Determination of Optimum Zn Content for Mg–xZn–0.5Mn–0.5Sr Alloy in Terms of Mechanical Properties and In Vitro Corrosion Resistance

This study investigated the microstructure, compressive properties and in vitro corrosion behavior of biodegradable Mg–xZn–0.5Mn–0.5Sr (ZMJ) alloy with Zn content of 0 to 5 wt% in the as-cast state. Increasing the Zn content in ZMJ alloy refined the grains from 215 to 95 µm and changed the secondary particles from Mg17Sr2 to Mg6Zn2Sr and MgZn phases. As the Zn content increased, the compressive yield strength increased from 44 to 67 MPa due to grain boundary strengthening. At immersion in phosphate-buffered saline for 7 days, the addition of Zn from 0 to 0.1 wt% reduced the corrosion rate from 0.71 to 0.48 mm/y, and 0.85 wt% Zn was alloyed to obtain the lowest corrosion rate of 0.45 mm/y. However, adding more Zn significantly increased the corrosion rate up to 3.31 mm/y. Thus, the best anti-corrosion performance can be obtained at 0.85 wt% Zn, which was attributed to its lowest Volta potential difference between the main secondary particles and the α-Mg matrix among ZMJ alloy. Based on this, the optimal Zn content for ZMJ alloy can be determined to be about 1 wt% by comprehensively considering the mechanical properties and in vitro corrosion behavior for biomedical applications. Micrographs of (a,c) the specimen as-built in vertical direction (Type I) and (b,d) the specimen as-built in horizontal direction (Type II) to the building platform.


Introduction
Magnesium (Mg) alloys are of increasing interest as potential metallic biomaterials for temporary biodegradable implants in orthopedic and vascular applications due to their mechanical, electrochemical and biological properties [1][2][3].
As an alternative to conventional inert metals, Mg alloys can be a promising solution to problems such as osteogenesis, stress shielding phenomenon, image distortion, and secondary removal surgery [1,4]. However, since Mg implants degrade rapidly in the physiological environment, they are more likely to lose mechanical integrity before bone tissue is fully restored [5][6][7]. Furthermore, the degradation of Mg alloys in human body fluids increases the surrounding pH and ionic concentration of alloying elements and produces excessive hydrogen gas, which impedes the healing of surrounding tissues [1].
For load-bearing implants, biodegradable Mg alloys require high strength to support the loads applied to the implants and a low degradation rate to induce bone regeneration until bone tissue is recovered [4,8]. One major approach to improving the comprehensive properties of Mg alloys is to introduce alloying elements into Mg without affecting biocompatibility [9]. Zinc (Zn) is one of the biologically essential elements in the human body, and adding Zn to Mg alloys can enhance strength and form a protective film on the surface [10][11][12]. As-cast Mg-5Zn alloy [10] and extruded Mg-6Zn alloy [11] were recommended as biodegradable materials based on their mechanical properties and corrosion resistance. Meanwhile, as-cast Mg-3Zn alloy was proposed as biodegradable implant due to its mechanical and in vitro corrosion properties [12]. Manganese (Mn) can promote the growth and development of bone, and the addition of Mn to Mg alloys can improve mechanical properties [3,14]. In addition, Mn helps to enhance the corrosion resistance of Mg by removing harmful impurities and forming Mn oxide film [15,16]. The oxidized Mn enhanced protective properties of Mg(OH) 2 surface layer, thereby reducing the degradation rate of Mg-2Zn-0.2Mn alloy [17]. It has also been reported that adding Mn in an amount of 1 wt% or 1 3 less is effective in increasing the in vitro corrosion resistance for Mn oxide film to inhibit chloride ion penetration of some Mg-Zn based alloys [16]. Mg-2Zn-1Mn alloy showed higher corrosion resistance and more stable degradation behavior than pure Mg and Mg-3Zn alloy [3]. The addition of 0.5 wt% Mn was beneficial to Mg-4Zn alloy by increasing tensile properties and reducing degradation rate [14]. Strontium (Sr) is effective in treating osteoporosis by promoting bone cell growth and increasing bone formation [18,19]. Moreover, Sr increases the tensile strength of Mg alloys through grain refinement [20]. With regard to uniform corrosion in Hank's solution, an optimal Sr content of 1.0-1.5 wt% was suggested for as-cast Mg-1Zn-1Mn alloy [5]. On the other hand, As-cast Mg-5Zn-0.2Sr alloy showed combined the optimum mechanical properties and corrosion resistance [20].
