Microstructure And Properties Of Biomedical Mg-Zn-Zr Based Magnesium Alloy

In recent years, the market of implant biomaterials is rapidly expanding for increasing bone trauma and cardiovascular disease. Biodegradable materials are becoming the dazzling stars as the new generation of biomaterials. Magnesium based alloys are experiencing a renaissance as potential choice for biodegradable implant materials in bone and cardiovascular surgery due to their outstanding biocompatibility, biodegradability, and mechanical compatibility. Unfortunately, up to date, even though much attention has been focused to optimize the performance of Mg-based alloys through various methods, the degradation behavior and mechanical properties are still not quite qualified for clinic applications.In order to avoid intensive galvanic corrosion, a novel single-phase biodegradable Mg-1.5Zn-0.6Zr alloy was developed in this paper. The degradation behaviors of this alloy in different environments were investigated and its uniform corrosion mechanism was uncovered. The rare earth element scandium (Sc) was creatively introduced into the Mg-1.5Zn-0.6Zr alloy. The effects of Sc addition on microstructure, mechanical properties, degradation behavior and biocompatibility of the alloy were systemically investigated. On this basis, extrusion was adopted to further improve its mechanical properties and corrosion resistance. The effects of albumin on degradation behavior of magnesium alloys were also investigated.It is found that, the Mg-1.5Zn-0.6Zr alloy exhibits a single-phase equiaxed grain structure with Zr enriching in the center of most grains. It shows a higher tensile strength and a higher elongation compared with AZ91D alloy. In both 5% NaCl and Hank’s solutions, it shows a lower degradation rate, which is because of the elimination of intensive galvanic corrosion reactions, the formation of much more compact and uniform corrosion films, and the stabilization of magnesium solid solution by Zr. Particularly, in both solutions, the Mg-1.5Zn-0.6Zr alloy exhibits prominent uniform corrosion characteristic, which is reasonably explained from an electrochemical viewpoint.Addition of Sc can obviously refine the grain sizes of the Mg-1.5Zn-0.6Zr-xSc (x= 0,0.2,0.5,1.0) alloys by heterogeneous nucleation mechanism. The kinds and amounts of precipitated particles both increased with the increasing of Sc content. The yield strength, hardness and wear-resistance of the alloys improved after Sc being added, which owes to the grain refining strengthening, solution strengthening, and Orowan strengthening. Low addition of Sc (x≤ 0.2%) can improve the degradation behavior of alloys, while higher addition content can deteriorate that. Addition of Sc can improve the hydrophilicity but deteriorate the hemocompatibility of the alloys. No obvious differences exist in cell viabilities for both L-929 cells and SP2/0 cells between different Sc contents. Compared with other developed magnesium alloys, Mg-1.5Zn-0.6Zr-0.2Sc alloy demonstrates excellent performance and suggests good potential for clinical application.After extrusion, the grain size of the Mg-1.5Zn-0.6Zr-0.2Sc alloy is obviously refined. The alloy cross section shows fine equiaxed grain structure with good structure uniformity, while relatively larger elongated grains formed on the longitudinal section that decreases the structure uniformity. The strength, elongation, and hardness of the alloy improved after extrusion. The fracture mode transfers from quasi-cleavage fracture to ductile and cleavage mixed fracture, and the strengthening mechanism belongs to grain refining strengthening. The cross section of the as-extruded alloy shows the lowest degradation rate compared with the longitudinal section of the as-extruded alloy and the as-cast alloy. After extrusion, the hydrophilicity improves, the hemolysis percentage decreases and the anticoagulation property enhances.The effects of albumin on degradation behavior of as-extruded Mg-l.5Zn-0.6Zr-0.2Sc alloy were also investigated. It is found that, at the initial stage of immersion (within the first 6 h), the adsorption is much stronger than the chelation, implying that the blocking effect of absorbed albumin is much more effective, which hinders the corrosion of magnesium alloy. However, with the increase of immersion time, the accelerating effect of chelation becomes more significant, which accelerates the corrosion of magnesium alloy. For the electrochemical behavior, the presence of albumin can increase the anodic Tafel slope, thereby inhibiting the anodic reaction of magnesium alloy.