Microstructures And Properties Of Zn-Based Biodegradable Alloys For Biomedical Applications

Biodegradable materials have become the frontier and hotspot in the field of biomaterials,because they could be decomposed in vivo after implantation,meanwhile,decomposed products can be absorbed or excreted from the body.As one of the indispensable elements in human body,Zinc involves major metabolic processes.Zinc and its alloys are considered to be the most potential biodegradable biomaterials in the 21st century for their good biocompatibility and appropriate corrosion rate.The main disadvantage of zinc alloy,however,is the poor mechanical properties.Therefore,it has been important to improve the mechanical properties of zinc alloy.In the present work,Zn-5A1,Zn-1.5Mg and Zn-3Al-1Mg alloys were prepared by sub-rapid solidification method using water-cooled copper mold.The microstructure of both alloys were characterized by Inductively Coupled Plasma Direct Reading Spectrometer(ICP),Optical Microscope(OM),X-ray Diffraction(XRD)and Scanning Electron Microscope(SEM)technique.The mechanical properties and corrosion rate of alloys were initially studied by quasi-static compressive testing,quasi-static tensile testing,fracture analysis and immersion corrosion.Results indicated that:The nanoscale lamellar eutectic structure of as-cast Zn-5A1 alloy prepared by watercooled copper mold was observed.The eutectic phases were ?-Zn phase and ?-Al phase.The lamellar eutectic structure came to thicker after homogenization.Lamellar ?-Al phase transformed into a short rod-like structure,and the eutectoid dissolution occurred locally.The grain size of rolled Zn-5A1 alloy was obviously reduced,and granular a-Al phase was found to evenly distribute around ?-Zn boundaries during rolling process.The results showed that the phase composition of Zn-1.5Mg alloy is mainly ?-Zn phase,Mg2Zn11 phase and MgZn2 phase which disappears after homogenization.As the as-cast Zn-1.5Mg alloy is homogenized,the lamellar morphology of the eutectic structure disappears and the microstructure becomes more uniform.The rolling deformation makes the dendrites refine.Results for Zn-3A1-1 Mg ternary alloy indicated four phases:?-Zn phase,?-Al phase,Mg2Zn11 phase and Mg2Al3 phase.The Mg2Al3 phase disappeared after homogenization.And Mg2Zn11 eutectic structure were indistinguishable,which indicated decreasing segregation in grain boundaries.The microstructure of rolled Zn-3Al-1Mg alloy was refined and some brittle hard phases were broken during the rolling process.Moreover,fibrous distribution for grains were observed along the deformation direction.The mechanical properties test shows that the compressive yield strength of the three kinds of as cast alloys is higher than that of 300MPa.The tensile test at room temperature shows that the three alloys have good mechanical properties after rolling and have the possibility of being a substitute for bone.The results of body fluid immersion test showed that the corrosion rate of alloys were kept at 0.15mm/year,all of which reached the degradation standard of bone repair equipment.The electrochemical experiments of three kinds of as cast alloys show that the corrosion sequence of three kinds of Zn-based alloys is,Zn-3Al-1Mg,Zn-1.5Mg and Zn5Al.By observing the corrosion morphology of the sample,it is found that the ?-Al phase in the eutectic structure of Zn-5A1 alloy is firstly corroded;Mg2Zn11 phase in Zn-1.5Mg alloy is corroded and then the ?-Zn is corroded;Observation for the corrosion morphology shows that the corrosion sequence of Zn-3Al-1Mg was,Mg2Zn11,?-Zn,and ?-Al.The reaction initially produced a small amount of zinc hydroxide,calcium salt,then produce phosphate,the product become more and more and even cover the surface of the two metals,making the corrosion rate becomes smooth and much smaller than the initial reaction.