In particular, addition of 0.5 wt% Sr exhibited the slowest corrosion rate in simulated body fluid [21]. Based on this, an appropriate combination of Zn, Mn and Sr can be useful for biomedical applications of Mg alloys with respect to the support of bone regeneration.
According to the above results, it is necessary to systematically analyze whether adding 0-3 wt% or 3-6 wt% of Zn to Mg-Mn-Sr alloy is effectual in biomedical applications. Furthermore, the underlying mechanism which Mg-Zn-Mn-Sr alloy degrades in the biological environment is not yet clear, especially when both Mn and Sr are added at 0.5 wt%. The reason for adding 0.5 wt% of Mn and Sr is that this content distinctly slows down the degradation rate in a physiological environment [14,21]. Moreover, when alloying with Zn, it aims to reduce galvanic corrosion due to secondary particles as much as possible while enhancing grain refinement and solid solution strengthening [10][11][12]. For this purpose, the present study investigates the microstructure, compressive mechanical properties, and in vitro corrosion behavior of biodegradable Mg-xZn-0.5Mn-0.5Sr (ZMJ) alloys with Zn content of 0 to 5 wt% in the as-cast state. Based on this, this study proposes the optimal Zn content for biodegradable ZMJ alloy by comprehensively considering the mechanical properties and in vitro corrosion resistance.

Materials and Methods
Mg-xZn-0.5Mn-0.5Sr alloys with the nominal composition (in wt%) were cast with pure Mg (> 99.9 wt%), Zn (> 99.9 wt%), Sr (> 99.5 wt%) and Mg-2Mn master alloy. The addition amount of Mn and Sr was fixed as about 0.5 wt%, and only the Zn content was changed to 0.00, 0.10, 0.85, 2.62 and 4.50 wt% (hereinafter these alloys are denoted as Z0, Z01, Z1, Z3 and Z5). All pure metals and master alloy were melted in a graphite crucible with an induction furnace at 730 °C under an inert atmosphere with a mixture of CO 2 and SF 6 . The molten metal was then stirred for 5 min and stabilized at 730 °C for 10 min. When the melt reached about 700 °C, it was cast into a steel book mold with dimensions of 200 × 145 × 20 mm 3 , preheated to 200 °C. Table 1 lists the actual chemical compositions of as-cast ZMJ alloys as measured by optical emission spectroscopy (OES, OBLF QSN 750-II).
Microstructures of as-cast ZMJ alloys were characterized by optical microscopy (OM, Olympus BX51M) and scanning electron microscopy (SEM, JEOL JSM-7900F) with backscattered secondary electron and energy dispersive spectrometer (EDS) detectors. For OM, samples were ground with SiC paper and etched with picric acid solution (3 g picric acid, 10 ml distilled water, 10 ml acetic acid and 100 ml ethanol). Average grain size (d avg ) was measured in three or more regions using the linear intercept method according to ASTM E112-12. Secondary particles and corroded surfaces were observed by SEM without etching. The area fraction of each secondary particle was quantitatively determined based on the composition ratio using AZtecFeature of Oxford Instruments' AZtec® 5.1, which is an SEM-based particle analysis solution. Since segregation may occur in the as-cast state, specimens were taken from the same position of all cast materials and an area of 250 × 250 μm 2 was observed in the center of the cast material to solve the localization problem.
Mechanical compressive test was performed at room temperature with a quasi-static strain rate of 1 × 10 -3 1/s using a universal test machine (INSTRON 5982). Solid cylindrical specimens with a diameter of 13 mm and a height of 38 mm were used in accordance with ASTM E09-19. The tests were carried out at least three times for each alloy. For immersion test, all samples with dimensions of 32 × 16 × 2 mm 3 were ground up to 2000 grit, ultrasonically cleaned in ethanol and air-dried. According to ASTM-G31-72, the immersion test was carried out in phosphatebuffered saline (PBS) solution (8.0 g/L NaCl, 0.2 g/L KCl, 1.42 g/L Na 2 HPO 4 and 0.24 g/L KH 2 PO 4 in 1 L distilled water). The ratio of the solution volume to the surface area was 0.25 ml/mm 2 . The pH value of PBS solution was adjusted to 7.4 and maintained at a physiological temperature of 37 ± 1 °C using a constant-temperature oven. All pre-weighed samples were hung with fishing line in PBS solution replaced with fresh solution daily to mimic the physiological environment. After immersion for 7 days, corrosion products were removed with a chromate solution (200 g/L CrO 3 , 20 g/L Ba 2 (NO 3 ) 2 and 10 g/L AgNO 3 in 1 L distilled water) and then washed in ethanol according to ASTM-G1-03. The corrosion rate (CR) in mm/y was calculated from the weight loss using the following equation: where the coefficient K = 8.76 × 10 4 , W is the weight loss (g), A is the sample area (cm 2 ), T is the immersed time (h), and D is the material density (g/cm 3 ). Three parallel tests were performed for each alloy to calculate the repeatability standard deviation.
