The synergistic effect of trace Sr and Zr on the microstructure and properties of a biodegradable Mg-Zn-Zr-Sr alloy
Introduction
Due to their excellent mechanical properties, such as a low density (1.7–2.0 g/cm3) and good elasticity (41–45 GPa) that are close to the values for the human bone, Mg alloys have drawn extensive attention around the world [1], [2]. Furthermore, Mg alloys also show a wide range of application prospects for use in internal fracture fixation and intravascular stents because of their excellent biocompatibility and unique biodegradability [1], [3]. Nevertheless, further applications of Mg alloys are limited by drawbacks such as their low mechanical strength and poor plasticity. Moreover, Mg alloys exhibit unsatisfactory corrosion performance in some special environments, especially in physiologic fluids and simulated body fluids (SBF) that are rich in chloride concentrations [4]. Consequently, the industrial magnesium alloys currently available cannot be used as internal fracture fixation materials because they either contain toxic elements (such as Al and Y) or are mechanically unreliable during clinical testing [5], [6]. Even more problematic, the hydrogen produced by the fast degradation of the magnesium alloys is released too quickly and cannot be absorbed by the human body, causing severe tissue necrosis [1].
It is well known that the addition of different alloying elements is an efficient way to enhance the properties of magnesium alloys. According to T. Homma and C.L. Mendis et al. [7], the addition of Zr into the Mg-6Zn-0.2Ca alloy increases the ultimate tensile strength from 327 MPa to 357 MPa. Additionally, the use of Mg-Cu [8], Mg-Zn-Sr [9], Mg-Nb-Zn-Zr [10], Mg-Ca-Mn-Zn [11], and Mg–3Sn–1Zn–0.5Mn [12] alloys in medical applications have been investigated by in vitro and in vivo testing. However, the materials reported in the literature that are used as internal fracture fixation didn't have suitable mechanical properties to avoid fracture failure, especially when used for load-bearing applications. Moreover, most of the literature focus on the effects of single element and ignore the synergistic effect of two, or even more elements.
In our previous studies, an Mg-3.2Zn-0.8Zr (MZZ) alloy was proved to be a safe and biocompatible material [13]. Additionally, zinc (Zn) and zirconium (Zr) in this alloy have good cytocompatibility and antibacterial properties [13], [14], [15], [16], [17]. In this study, Sr is selected as an additive into the MZZ alloy due to its ability to diffuse to the solid-liquid interphase and improve the undercooling of the alloy, refining the grains and dramatically improving the alloy yield strength [18]. Li et al. [19] have reported that the addition of Sr increases the ultimate tensile strength of Mg–Zn–Ca alloy from 302 MPa to 327 MPa. Yan et al. proved the addition of Sr could enhance the mechanical properties of the Mg-4Al-4La alloy by refining the thick Al11La3 secondary phase [20]. Furthermore, as a trace element in the human body, elemental Sr does not exhibit biotoxicity and can even enhance osteoblast proliferation and inhibit bone resorption [21], [22], [23]. Thus, the main purpose of this study is to design and prepare novel MZZS alloy with superior strength and demonstrate the synergistic effect of trace Sr and Zr on the microstructure, mechanical properties and corrosion behavior of Mg alloy, and these kinds of behaviors cannot be found in most binary and ternary Mg alloys. Therefore, such novel Mg-Zn-Zr-Sr (MZZS) alloys can potentially be a safe and mechanical reliable internal fracture fixation material, especially when used as load-bearing applications.
Section snippets
Sample preparation
Pure Mg (99.99%) ingot, Mg-Zr master alloy (with 30.89 wt% Zr) and Mg-Sr master alloy (with 11.11 wt% Sr) (Hunan rare earth metal research institute Co. Ltd, China) and Zn (99.99%) particles (Ke Wei company of Tianjin University Co. Ltd, China) were used as raw materials to prepare the MZS, MZZ and MZZS alloys, with the chemical composition of the MZS, MZZ and MZZS alloys summarized in Table 1. All the alloys were melted in a vacuum induction furnace (ZG-10) with magnetic agitation under argon
Microstructure analysis of as-cast MZS, MZZ and MZZS alloy
The XRD patterns shown in Fig. 1 reveal the main phases present in as-cast MZS, MZZ, and MZZS alloys. In the MZS alloy, diffraction peaks of α-Mg were evident as well as distinct diffraction peaks for Mg17Sr2 appearing in the 2-theta degree range of 15–25. But in the MZZS alloy with the same Sr content, the intensity of diffraction peaks of Mg17Sr2 at 38, 45 and 65° increased dramatically. This is indicates that, in contrast to Mg17Sr2 in the MZS alloy, the crystallinity of Mg17Sr2 in the MZZS
Discussions
In this paper, it is well established that the synergistic effects of Sr and Zr alloys can result in grain refinement dramatically in as-cast MZZS alloys and its mechanism is shown in Fig. 13. Firstly, α-Zr acted as core of heterogeneous nucleation which increased the nucleation rate during the solidification progress. Secondly, a low solid solubility of Sr in the Mg matrix (0.11%) made it difficult for the added Sr to dissolve into the Mg matrix, thus giving rise to a high Sr concentration at
Conclusions
The current study presented a systematically testing of MZZS alloys for orthopedic applications, analyzing the synergistic effect of trace Zr and Sr on the alloy performance criteria of mechanical properties, corrosion and cytocompatibility. The addition of Zr and Sr presented in as-cast MZZS alloy were leading to a separation of the Mg17Sr2 phase, a new stripe-shaped secondary phase, at the GBs, and contributed to a dramatically decrease in grain size from over 60 μm–19.6 μm. Moreover, the
Acknowledgements
The authors acknowledge the financial support for this work from the National Nature Science Foundation of China (No. 51371126 and No. 51271131), Major science and technology projects in Tianjin (No. 15ZXQXSY00080) and Science and Technology developing Foundation of Tianjin High Education (No. 20110301). And we thank Pro.Bradley Fahlman for the editorial assistance.
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