the layer spacing of 4#alloy is 0.43 μm and the layer thickness is 0.28 μm.In this case,the eutectic structure is the finest,and the compressive strength is the largest,which is 821.1 MPa;The fracture mode of the alloy is cleavage fracture,and the fracture morphology shows typical river patterns,cleavage steps,slip zones and dimples.