Potentiodynamic polarization (PDP) tests were conducted using a potentiostat (Interface 1010E, Gamry instruments). Samples with dimensions of 16 × 16 × 2 mm 3 were prepared in the same manner as the immersion specimen. A threeelectrode flat cell kit was used for the PDP tests. The vessel was filled with 500 ml PBS solution and the temperature was adjusted to 37 °C using a water bath. A graphite electrode, an Ag/AgCl electrode with a saturated KCl solution and a specimen were set as auxiliary electrode, reference electrode, and working electrode, respectively. All samples were connected to a stainless-steel holder and the area of the working electrode exposed to the electrolyte was 1.0 cm 2 . The potential was increased from −0.5 V versus open circuit potential (OCP) to 0.7 V versus OCP at a potential sweep rate of 1 mV/s. Volta potential difference (VPD) between the secondary particles and the α-Mg matrix was measured using a scanning kelvin probe force microscopy (SKFPM, Park Systems XE-100). All measurements were carried out at a temperature of ~ 22 °C and a relative humidity of 20%. For each alloy, VPD value of the second phase was measured at 3-8 points in two regions with an area of 50 × 50 μm 2 , and the maximum-minimum values were presented as a result. Figure 1 shows the microstructures of as-cast ZMJ alloys observed by OM (a 1 -e 1 ) and SEM (a 2 -e 2 ). The average grain size of the as-cast microstructure decreased from 215 to 95 µm with increasing Zn content. The addition of 0.1 wt% Zn in Mg-0.5Mn-0.5Sr alloy had no effect of grain refinement, but the average grain size definitively decreased from 215 to 100 µm as the Zn content increased to 2.62 wt%. When the Zn content exceeded 2.62 wt%, the effect of grain refinement was weakened, and Z3 and Z5 alloys exhibited similar grain sizes ranging from 100 to 95 µm ( Fig. 1d 1 and e 1 , respectively). Without Zn, the second phases were distributed along the grain boundaries (Fig. 1a). On the other hand, when 0.1 wt% or more of Zn was added, Fig. 1b-e show not only the granular secondary particles inside the grains, but also the secondary particles in the form of strips along the grain boundaries. The EDS analysis in Fig. 2 confirms that the second phase of Z0 alloy was Mg 17 Sr 2 (red dots in Fig. 2a). When the Zn content was added up to 0.85 wt%, the dissolved Zn in Mg 17 Sr 2 also increased (red dots in Fig. 2b and c). The formation of Mg-Zn-Sr ternary phase and Mg-Zn binary phase was observed in Z3 and Z5 alloys. Liu et al. [22] reported the formation of Mg 6 Zn 2 Sr in Mg-3Zn-1Y alloy containing Sr of 0.3-1.5 wt%. Based on this, it is possible to identify Mg-Zn-Sr ternary particles marked with orange dots in Z3 and Z5 alloys as Mg 6 Zn 2 Sr. Furthermore, Cai et al. [10] found MgZn intermetallic compound with an atomic ratio of Mg to Zn (7:3) in Mg-Zn alloys containing 5-7 wt% Zn. They estimated that the high concentration of Mg atoms in this secondary particle could be attributed to the existence of α-Mg in the eutectic MgZn + α-Mg. For this reason, the secondary particle marked with yellow dot can be identified as MgZn in Z5 alloy (Fig. 2e).

