Study On Microstructure And Properties Of A Novel Biodegradable Fe-Mn-N Alloy

As an essential trace metal element,iron(Fe)participates extensively in metabolic processes of human body and thus shows good biocompatibility.Compared with magnesium and zinc alloys,biodegradable Fe alloys have obvious advantages on mechanical properties and show great potential in the application of load-bearing medical devices such as biodegradable stents and gastrointestinal anastomosis staples.It has been found that the biodegradable Fe alloy with twinning induced plasticity(TWIP)effect showed excellent matching of strength and plasticity,which was even better than some permanent medical metal materials.However,their slow degradation rates have become a critical problem which is needed to be solved urgently.Previous studies achieved no significant increase in degradation rate through alloying,while large deformation and second phase precipitation usually resulted in aggravated the local corrosion with an increased risk of implant failure.Therefore,in this study,a novel series biodegradable Fe-30Mn-xN(x=0,0.3,0.6,wt.%)alloys was designed and developed to improve the above problems through nitrogen(N)alloying,and the effects of N content on microstructure,mechanical properties and degradation properties were investigated.The degradation behaviors of the wires made of the 0.6N alloy with excellent comprehensive performance and the gastrointestinal anastomosis staples made of the wires were also preliminarily investigated in vitro and in vivo,respectively.In order to elucidate the reasons for their differences,the degradation behavior of the Fe alloys in the simulated vascular high-oxygen environment was studied,which provided a new design approach and theoretical guidance for the development of high performance degradable Fe alloys and their implants.The main results and conclusions were drawn as follows:1.X-ray diffraction,electron backscatter diffraction and transmission electron microscopy were used to evaluate the cold deformed microstructure of the Fe-Mn-N alloys at different deformation degrees,and the effects of N content on the stacking fault energy(SFE),twinning evolution and mechanical behavior were systematically studied.The results showed that N could effectively reduce the SFE of Fe-Mn-N alloys.The breakthrough reduction of SFE not only led to a significant increase in the number of twins,but also obviously reduced their thickness,obtaining numerous ultrafine nanotwins below the critical size(15 nm).The improvement of strength for Fe-Mn-N alloy resulted from the solid solution strengthening effect of N,the increase in twin number and the decrease in twin size,and the enhancement of plasticity mainly originated from the additional work-hardening contributed by ultrafine nano-twins.2.Electrochemical,ion dissolution and weight loss tests were used to evaluate the effects of N content on the degradation rate and local corrosion of the Fe-Mn-N alloys.The 0.6N alloy with excellent comprehensive performance was used to manufacture wires and then anastomosis staples,and their degradation behaviors in vitro and in vivo were preliminarily investigated by mechanical properties reduction,weight loss,electrochemical test and animal experiment,respectively.The results showed that the degradation rate of Fe-Mn-N alloys was improved significantly with increase of the N content,while the local corrosion was obviously suppressed.The wires of 0.6N alloy with high strength,high plasticity and rapid degradation rate maintained sufficient mechanical properties in the early stage of degradation.Different from the degradation behavior in vitro,anastomotic staples made of 0.6N alloy still maintained a high degradation rate in the late stage of implantation in vivo.Both blood and pathological results showed good biocompatibility of the high-N Fe alloy.3.The effects of dissolved oxygen on the degradation rate and degradation products of Fe-Mn-N alloys were studied,and the degradation behaviors of some commonly seen Fe alloys in high-oxygen environment were also evaluated.The results showed that the degradation rate of 0.6N alloy was enhanced with increase of the dissolved oxygen content.When the dissolved oxygen content exceeded a certain critical value(24 mg/L),degradation rate of the alloy exhibited an explosive increase,while its local corrosion was obviously inhibited.Compared with the low-oxygen environment,the 0.6N alloy immersed in high-oxygen environment could continuously degrade,which was consistent with the degradation behavior in vivo.Besides being able to accelerate degradation by affecting the cathodic reaction of Fe alloy,increase of the dissolved oxygen could also promote the transformation of degradation products.In high-oxygen environment,N was the most effective in enhancing the degradation rate of Fe alloy compared to Mn and C,which also caused the weakest local corrosion.