Dynamic Corrosion And Corrosion Fatigue Behavior Of Fine-grained Magnesium Alloy For Vascular Stents

Magnesium alloy has good biocompatibility and biodegradability,so it becomes an ideal material for degradable vascular stent.However,magnesium alloy vascular implant materials will be jointly affected by the periodic radial force generated by the contraction and relaxation of the blood vessel wall and the physiological medium environment during the service process.The resulting stress concentration and corrosion fatigue are the deterioration of their mechanical properties and the main cause of implantation failure.In order to promote the development of magnesium alloy materials for vascular stents,the microstructure of magnesium alloys is regulated by means of reasonable processing and heat treatment,and the effect of grain structure on the corrosion properties of magnesium alloys and the effect of the coupling of stress and corrosive medium on its corrosion fatigue behavior are respectively studied.In this paper,ZE21B fine-grained magnesium alloy was prepared by cyclic extrusion compression（CEC）,and the ZE21B magnesium alloy in CEC state was then annealed.And the microstructure,dynamic corrosion properties and corrosion fatigue behavior of ZE21B magnesium alloy in CEC state were analyzed by metallographic microscope（OM）,scanning electron microscope（SEM）,electron backscatter diffractometer（EBSD）,X-ray diffraction（XRD）and other analytical methods.The research results show that the cyclic extrusion compression can significantly refine the grain size of ZE21B magnesium alloy and improve its microstructure uniformity.The average grain size of the alloy decreases with the decrease of extrusion temperature and the increase of accumulated deformation.When the CEC process parameters are 360°C and 4 passes,a microstructure with a small average grain size and good uniformity can be obtained.The average grain size of the alloy prepared under this process parameter is 2.58μm.The fine and uniform microstructure improves the comprehensive mechanical properties of the magnesium alloy.The yield strength,tensile strength and elongation of ZE21B magnesium alloy after cyclic extrusion compression are 191.5MPa±3.1MPa,254.1±3.4MPa and 28.4±3,respectively.Compared with the yield strength of the extruded ZE21B magnesium alloy billet used in the experiment,the tensile strength and elongation were increased by 33.6%,13.8%and 16.4%,respectively.The microstructure and corrosion properties of ZE21B magnesium alloy extruded at 360℃and 4 passes after annealing with different parameters were tested.The microstructure characterization results show that the alloy has the best microstructure uniformity after annealing at 280℃for 30min,and the average grain size is 2.86μm.The electrochemical experiment results show that the alloy has the highest corrosion potential and the lowest self-corrosion current density in Hanks’solution after annealing at 280℃for 30min,which are-1.42V（vs,SCE）and 2.6×10^-5A·cm^-2respectively..The results of dynamic corrosion experiments show that the average corrosion rate of the alloy after annealing at 280℃for 30min is the lowest,and the corrosion products formed on the surface are protective precipitation layers mainly composed of calcium phosphate salts.The results of the corrosion fatigue test show that different deformation processing methods and corrosion media will affect the corrosion fatigue behavior of magnesium alloys.The corrosion fatigue strength of as-cast ZE21B magnesium alloy in physiological saline under 3.1×10⁵ cycles is 50MPa;the corrosion of ZE21B magnesium alloy in reciprocating extrusion state after annealing at 280℃for 30min under 9.0×10⁵ cycles in physiological saline The fatigue strength was 60 MPa,and the corrosion fatigue strength at 1.7×10⁶ cycles in Hanks’solution was 60 MPa.When the corrosion medium of corrosion fatigue test is the same,the processing method of magnesium alloy will significantly affect its corrosion fatigue life.The finer and uniform the magnesium alloy structure,the higher the corrosion fatigue life under the same stress amplitude.For magnesium alloys in the same processing state,when the stress amplitude is lower,the corrosion product layer generated on the side surface in different corrosive media has better protection to the substrate,and the corrosion fatigue life is higher.