Microstructure Characterization
The quantitative analyses of the second phases in Fig. 3 support this explanation. In Z0 alloy, only Mg 17 Sr 2 was detected with an area fraction of 1.70% (Fig. 3a). Z1 alloy consisted of Mg 17 Sr 2 and Mg 6 Zn 2 Sr with area fraction of 1.58% and 0.16%, respectively (Fig. 3c). When more than 2.62 wt% Zn was added, most of Mg 6 Zn 2 Sr was formed instead of Mg 17 Sr 2 , and Mg 17 Sr 2 was hardly observed (Fig. 3d). As Zn increased from 0.85 to 4.50 wt%, the area fraction of MgZn gradually increased from 0.02 to 1.60%. As a result, the total area fraction of the second phases reached from 1.73% in Z0 alloy to 3.69% in Z5 alloy (Fig. 3f). On the other hand, secondary particles composed of only Mn element were rarely observed. The maximum solubility of Mn in Mg alloy is 2.2 wt%, indicating that almost all Mn elements were uniformly dissolved in the α-Mg matrix regardless of the increase in the Zn content. One of the widely accepted causes for grain refinement by Zn in Mg alloys is that segregation of Zn induces intensive constitutive supercooling in the diffusion layer in front of the solid/liquid interface, thereby restricting grain growth and promoting nucleation of primary Mg to refine grain size. [10,23]. For this reason, it can be explained that the addition of Zn basically makes grain size refiner compared to Z0. Nevertheless, the refinement efficiency decreases with increasing Zn content from 2.62 to 4.50 wt%. This is presumed to be related to changes in the major second phase from Mg 17 Sr 2 to Mg 6 Zn 2 Sr. When the Zn content is less than 1 wt%, Mg 17 Sr 2 was mostly present at the grain boundary, but did not inhibit grain growth. On the other hand, the grain growth was suppressed as Mg 6 Zn 2 Sr was substituted at the grain boundaries. When the Zn content is 4.5 wt%, it is assumed that the reason for the decrease in the refinement efficiency is that the increase in the fraction of Mg6ZnSr is relatively decreased in Fig. 3f. Figure 4 presents the influence of Zn addition on the compressive properties at room temperature in as-cast ZMJ alloys. As-cast Mg alloys generally have low yield strength due to large grain size as well as low critical resolved shear stress of basal < a > slip and tensile twinning, which are the main deformation modes in Mg alloys. As the Zn content increased from 0.0 to 4.5 wt%, compressive yield strength (CYS) and ultimate compressive strength (UCS) increased from 44 ± 0.87 to 67 ± 0.12 MPa and from 212 ± 2.71 to 331 ± 3.64 MPa, respectively. All ZMJ alloys exhibited a fracture strain (FS) higher than 20%. When only 0.1 wt% Zn was added, there was no significant difference in compressive behavior compared to Z0. This was primarily because the grain sizes of Z0 and Z01 were similar as shown in Fig. 1. On the other hand, when more than 0.85 wt% of Zn was added, work hardening occurred remarkably and Z1 alloy showed the largest FS of 27 ± 1.06%. Subsequently, when the Zn content increases up to 4.50 wt%, the compressive strain decreases to 22.3 ± 0.86% due to the increase of secondary particles. The change in compressive strength of as-cast ZMJ alloys can be principally attributed to grain refinement by addition of Zn. Figure 5 compares the PDP curves of as-cast ZMJ alloys obtained in PBS solution. The PDP curves indicated that all ZMJ alloys exhibited passivity at their corrosion potential (E corr ). Moreover, as the Zn content increased from 0.0 to 4.5 w%, the passive current density was reduced and the cathodic current density generally increased. As a result,   Fig. 5 demonstrated the beneficial effect of Zn on the initial passivation behavior. Hereby, it was also possible to measure the corrosion current density (i corr ) of ZMJ alloys covered with the passive film. Each i corr of Z0, Z01, Z1, Z3, and Z5 alloys was measured to be 67.17, 65.30, 45.87, 43.22, and 61.84 μA/cm 2 , respectively. This indicates that i corr reflects CR when the alloy surface has a passive film. That is, if the passive film of as-cast ZMJ alloys remains stable, their electrochemical CRs can be calculated to be ~ 0.001 mm/y. Moreover, the passive state of the alloys was stable only in a very limited potential range. The passive film of Z0 alloy was broken around -1.43 V versus Ag/AgCl, and the passivity breakdown potential for Z5 alloy was lowered to -1.47 V versus Ag/AgCl. Therefore, it can be expected that the main corrosion type of ZMJ alloys in PBS solution is localized corrosion, and the addition of Zn will reduce the resistance of the alloys to localized corrosion.

