Microstructure And Corrosion Fatigue Behavior Of Double Extruded ZE21B Mg Alloy Bars And Micro-Tubes

Mg alloys are regarded as the suitable candidate for absorbable vascular stents because of their superior biocompatibility and biodegradability.However,Mg alloys are prone to stress corrosion(stress corrosion,SC)and corrosion fatigue(corrosion fatigue,CF)under the combined action of cyclic mechanical loading and corrosive physiological environment.Localized corrosion will occur when the stress is concentrated and the maximum principal stress exceeds a certain value.It probably reduces the fracture resistance significantly and results in severe or catastrophic failure of the implants.Consequently,to avoid the occurrence of such events and to achieve uniform and controllable degradation of Mg alloy materials for vascular stents,it is necessary to study its corrosion fatigue behavior and related mechanisms,and provide theoretical guidance for the design,processing and microstructure regulation of Mg alloy materials.This study was devoted to preparing ZE21B Mg alloy bars by double extrusion,and then Mg alloy micro-tubes were prepared by hot extrusion and cold drawing methods using the double extruded alloys.Microstructure,mechanical properties and corrosion fatigue behavior of double extruded ZE21B Mg alloy bars and Mg alloy micro-tubes performed by optical microscope(OM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD)and MTS fatigue testing machine.Double extrusion can significantly eliminate the inhomogeneity of microstructure of once extruded ZE21B Mg alloy and obtain uniform and fine microstructure.The average grain size decreased from 4.1μm to 3.5μm when the double extrusion ratio increased from 12 to 23.Meanwhile,it also brought a change of texture.The deformation texture of<011 0>∥ED was observed in once extrueded alloy.After double extrusion,the texture intensity was weakened.The deformation texture of<011 0>∥ED and rare earth texture of<1 21 1>∥ED were found in the alloy when the double extrusion ration was 12.The deformation texture of<011 0>∥ED was observed in the alloy when the double extrusion ration was 23.The changes of microstructure and texture brought mechanical properties improvements.The yield strength,tensile strength and elongation of once extruded ZE21B Mg alloy were 159.7(?)8.1 MPa,226.2±2.0 MPa and 24.5±0.8%,respectively.Compared with once extruded alloy,double extruded alloy obtained improved mechanical properties.When the double extrusion ratio was 12,the yield strength,tensile strength and elongation of the alloy were 196.5±4.8 MPa,241.8±5.3 MPa and29.7±0.3%,respectively.When the double extrusion ratio was 23,the yield strength,tensile strength and elongation of the alloy were 192.6±3.2 MPa,251.5±4.5 MPa and30.1±0.2%,respectively.The fatigue strength of once extruded ZE21B Mg alloy in air was 65 MPa and in simulated body fluid was 50 MPa(3.5×10~6cycles).Compared with once extrusion,the fatigue and corrosion fatigue strength of ZE21B alloy prepared by double extrusion were improved.When the double extrusion ratio was 12,its fatigue strength in air was 80 MPa and that in simulated body fluid was 60 MPa(3.3×10~6cycles).When the extrusion ratio was 23,its fatigue strength in air was 85 MPa and that in simulated body fluid was 70 MPa(3.6×10~6cycles).The fatigue crack sources of double extruded alloy in air were the microstructure defects,while the fatigue crack sources in simulated body fluid were generally corrosion pits.The improvement of fatigue strength of ZE21B Mg alloy after double extrusion was mainly due to the refinement and homogenization of microstructure.The average grain size of ZE21B Mg alloy tube blanks prepared by hot extrusion and artificial water cooling was 3.9μm.After two passes of cold drawing and annealing at 320℃for 30 min,the average grain size of Mg alloy micro-tubes was about 4.8μm.A weak texture peak was observed between<1 21 1>and<011 1>in the tube blanks.After 2 passes cold drawing and annealing at 320℃for 30 min,the texture intensity enhanced,and the texture peak between<1 21 2>and<011 1>could be observed in Mg alloy micro-tubes.The yield strength,tensile strength and elongation of ZE21B Mg alloy blanks were 122.3(?)3.1 MPa、222.1(?)2.7 MPa and 24.7(?)0.5%,respectively.After two passes of cold drawing and annealing at 320℃for 30 min,the yield strength,tensile strength and elongation of Mg alloy micro-tubes were 161.3±4.0 MPa,296.5±6.1MPa and 25.9±0.2%,respectively.Fatigue testing results showed that the fatigue crack sources of Mg alloy micro-tubes in air were their own microstructure defects,and the fatigue crack sources in simulated body fluid were corrosion pits.