A Research On Microstructure And Properties Of Magnesium-Zinc Alloy Micro-Tubes For Coronary Vascular Stents Use

With the development of therapeutic methods for cardiovascular diseases,the application of implanted vascular stents has become a research hotspot in this field.Due to excellent biodegradable properties,good biocompatibility and mechanical properties,magnesium alloy vascular stent has attracted more and more attention.However,it is hard to maintain the integrity of pure Mg in human due to its high corrosion rate.Therefore,it is necessary to improve the comprehensive performance by suitable alloying and processing technology.In this study,Mg-Zn alloys with three compositions(Mg-2Zn,Mg-4Zn,and Mg-6Zn)were selected as the research objects.The alloy ingots were fabricated into micro-tubes(outer diameter 3.000±0.050 mm,wall thickness 0.150±0.015 mm)through hot extrusion and hot drawing,which can be used for blood vessel stents.The effects of drawing temperature and Zn content on the microstructure,and mechanical properties were studied.Further,the corrosion resistance in artificial plasma under static condition and dynamic condition were explored.The results show significant value for the application of magnesium alloy in the field of vascular stent.The as-cast Mg-Zn alloys exhibited coarse grains.After hot extrusion and hot drawing at different temperatures,the microstructures of the micro-tubes are significantly refined due to the dynamic recrystallization of different degrees.In addition,as the zinc content increases,the average grain size decreases and the volume fraction of the precipitated second phase increases,resulting in higher strength,higher hardness and lower elongation rate.It was found that an appropriate Zn addition effectively improved the corrosion resistance of Mg-Zn alloy microtubes in artificial plasma in vitro degradation experiments under stastic condition.Among the three Mg-Zn alloys,the Mg-4Zn micro-tube exhibited the best corrosion resistanceHot drawing temperature has a significant effect on the properties of the alloy.When the drawing temperature is 290?,the microstructures of Mg-2Zn and Mg-4Zn were closest to the ideal full dynamic recrystallization state with uniform grain size distribution and clear grain boundaries.For Mg-6Zn,the temperature is 320?.When drawn at the above temperature,the tensile strength of Mg-2Zn,Mg-4Zn and Mg-6Zn micro-tubes were 299 MPa,342 MPa and 378 MPa,and the yield strength were 278 MPa,305 MPa and 328 MPa,respectively,all of which meet the requirements in the human body.In addition,the micro-tubes of different compositions exhibited the best static corrosion resistance when drawn at these temperatures The corrosion rates of Mg-Zn alloys measured by hydrogen evolution were 4mm/year,3mm/year,and 7mm/year,respectively after five days immersion.When increasing drawing temperature,the grains of the tubes will continue to grow after the dynamic recrystallization,resulting in lower strength and higher plasticityIn the dynamic artificial plasma flow environment,the Mg-4Zn micro-tube exhibited the best corrosion resistance.When the Zn content increased to 6wt.%,the corrosion resistance decreased,which is even lower than that of the Mg-2Zn alloy.This is consistent with the law of corrosion resistance in static environments.In the first eight hours of immersion under the dynamic flow environment,the corrosion resistance of micro-tube is increased with immersion time and flow rate,due to the thickening of Ca-P phase deposit on the surface of the micro-tube.