| Due to good biocompatibility and being absorbed by the human body through biodegradation,magnesium and its alloy have shown great application potential as surgical staples by replacing titanium alloys or tantalum metals.The material used for surgical staples is required to be a wire with a diameter of 0.18-0.35 mm.When applied for surgical staples,magnesium alloy should have good mechanical properties and corrosion resistance to ensure anastomosis process to go on well and provide sufficient mechanical support after anastomosis.However,drawing,which is commonly used to fabricate wire,has a negative influence on the ductility due to work-hardening.As a hexagonal close-packed structural metal material,magnesium alloy is very sensitive to work-hardening.Therefore,this study was focused on improving the mechanical properties and corrosion resistance of a Mg-2Zn-0.5Nd(ZN20)alloy by microstructure modification.The main researches and conclusions are drawn as follows:1.Optical microscope,tensile test,immersion test and electrochemical test were carried out to evaluate the evolution of microstructure,mechanical properties and corrosion resistance of ZN20 alloy during the drawing process.The effect of microstructure evolution on mechanical properties and corrosion resistance was investigated systematacially.The results showed that obvious grain refinement could be achieved during drawing process due to dynamic recrystallisation.Both strength and ductility of ZN20 alloy could be improved by grain boundary strengthening and providing more grains to coordinate the deformation.Grain refinement also had a positive influence on corrosion resistance by eliminating residual stress and stabilizing the corrosion product layer.Basal plane was found to tilt towards drawing direction(DD)until it was parallel to DD during single direction drawing processing.The rotation of basal plane had a negative influence on ductility and corrosion resistance,because the hard orientation would form during tensile test and the number of basal plane was decreased.The ductility and corrosion resistance of ZN20 alloy could be effectively improved by bi-direction drawing through inhibiting the rotation of basal plane.2.EB SD,SEM,tensile test and imm ersion test were carried out to study the effects of equal channel angular extrusion(ECAP)and friction stir processing(FSP)on the mechanical properties and corrosion resistance of ZN20 alloy.The results showed that the bimodal grained structure in extruded ZN20 alloy was composed of coarse grains with strong texture and fine grains with random texture.Besides grain refinement,ECAP could also weaken the texture intensity of coarse grains,which caused the deformation could be coordinated by basal slip easily.The ductility of ZN20 alloy was increased obviously after ECAP.FSP could refine grains and decrease the amount of second phases in ZN20 alloy.The corrosion resistance of ZN20 alloy could be improved by reducing the galvanic corrosion.3.ZN20 alloy wires with high strength,high plasticity and good corrosion resistance were fabricated by a combined process of severe plastic deformation techniques(ECAP and FSP),second extrusion and hot drawing.ECAP could improve the formability of ZN20 alloy by texture weakening.FSP could improve the corrosion resistance by decreasing the amount of second phases.Second extrusion had a positive influence on grain refinement.The bi-ultrafine microstructure was formed in ZN20 alloy wires after a combined process of ECAP,second extrusion and hot drawing.Second phase in bi-ultrafine microstructure could promote the nucleation of shear bands,and the ductility of ZN20 alloy wire was increased by 3.5 times,because shear bands could coordinate the deformation.,ZN20 alloy wires prepared by a combined process of FSP,second extrusion and hot drawing had good corrosion resistance,and the ultimate tensile strength was 348 MPa under the influence of grain refinement,texture evolution and less second phase.4.The application potential of ZN20 alloy wires for surgical staples were evaluated by a series of in vivo and in vitro experiments,such as immersion test,tensile test,burst pressure test and animal test.The results showed that ZN20 alloy wires with an ultimate tensile strength of 248 MPa and the burst pressure of 40 kPa had good biocompatibility,which can provide sufficient mechanical support after anastomosis.After drawing,the residual tensile stress on the outside of the wires finally caused the wires peeling off during corrosion,and the diameter of the wires was decreased.The corrosion rate in vivo was much slower than that in vitro.Because the adhesion of macrophages and fibroblasts after implantation would cause new tissue formed on the surface of the wire,the new tissue could protect ZN20 alloy wire form reacting with corrosion medium in stomach or intestine. |
