| Mg-Zn-Sn alloys have good aging strengthening and high-temperature mechanical properties,and it is a promising magnesium alloy with high strength,toughness,and heat-resistance.To develop a deformed Mg-Zn-Sn alloy,the plasticity of the alloy needs to be further enhanced.Mg-Zn-Sn alloys with good plasticity were obtained by adjusting the contents of Zn and Sn,based on which Zr refinement treatment was performed to obtain Mg-Zn-Sn-Zr alloys with better strength and toughness.First,Mg-Zn-Sn alloys with different Zn and Sn contents were prepared by the metal-mold casting method.The effects of Zn and Sn contents on the as-cast composition and mechanical properties of the alloys were investigated,and the effects of Zn and Sn contents on the transformation of columnar crystal to equiaxed crystal organization were analyzed by the Hunt model.Secondly,Zr was added to the Mg-Zn-Sn alloy with good plasticity for refinement,the effects of Zr addition on the as-cast composition,grain size,and mechanical properties of Mg-Zn-Sn-Zr alloys were investigated,and the refinement mechanism of Zr was explored using the edge-edge matching model and the microstructure analysis.Finally,compression tests were performed for the selected Mg-Zn-Sn-Zr alloys at different strain rates and temperatures,activation energy,and activation volume were calculated,and the thermal deformation constitutive equation was constructed.The plastic deformation mechanism of the alloy was analyzed.Results of the microstructure study showed that with the increase of Zn and Sn content,the type of microstructure of the Mg-Zn-Sn alloys transformed from columnar to equiaxed,the grain organization became refined,and the number of secondary phases increased,which was related to the increase of the growth inhibition factor Q value.Quantitative calculations based on the Hunt model also indicated that the composition of Mg-Zn-Sn alloys were fine and equiaxed crystals in the condition of Zn+Sn content>2 wt.%,which was in line with the experiment.Refined by Zr,the grains in the as-cast Mg-Zn-Sn alloys were significantly refined,and the good grain refinement was obtained when the Zr content was 1wt.%.The XRD and EDS analysis indicated that Zr might become a nucleation core.Calculations of the edge-edge matching model revealed the crystal orientation relationship betweenα-Zr andα-Mg,α-Zr and Mg2Sn.Therefore,α-Zr particles can serve as the heterogeneous nucleation core ofα-Mg to refine the magnesium alloy grains,andα-Zr can refine the secondary phase Mg2Sn to further improve the mechanical properties.Results of the mechanical properties study showed that the strength of Mg-Zn-Sn alloys increased with the increase of Zn and Sn content,but its compressibility（ie plasticity）reached to the maximum when both Zn and Sn content were 2wt.%,which was related to the refinement of the secondary phase Mg2Sn and Mg Zn as well as their uniform distribution at the grain boundaries.The hardness,compressive strength,and compressibility of Mg-2Zn-2Sn-Zr alloys showed a trend of increasing and then decreasing with the Zr content.The mechanical properties of the alloy were optimal at the Zr addition of 1 wt.%at room temperature.The yield stress and peak stress of Mg-2Zn-2Sn-1Zr alloy increased with the decrease of deformation temperature and the increase of strain rate within the deformation temperature of 200℃～350℃and strain rate of 10-3s-1～2×10-2s-1.The deformation activation energy of Mg-2Zn-2Sn-1Zr alloy was calculated based on the Sellar model,which was 167.8k J?mol-1,and the thermal deformation constitutive equation of the alloy was then obtained.The activation volume of Mg-2Zn-2Sn-1Zr alloy was calculated at different temperature and strain rates,which was 11.5～89.5b3.With the deformation activation energy and the activation volume,the deformation mechanism of the alloy was analyzed,indicating a shift from basal to non-basal surface slip deformation mechanism of Mg-2Zn-2Sn-1Zr alloy during deformation at temperatures from 200℃ to350℃. |
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