Research And Development Of A New Mg-Zn-RE Alloy With High Strength High And Toughness

Magnesium alloy as the lightest structural metal materials is becoming more and more attractive for material researchers.The existing magnesium alloys with high strength and toughness always contain relatively high concentration of RE elements, rusting in high cost.Therefore,research and development of new low cost magnesium alloys with high strength and toughness are staring our in the face.Based on the theory of improving the strength and toughness of magnesium alloy, the concentrations of RE,Sn,Zn,Ca and Zr in the Mg-Zn-Zr alloy were studied and optimized by using orthogonal tests.The effects of Y,Nd,Sn,Zn,Ca and Zr on the microstructures and mechanical properties of the Mg-Zn-Zr alloys were investigated. Simultaneously,the effect of heat treatment on the microstructure and mechanical properties was also discussed.The results indicated:1.A new Mg-Zn-RE magnesium alloy,named alloy ZW21,was developed and its compositions were 2%Zn,1%Y,0.5%Nd,0.5%Sn,0.3%Zr,0.05%Ca and balance of Mg(in wt.%).This alloy had tensile strength(σ_b)of 201MPa,elongation rate(δ)of 19.4%and hardnessof 50.69HV.Its density was ablut 1.79g/cm³.The strength and toughness of ZW21 are all higher than the series alloy of Mg-Zn-Zr(RE),because its content of Zn and RE is lower than that of he series alloy of Mg-Zn-Zr(RE),the price will be cheaper and the density will be lighter.2.The addition of Zn and RE formed some Mg-Zn-RE ternary phases.When a large amount of H phase and a small quantity of W phase formed in the alloy,it would have high mechanical properties.But the excessive ternary phases would distribute in a continuous network,which could obviously decrease its mechanical properties.Element Sn could for an intermetallic compound,Mg₂Sn,with Mg.The dipersive distribution of this compound increased the mechanical properties due to its pining role.Element Ca mainly dissolved in the precipitates containg Mg,Zn and RE which distributed in the interdendrites.A trace of Ca could refine grains.When the addition amount of Ca was slower than its solubility could also obviously refine grains.But its amount was larger,the excessive Zr element would precipitate in form of pure Zr phase and also repulsed Zn and RE to the grain boundaries to form a large amount of precipitates and thus decreased the mechanical properties.3.A proper heat treatment technique for the WZ21 alloy was obtained, solutionized at 525℃for 4h and water-quenched at 63℃,then aged at 250℃for 24h and air-cooled.After heat treatment,the W phase decomposed,the dispersed precipitation of phaseγandβ′would appear,Mg₂Sn and phase H would diffuse into the grain,all of these could lead to the increase of its mechanical properties,the tensile strength could be 243MPa,the percentage elongation could be 23.75%.