Immersion Test
The effect of Zn on CR of as-cast ZMJ alloys was quantitatively evaluated through immersion tests in PBS solution. Figure 6 compares the corrosion morphology before and after removal of corrosion products of the specimens immersed in PBS solution for 7 days. In Fig. 6a, the surface color of the corroded specimen changed depending on the Zn content. The surface of the corroded Z0 specimen showed a dark gray color, but the corroded surface was noticeably brightened by adding only 0.1 wt% Zn (Z01). Moreover, the number and size of the corrosion spots were gradually reduced when comparing ZMJ alloys containing less than 1.0 wt% Zn (denoted as Z0-Z1 alloys). Z0-Z1 alloys exhibited a relatively smooth surface even after immersion for 7 days, whereas a significant amount of white corrosion products was observed on the surface of Z3 and Z5 alloys. Figure 6b compares the corroded area more clearly. Z0-Z1 samples maintained a fairly clean surface under the corrosion product layer after immersion in PBS solution for 7 days. Moreover, Z0-Z1 alloys exhibited very few traces of localized corrosion. On the other hand, Z3 and Z5 specimens were severely damaged during immersion, and the corrosion area expanded as the Zn content increased from 2.62 to 4.50 wt%. Figure 6c depicts the magnified photograph of the surface of each specimen. The pitting corrosion of Z0-Z1 alloys was highly localized and little lateral growth was also observed, whereas that of Z3 and Z5 alloys propagated both laterally and vertically. In this regard, Fig. 7 supports that Z0-Z1 alloys exhibited similar corrosion behavior, and CR of Z3 and Z5 alloys were much faster than those of Z0-Z1 alloys. The cross-section of the corrosion layer was investigated after immersion in PBS solution for 24 h. Specimens for inspection were prepared by immersion for a relatively short time to avoid severe corrosion attack, especially on Z3 and Z5 alloys. Figure 7a presents that the α-Mg matrix was preferentially corroded rather than the secondary particles. In addition, Fig. 7b compares the thickness of the corrosion product layer among ZMJ alloys. As the Zn content increased from 0.0 to 0.85 wt%, the thickness of the oxide layer decreased from ~ 4 µm (Z0) to less than 1 µm (Z1). However, adding more Zn thickened the corrosion product layer up to 8 µm (Z5). Therefore, Fig. 7 demonstrates that CR increased in the order of Z1 < Z01 < Z0 < Z3 < Z5.
The corrosion rates were calculated from the measured weight loss after the immersion tests for 7 days and plotted as a function of the Zn content in Fig. 8. According to Eq. (1), CR of Z0, Z01 and Z1 alloys were 0.71 ± 0.02, 0.48 ± 0.05 and 0.45 ± 0.04 mm/y, respectively. This indicates that CR decreases slightly with increasing Zn content. However, the addition of 2.6 and 4.5 wt% Zn accelerated corrosion 4-6 times (from 2.28 ± 0.36 to 3.31 ± 0.18 mm/y) compared to Z0 alloy. As a result, the graph of CR versus Zn content showed a V shape. The immersion test confirmed that Z1 alloy exhibited the highest corrosion resistance in PBS solution among ZMJ alloys containing 0-4.5 wt% Zn. Figure 9 investigates the pit initiation sites of ZMJ alloys in detail. Corrosion started primarily in the form of pitting corrosion within 5 min of immersion, as expected from the PDP curves (Fig. 5). The pits in Z1, Z3, and Z5 alloys were mostly originated from the α-Mg matrix adjacent to the secondary particles (Fig. 9). As discussed in Figs. 2 and 3, the dominant second phase in Z0-Z1 alloys was Mg 17 Sr 2 , and those in Z3 and Z5 alloys were the Zn-related intermetallic compounds Mg 6 Zn 2 Sr and MgZn. The pit marked with yellow arrow initiated from Mg 17 Sr 2 particle in Z1 alloy (Fig. 9a) and the pits occurred at Mg 6 Zn 2 Sr in Z3 alloy (Fig. 9b). Figure 9c and d show the pits nucleated near Mg 6 ZnSr and MgZn in Z5 alloy, respectively. Thus, this indicates that these three second phases (Mg 17 Sr 2 , Mg 6 Zn 2 Sr and MgZn) serve as pit initiation sites in ZMJ alloys, suggesting that the secondary particles act as electrochemical cathode than the α-Mg matrix.

