| Biodegradable magnesium (Mg) alloys are attracting most attention in the field of vascular stents all over the world; however, their corrosion and mechanical properties of Mg alloys haven’t fully satisfied the corresponding requirements. Most current studies focus on grain refinement of Mg alloys through alloying and plastic deformation, but deformation technologies, like extrusion, will introduce extrusion stress which badly affects the corrosion resistance of Mg alloys. Therefore, to develop new Mg alloys with both excellent corrosion resistance and exceptional mechanical properties for degradable stents, how to optimize alloying and new processing technologies is an urgent and cruicial problem.Based on Mg-Zn-Y alloy with low Zn content and only biocompatible elements, which has good mechanical and corrosion properties, this paper micro alloyed rare earth element Nd wich is nontoxicious to human, can improve the anticoagulant property and has obvious refinement on Mg alloys, and chosen subrapidly solidification (copper mold casting) to refine microstructure, avoid the deformation stress, enhance the solid solutin ability of α-Mg phase, to obtain Mg-Zn-Y-Nd alloys with excellent properties and comparable size with stents. Microstructure, in vitro degradation and biocompatibility and mechanical properties of subrapidly solidified Mg-Zn-Y-xNd alloys were studied through Optical microscopy, X-ray diffraction, electronchemical tests, hemolysis tests, and so on, hoping to provide useful practical reference to the research on the best match of alloying and processing techniques, and basis for next step of preparation of small tubes for stents.The results showed that:After subrapid solidification, microstructures of Mg-Zn-Y-xNd (x=0,0.5,1.0,1.5, wt%) alloys changed obviousely:most were rosette-like dendrites, the secondary dendrite arm space (SDAS) was around4-5μm, secondary dendrites of alloys with certain Nd content turned not obvious, and most of Zn and Nd elements were solid soluted into the a-Mg substrate, which led to that there are only peaks of α-Mg phase in XRD, and they showed offset to the right compared to those of cast alloysFor the as-cast Mg-Zn-Y-xNd (x=0,0.5,1.0,1.5, wt%) alloys, with the increase of Nd content, grains became firstly smaller and then bigger,400→90→70→200μm; the amount of second phase along grain boundries increased, and their morphologies were rodlike→lineship; excessive amount of Nd increased the tendency of dendrite formation, and there were NdZn and NdZn2phases. For the subrapidly solidified (Φ3mm) Mg-Zn-Y-xNd (x=0,0.5,1.0,1.5, wt%) alloys, rosette-like dendrites appeared. With the increase of Nd content, dendrites became more obvious, but the SDAS remained nearly the same, about5p.m. For subrapidly solidified (Φ2mm) Mg-Zn-Y-xNd (x=0,0.5,1.0,1.5, wt%) alloys, typical microstructure of three-crystal zone easily appeared in the cross section. Nd addition can homogenize the size of secondary dendrite arms, excessive addition would promote dendrites to become longer, but the SDAS was about4μm for all Φ2mm alloys. The main phase for subrapidly solidified alloys (Φ3mm and Φ2mm) was α-Mg solid solution.For Mg-Zn-Y-xNd (x=0,0.5,1.0,1.5, wt%) alloys, after subrapid solidification, corrosion potential and open circuit potential increased, corrosion current density decreased, and charge transfer resistance increased, indicating that the corrosion resistance of alloys were improved, and this improvement was most effective for Mg-Zn-Y-1.ONd alloy.With the increase of Nd content, for as-cast and subrapidly solidified (Φ3mm and Φ2mm) alloys, the corrosion potential and open circuit potential increased, corrosion current density decreased firstly and then increased, and charge transfer resistance increased firstly and then decreased. The corrosion resistance got best when the amount of Nd addition is1.0%. Nd can decrease the tendency of corrosion reaction; a moderate amount of Nd can reduce the corrosion rate and improve the corrosion resistance.The effects of Nd in improving corrosion resistance of Mg-Zn-Y-Nd alloys were strengthened by subrapid solidification. In as-cast and subrapid solidification (Φ3mm) groups, the improvement in corrosion resistance of Mg-Zn-Y-1.0Nd alloy was not obvious; while in subrapid solidification (Φ2mm) group, that improvement was great. Among the alloys studied, subrapidly solidified (Φ2mm) Mg-Zn-Y-1.0Nd alloy possessed best corrosion resistance, which was better than WE43alloy that has been used as vascular stent material.Considering the microstructural characteriastics of subrapidly solidified (Φ2mm) alloys, and that it is the properties of cylindrical surface of small tube material that matter most for vascular stents, the corrosion properties of cylindrical surface (CS) and sectional surface (SS) of subrapidly solidified (Φ2mm) Mg-Zn-Y-1.ONd alloy were studied. Potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) and immersion tests indicated that CS had better conrrosion resistance than SS, which is beneficial for subrapidly solidified Mg alloys to maximize its strengths and avoid its disadvantages to be used as stents.After subrapid solidification, hemocompatibility of Mg-Zn-Y and Mg-Zn-Y-l.ONd alloys were improved; an addition of1.0%Nd can decrease the hemolysis and make APTT longer, and those effects were more obvious in subrapidly solidified alloys; hemolysis and APTT of as-cast and subrapidly solidified Mg-Zn-Y-1.ONd alloys were better than those of WE43and316L stainless steel; the cells cultured in extracts of subrapidly solidified Mg-Zn-Y-l.ONd alloys showed good morphology and had high cell viability, the cytotoxicity was1class, indicating the alloy has met national requirement for biomaterials.With the increase of Nd content (from0to1.0%), microhardness and compressive strength of subrapidly solidified Mg-Zn-Y-Nd alloys increased, but strain at compressive strength had little difference; microhardness, tensile strength and compressive strength of as-cast alloys increase, elongation increases firstly and then decreases. After subrapid solidification, microhardness and compressive strength of Mg-Zn-Y-xNd (x=0,0.5,1.0%) were improved, and the brittle fracture in as-cast alloys turned into plastic failure for subrapidly solidified alloys. These effects were more obvious for Mg-Zn-Y-1.ONd alloy. |
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