Preparation Technology And Properties Of Mg-RE Alloy Seamless Tube As Biodegradable Stent

Among medically-applied metal vascular stents are stainless steel, nickel titanium alloys and cobalt alloys, which are prone to stimulate the surrounding tissue so that inflammatory response, vascular intimal hyperplasia and restenosis occur when implanted as permanent foreign body. Fortunately, Magnesium alloys with non-toxic and biodegradable properties have been grown to vascular stent candidate.The objective of the thesis is the preparation of biodegradable Mg-Y-Nd-Zr alloy seamless tube stent. In this thesis,the influence of the rare earth elements on microstructure, mechanical properties and corrosion performance was investigated and the optimal alloy composition was acquired based on above work;a steady flow of material stress constitutive equation and the processing maps were established;a small thin-walled seamless tube extrusion die was designed and extrusion process was modeled;the effect of deformation conditions and heat treatment on microstructure and properties of tube was investigated;capillary vascular stent was prepared;the as-extruded alloys were biocompatibility evaluated.This paper has conclusions as follow:(1) Cast Mg-Y-Nd-Zr alloy was mainly composed of?-Mg,?and Mg₂₄Y₅. As for Nd content increase from 0 to 4.2% in Mg-5.0Y-xNd-0.6Zr alloy, the average grain size of the as-cast alloys reduce from 33.5?m to 19.2?m, the tensile strength increase and then decrease, the yield strength increase first and then tend to be constant whose maximums were 214.5MPa and 102.8MPa respectively, and the elongation and area reduction rates are plummeted from16.4% and 23.8% to 4.9% and 2.7% respectively. As for Y content increase from 0 to 7.4% in Mg-yY-2.6Nd-0.6Zr alloy, the average grain size of the alloys reduce from 41.6?m to 19.1?m, the tensile strength first increase and then decrease and the yield strength increase first and then tend to remain unchanged with the maximums of 214.5MPa and 102.8MPa respectively, and the elongation and area reduction rates slightly increase and then decrease rapidly with the maximums of 12.3% and 14.8% when Y contents reach 2.5%.(2) With the increase of Nd content in the extruded Mg-5.0Y-xNd-0.6Zr alloy, the volume fraction of dynamic recrystallization increases, the recrystallization grain size decreases, the tensile strength and yield strength improve, and the elongation and area reduction decreased gradually. 2.6% Nd content in the extruded Mg-5.0Y-xNd-0.6Zr alloy sees the lowest corrosion rate, 0.108mg/cm2h. With the increase of Y content in the extruded Mg-yY-2.6Nd-0.6Zr alloy, the volume fraction of dynamic recrystallization increases, the recrystallization grain size decreases, the tensile strength and yield strength increased first and then tended to remain unchanged, and the elongation and area reduction slightly increased and then decreased gradually. 6.2% Y content in the extruded Mg-yY-2.6Nd-0.6Zr gives alloy the lowest corrosion rate, 0.103mg/cm2h.The results show that Mg-6.2%Y-2.6%Nd-Zr alloy with both good mechanical properties and corrosion resistance was selected experimentally as optimal rare earth composition alloy.(3) The flow stress behavior of Mg-6.5Y-2.5Nd-0.6Zr alloy during hot compression deformation can be well described by the hyperbolic sine function which contains Arrhenius relationship and Z parameters, the constitutive equation is: (?)Using this equation, the peak stress and the revised experimental data coincide very well, and the relative error is less than 5%. During hot deformation, the dynamic recrystallization size d and Z parameters have the following relationship: d = 1 .61×10⁵Z^-0.292(4) At the strain rate of 0.1¹.0s^-1 and the deformation temperature below 400?, Mg-6.5Y-2.5Nd-0.6Zr alloy tend to flow instability, and in the development of thermal processing, parameters should avoid these processing conditions. When the deformation temperature is 400 450?and deformation rate 0.01⁰.1 s^-1 as prior deformation condition, the alloy has high efficiency of power dissipation, dynamic recrystallization as the main deformation mechanism, and fine grain size.(5) The designing small thin-walled seamless tube extrusion die mainly composed of the punch, die, extrusion container, insulation sets, upper and lower mold base and top pressure limit. Extrusion use two prestressed composite structure with temperature opening; die through the extrusion cylinder model with a fixed in the next seat, model angle of 60o, working with a length of 2mm, the entrance radius is 3mm; Punch composed of extrusion shot, extrusion axis and extrusion needle, extrusion axis direct with shaft, front-end to 25o with the extrusion needle cone and the arc transition. (6) The numerical simulations of tube extrusion show that during extrusion process, the concave cavity flow fast near the inside of the hollow billet, while lateral flow in the cavity slow so that the squeeze transition is easy to form at the "dead zone ", reduced by increasing the compression ratio. After extrusion billet, the equivalent strain, equivalent strain rate, flow rate, temperature, equivalent stress of the die's working band distribute for concentration with the larger value; with increasing extrusion ratio, the equivalent strain rate, flow speed, temperature, equivalent stress are increased, while the equivalent strain decreases; working with a blank area inside the flow velocity and equivalent stress is greater than the outside, but inside of the stock is less than the outside temperature.(7) The increase of the Extrusion temperature and extrusion speed and the decrease of the extrusion ratio spoil the tube's recrystallized grain size, decrease the tensile strength and increase elongation. At the extrusion temperature of 440, extrusion speed of 28mm/s, extrusion ratio of 143, the tube with outside diameter 3mm and inside diameter 2.5mm can be smooth out. After 460?×2h +200?×20h solution treatment and aging treatment, the tensile strength and yield strength reached 291.4MPa and 214.2MPa, extension rate of 16.3%, which can meet the performance requirements of the capillary.(8) Extruded Mg-6.5Y-2.5Nd-0.6Zr alloy extracts were adherent cultured cells grew well, normal morphology, no significant difference with the negative control group, and cell toxicity for the 0 and 1, and has good cell Compatibility. Alloy has some hemolysis with 8.22% hemolysis rate; After incubating 1h in platelet-rich plasma, surface of materials adhered more platelet and out pseudopodia; dynamic clotting time was 36min, indicating that the blood compatibility of the alloy need to be improved.