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Investigation on Microstructures and Mechanical Properties of Extruded Biodegradable Zn-1Mg-xZr (x = 0−0.4 %) Alloys

In this work, Zn-1Mg-xZr alloys were cast, annealed and hot-extruded to investigate the microstructures and/or mechanical properties. The results show that, besides the primary Zn-rich dendrites and lamellar eutectic Zn + Mg2Zn11 mixture, the addition of Zr produces extra blockor bar-like Zn22Zr intermetallic compounds in the as-cast Zn-1Mg-xZr alloys. The annealing process results in that the interdendritic eutectic mixture changes from lamellar morphology to discrete Mg2Zn11 particles. The hot extruding process seriously deforms the primary Zn-rich crystals (with twinning inside), and breaks the Mg2Zn11 and Zn22Zr intermetallic compounds into small particles. Adding Mg and Zr to pure Zn significantly improves the micro-hardness, ultimate tensile strength (UTS), tensile yield strength (TYS) and compression yield strength (CYS). The addition of Zr to binary Zn-1Mg alloy slightly improves the micro-hardness, UTS, TYS and CYS, but contributes more to the elongations. The modification of the mechanical properties is related to the change of the microstructures due to the addition of alloying elements and hot-working deformation. Introduction As temporary implants in human body, biodegradable metallic materials have aroused great interest of researchers owning to their high mechanical properties and good biocompatibility. Among these materials, Mg [1-6] and Fe [7-11] alloys are two major systems that have been extensively investigated in the last decade. However, these two alloy systems possess obvious disadvantages that limit their clinical applications. Mg alloys corrode too fast in physiological environments so that the mechanical strength loses rapidly [6]. Moreover, the fast corrosion also results in production of excessive amounts of hydrogen that retard the healing process. In contrast, Fe alloys generally corrode slower than clinical needs. This means that large portions of the implant materials may stay in human body for a long time even after finishing their clinic roles [11]. Considering these drawbacks, attention is being distracted from Mg and Fe alloys to alternative Zn and its alloys. Zn is a biologically tolerable element. Its standard potential (-0.8 V) is between Fe (-0.4 V) and Mg (-2.4 V), indicating an intermediate corrosion rate [12]. Moreover, Zn has a low melting point, lower chemical reactivity and good machinability. These characters permit that they can be easily prepared by conventional processing technologies, such as gravity or die casting, hot rolling or hot extrusion [13]. Despite of these advantages, however, the mechanical properties of pure Zn are unsatisfactory. Therefore, Zn alloys with various elements were more extensively studied, e.g. Zn-Mg, Zn-Sr, Zn-Ca, Zn-Al, Zn-Cu, etc [13-16]. Moreover, some ternary Zn alloys were also developed, such as Zn-Mg-Mn [17] and Zn-Mg-Sr [18]. Till now, further improving the mechanical properties of biodegradable Zn alloys is still a hot topic. In the research of biodegradable Mg alloys, Zn and Zr elements are commonly added to improve the mechanical and corrosion properties. The most important reason is that these elements are non-toxic. Mg exists in the human body as fourth most prevalent cation [19] and Zn is an essential