Study On Microstructure And Properties Of Mg-Zn-Y-Nd Alloy For Degradable Vascular Stent Application

At present, the corrosion and mechnaical properties of commercial Mg alloys can’t fully satisfy the requirement of vascular stent. Hence on, it is a difficult problem to develop a kind of Mg alloy which has both perfect corrosion resistance and excellent mechnaical properties for vascular stent application.Aiming at the requirement of endovascular stent in clinic, a new biomedical Mg-Zn-Y-Nd alloy with low Zn and Y content (Zn/Y atom ratio 6) was designed in this study. The design concept of the alloy was that:when the Zn/Y atom ratio was 6:1 in Mg-Zn-Y alloys under proper solidification rate, the microstructure contained I-phase which possessed many interesting properties, such as high corrosion resistance, low interface energy, it was favorable for improving the anticorrosion properties of alloy. Because of the brittleness of I-phase, Nd was added into the alloy to improve the plasticity. Furthermore, as cast、sub-rapid solidification and extrusion processing were used for these alloys. The microstructure、corrosion property and mechnaical property of Mg-Zn-Y-Nd alloy were studied by OM、SEM&EDS、XRD、electrochemical test and tensile test, which opened a new window for biomedical materials design, especially for vascular stent application.The results showed that:along with the increasing of Zn and Y, the size of dentrite was bigger→smaller→bigger, the number of the second phase was more→less→more, and the distribution of the second phase was continuous distribution→diffusion→semi-continuous distribution; As the content of Nd increased, the number of the second phase increased, and the distribution of the second phase was also continuous distribution→diffusion→semi-continuous distribution. On the other hand, the rate of solidification had a great influence upon the form of I-phase. The microstructure of as-cast Mg-Zn-Y-Nd alloy consisted ofα-Mg+ Nd3Zn11, while in sub-rapidly solidified Mg-Zn-Y-Nd alloy, spherical I-phase was formed. After sub-rapid solidification, the average grain size of WE43 alloy and Mg-Zn-Y-Nd alloy significantly reduced, from about 700μm to about 15μm. Analysis showed that I-phase was only discovered in as sub-rapid solidification alloy, but was not discovered in as cast and as extrusion alloys.The corrosion resistance of alloys was characterized by M352 electrochemical system. The results showed that the corrosion potential of as cast Mg-1Zn-0.23Y-0.5Nd and Mg-3Zn-0.69Y-0.5Nd alloys was -1.80V and -1.79V, the current density was 4.36×10-4 A·cm-2 and 3.13×10-4A·cm-2, while the corrosion potential of Mg-2Zn-0.46Y-0.5Nd alloy was -1.57V, and current density was 5.30×10-5 A·cm-2. That was, the Mg-2Zn-0.46Y-0.5Nd alloy had better corrosion resistance than Mg-1Zn-0.23Y-0.5Nd and Mg-3Zn-0.69Y-0.5Nd alloys for as cast alloys. After sub-rapid solidification processing, the corrosion potential of Mg-Zn-Y-Nd alloy rised to the -1.57V and the current density decreased to 2.62×10-5 A·cm-2, which was attributed to I-phases existing and grain refinement. Moreover, after extrusion processing at 320℃, Mg-2Zn-0.46Y-0.5Nd alloy was refined considerably, from about 700μm to about 3μm. However, after extrusion processing, the corrosion resistance of alloy didn’t increase for the reason that there was residual strain in as extrusion alloy. Besides, compared with WE43, Mg-2Zn-0.46Y-0.5Nd alloy had better corrosion resistance for the reason that Zn was beneficial to improve the corrosion resistant. Loss mass test was conducted to investigate degradation behavior, which was agreement with the results of electrochemical test.Dynamic simulated body fluid (SBF) came nearer the true condition of vascular stent, then, corrosion behavior of alloys in dynamic SBF (the speed of body fluid: 16ml/800ml·min-1) were investigated. The results showed that, for the same alloy, either Mg-Zn-Y-Nd or WE43 alloy, the corrosion potential improved about more 80mv in dynamic SBF than that in static SBF, which mainly related to the concentration of Cl-.Mg-2Zn-0.46Y-0.5Nd had different corrosion surface under different preparation processes after immersion in SBF for 24 hours. As sub-rapid solidification alloy belong to general corrosion, and I-phase dispersed in a matrix of Mg, which was beneficial to improve the corrosion resistance of alloy. However, in the same test condition, the surface of as cast and as extrusion Mg-2Zn-0.46Y-0.5Nd had varying degrees of pitting. The mechanical property of Mg-2Zn-0.46Y-0.5Nd was detected, and the results showed that, for as extrusion Mg-2Zn-0.46Y-0.5Nd alloy, the tensile strength was 292.6MPa, yield strength was 168.2MPa and elongation was 21.1%, compared with the as cast alloy, which were increased by 133.3%、27.4% and 160.0% respectively, which met the requirements of mechanical property for vascular stent application.