Effects Of Zr And Sn On Microstructures And Properties Of Biodegradable Zn-Mg Alloy

Based on good mechanical properties and biocompatibility,metallic biodegradable materials have aroused great interest of researchers.Previous studies focused more on Mgand Fe-based alloys.However,these two alloys have the drawbacks of degrading too fast or too slow,which limit their clinical application.In recent years,Zn and its alloys have attracted researchers' much attention due to their suitable degradation rate and good biocompatibility.At present,exploring new alloy composition and processing technology to further improve the mechanical properties of Zn-based alloys and effectively control their degradation rate have become an important research direction.Based on this,the Zn-Mg-X(X=Sn,Zr)ternary alloys were studied by adding different contents of Sn and Zr into the extensively studied Zn-Mg binary alloys to verify their feasibility as biodegradable materials.After casting,homogenization and hot extrusion,the microstructures,mechanical properties and corrosion resistance of the Zn-Mg-X ternary alloy were systematically studied by OM,SEM,XRD,hardness test,tensile and compression test,electrochemical and immersion test.The mechanism of the corresponding changes in microstructures and properties was discussed.(1)Mircostructures:As-cast Zn-1Mg alloy consited of dendritic primary Zn phase and lamellar eutectic Zn+Mg2Zn11 phase.After adding Sn and Zr elements,Mg2Sn and Zn22Zr intermediate phases were formed between the interdendritic primary Zn crystals in the Zn-Mg-Sn and Zn-Mg-Zr alloys,and their amounts increased with increasing the element content.After homogenization treatment,the morphology and size of the Mg2Zn11 phase in Zn-Mg-Sn alloy did not change significantly,but the decomposition of Mg2Sn phase resulted in a wider distribution of Sn element.The Zn22Zr phase did not change significantly either,but the eutectic Mg2Zn11 phase was transformed into a discrete structure.After hot extrusion,the primary Zn phase in the Zn-Mg-X alloy was distorted and refined,and a large number of twins were formed.And the eutectic Mg2Zn11 phase,Mg2Sn and Zn22Zr phase were crushed into small particles,and evenly distributed in the Zn matrix.(2)Mechanical properties:The Vickers hardness(HV),compressive yield strength(CYS),tensile yield strength(TYS)and tensile strength(UTS)of extruded binary Zn-Mg alloy were much higher than that of pure zinc,and reached 91.85,298.1 MPa,243.7 MPa and 307.4 MPa,respectively,but the elongation decreased from 9%to 0.84%.The HV,UTS,TYS and CYS of Zn-Mg-Sn alloy decreased slightly compared to those of the Zn-1Mg alloy,and the decrease range increased with Sn content,but the elongation increased gradually.The mechanical properties of Zn-Mg-Zr alloy were improved compared with Zn-1Mg alloy,but the increasing range decreased firstly and then increased with increasing the Zr content.The HV,UTS,TYS,CYS and elongation of Zn-1Mg-0.4Zr reached 93.5,301.3 MPa,241.1 MPa,317.5 MPa and 4.69%respectively.Fracture analysis showed that cleavage river pattern appeared on the fracture surfaces of Zn-Mg-Sn and Zn-Mg-Zr alloys,which had the typical brittle fracture.(3)Corrosion properties:The corrosion rate of extruded Zn-1Mg alloy was faster than that of pure zinc.Zn-Mg-Sn alloy corroded slower with increasing the Sn content,but it was still higher than that of Zn-1Mg alloy.Among them,the corrosion rate of Zn-1Mg-0.8Sn was the fastest(0.363mm/y).The corrosion rate of the alloys with Zr was lower than Zn-1Mg,and decreased with increasing the Zr content.The corrosion rate of Zn-1Mg-0.4Zr was 0.270mm/y,which was the slowest among all the groups.The Zn-Mg-Sn alloy in SBF corroded more uniformly,and primary Zn phase was left completely which would be beneficial for maintaining the mechanical properties.Crrosion pits appeared in Zn-Mg-Zr alloy,with Zn22Zr distributed at the peripheral region.The properties of the as-extruded Zn-1Mg-0.8Sn alloy were more close to those of some bio-bone substitutes,and the as-extruded Zn-1Mg-0.4Zr alloy to those of some biodegradable vessel stents.However,their elongation still needs to be further improved in the future study.