| Among the candidate materials for biodegradable scaffolds,magnesium（Mg）alloys are ideal materials for biodegradable scaffolds due to their excellent biocompatibility and biodegradability.However,due to the unique hexagonal close packed crystal structure of Mg alloy,there are only few sliding systems can be activated at room temperature,which lead to poor plastic deformation ability and long process of preparing Mg microtubes and cannot effectively control the microstructure of Mg alloy microtubes,resulting in many problems such as too fast degradation rate and inhomogeneous degradation in the processing of Mg alloy microtubes,which greatly limit the clinical application of Mg alloy vascular stents.Therefore,it is necessary to study the microstructure evolution of Mg alloy microtube processing,realize the refinement of microtube structure by designing technological process and optimizing process parameters,and analyze its influence on the degradation behavior of microtubes to improve the corrosion resistance of Mg alloys.In order to refine microstructure and improve the workability of Mg alloy microtubes,this paper proposed to prepare fine-grained ZE21B Mg alloy rods by CEC（Cyclic Extrusion Expression,CEC）at 360-380℃for 4 passes,and regarded CEC rods as raw materials to prepare fine-grained ZE21B Mg alloy microtubes by hot extrusion and cold drawing.Results showed that the changes of microstructure and texture could significantly improve the properties of Mg alloys.CEC could improve the microstructure of ZE21B Mg alloy and obtained fine and uniform equiaxed grains.Metallographic analysis showed that the average grain size of ZE21B Mg alloy bars after CEC was 2-3μm,the tensile test showed that the yield strength of ZE21B Mg alloy after CEC was 220±3.6 MPa,the tensile strength was263±3.9 MPa and the elongation was 20.5±0.4%,the strength and plasticity were obviously improved compared with as-extruded alloy,electron backscatter diffraction（EBSD）analysis showed that the texture type of CEC bar was<01（?）1>∥ED of the rare earth texture.The tube billet with an outer diameter ofΦ3.2 mm and a wall thickness of 0.2mm was prepared by hot extrusion and water cooling of the CEC bar at 380-400℃.The metallographic analysis showed that the average grain size of the tube billet was3-4μm.The tensile test showed that the yield strength of the tube billet was145.5±5.6 MPa,the tensile strength was 270.1±3.5 MPa and the elongation was24.4±0.7%.Electron backscatter diffraction（EBSD）analysis showed that the texture type of the tube billet was<01（?）1>∥ED of the deformation texture.ZE21B Mg alloy microtubes with an outer diameter ofΦ2.73 mm and a wall thickness of 0.135 mm were prepared by 3 passes of cold drawing and intermediate annealing at 320℃.Metallographic analysis showed that the grain size was almost unchanged during the cold drawing process,with an average grain size of about 5μm,and electron backscatter diffraction（EBSD）analysis showed that the texture type of microtubules was<01（?）1>∥ED of the deformation texture,with the increasing of drawing passes,the average schmid factor of as-drawn ZE21B Mg alloy microtubes gradually decreased,but increased in annealed microtubes,and the average schmid factor after annealing was larger than that in the as-drawn microtubes,which indicated that the plastic deformation ability of ZE21B Mg alloy microtubes could be recovered after annealing.Electron backscattered diffraction（EBSD）analysis showed that the texture type of as-drawn ZE21B Mg alloy microtubes was<01（?）1>∥ED of the deformation texture,the type of texture was unchanged after annealing at 250℃for 10 min,30min and 60 min,respectively.With the increasing of annealing time,<（?）2（?）0>∥ED of a weak rare earth texture gradually appeared after annealing at 250℃for 60 min.The texture type changed from<01（?）1>∥ED of the deformation texture into<（?）2（?）0>∥ED of the recrystallized texture after annealing at 150℃,250℃and 320℃for 30 min,especially the annealing temperature increased to 320℃.The tensile test showed that the yield strength,tensile strength and elongation of as-drawn ZE21B Mg alloy microtube were 302.6±2.9 MPa,318.6±4.7 MPa and3.9±0.4%,respectively.With the increasing of annealing time,the strength of the microtubes gradually decreased,and the elongation gradually increased.After annealing at 250℃for 60 minutes,the yield strength,tensile strength and elongation of microtubes were 242.4±2.3 MPa,298.5±1.9 MPa and 15.8±0.5%,respectively.After annealing at 150℃,250℃and 320℃,it was found that with the increasing of annealing temperature,the strength of the microtubes gradually decreased and the elongation increased gradually.Electrochemical analysis showed that the self-corrosion potential and corrosion current density of the as-drawn ZE21B Mg alloy microtubes were-1.48 V and6.70×10-5 A·cm-2,respectively.The self-corrosion potential of the microtubes increased after annealing,corrosion current density decreased,which indicated that the corrosion resistance of the microtubes was enhanced after annealing.The self-corrosion potential and corrosion current density were-1.42 V and 3.50×10-5 A·cm-2when annealed at 320℃for 30 min,respectively,and the self-corrosion potential and corrosion current density were-1.41 V and 3.44×10-5 A·cm-2 when annealed at 250℃for 60min.The static degradation and dynamic degradation experiments of ZE21B Mg alloy finished pipes with different annealing processes showed that the degradation rate was the slowest after annealing at 250℃ for 60 min,which indicated that the microtubes have higher corrosion resistance after annealing at 250℃ for 60 min. |
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