VPD Measurement
VPD measurement supports the observations in Fig. 9.  respectively. Figure 10 reveals that all second phases exhibited higher Volta potential than the α-Mg matrix, which can be understood as the selective dissolution of the α-Mg matrix around the secondary particles. It is worth noting that the Zn content of each alloy affects the VPD value of the second phase. The measured VPD values of Mg 17 Sr 2 , Mg 6 Zn 2 Sr, and MgZn in Z0, Z1 and Z3 alloys were displayed in Fig. 10. Mg 17 Sr 2 in Z0 alloy exhibited a higher Volta potential of 114-120 mV than the α-Mg matrix, while VPD between Mg 17 Sr 2 and the α-Mg matrix decreased to 54-75 mV in Z1 alloy. Furthermore, VPD between Mg 6 Zn 2 Sr and the α-Mg matrix was 120 mV. On the other hand, Mg 6 Zn 2 Sr and MgZn coexisted in Z3 alloy, and the VPD value between Mg 6 Zn 2 Sr and the α-Mg matrix was 84-104 mV. In particular, the VPD value between MgZn and the α-Mg matrix was 142-154 mV, which was the highest VPD value among three secondary particles. Comparing the VPD values of Mg 17 Sr 2 in Z0 and Z1 alloys, it was found that VPD between Mg 17 Sr 2 and the α-Mg matrix decreased with increasing Zn content of the α-Mg matrix. In the same way, VPD between Mg 6 Zn 2 Sr and the α-Mg matrix decreased with increasing Zn content in the α-Mg matrix in Z1 and Z3 alloys.

Effect of Zn Content on Strengthening Mechanisms
The increase in strength of as-cast ZMJ alloys with increasing Zn content can be principally attributed to grain boundary and solid solution strengthening. Reducing the grain size finely suppresses the movement of dislocation by grain boundaries, increasing the strength and hardness to high values [24]. The effect of grain boundary strengthening (∆σ GBS ) can be estimated according to the following Hall-Petch relation [25,26]: where d is the average grain size and k is the Hall-Petch coefficient representing the magnitude of boundary obstacle against deformation propagation. Based on the data in Table 2, k can be calculated as 600 MPa mm 0.5 . Hence, the grain boundary strengthening contributes to the increase in CYS of as-cast ZMJ alloy from 41.0 to 61.7 MPa. This demonstrates that the addition of Zn made grain much finer and the strength improved with grain refinement. Solid solution strengthening arises from the resistance of solute atoms to dislocation glide [24]. In this study, the solution strengthening of as-cast ZMJ alloy was mainly caused by the Zn solute in the α-Mg matrix. According to the model proposed by Gypen and Deruyttere [27], the increase in strength by solid solution (∆σ ss ) can be expressed as follows, focusing on the Zn solute:  where k i and C i represent solution strengthening constant and atom fraction of solute i, respectively, n is constant and generally treated as 1/3 or 2/3 [27,28]. For Mg alloys, n is adopted as 2/3 and k Zn is 40 MPa (at.%) −2/3 [28,29]. The EDS analysis in Fig. 2 shows that the amount of Zn dissolved in the α-Mg matrix gradually increased from 0.00 to 1.52 at.%. In the case of Z01 alloy, the Zn content was originally 0.1 wt% (= 0.037 at.%), and a significant amount of Zn was dissolved in Mg 17 Sr 2 , so that the amount of Zn solute was measured to be almost zero. Based on this, the contribution of solid solution strengthening by Zn to CYS is estimated to be 0.0 to 2.5 MPa. Two strengthening contributions are summarized in Table 2. The calculated values (σ cal ) and the measured values (σ CYS ) are compared in terms of Zn content, considering the assumptions and deviations of the parameters used in the above equations. As a result, the calculated CYS is approximately equal to the measured CYS. The comparative analysis indicates that the grain boundary strengthening is the most effective mechanism for strengthening as-cast ZMJ alloys. This is supported by the increased CYS of Z5 alloy due to finer grains compared with Z0 alloy. In addition, it is estimated that the strengthening effect of the secondary particles contributed to the grain boundary strengthening by suppressing grain growth rather than dispersion strengthening by themselves. The reason is that, as shown in Figs. 1 and 3, the secondary particles were mostly distributed along the grain boundaries rather than inside the α-Mg matrix, and the grain size decreased as the Zn content increased.

Effect of Zn Content on In Vitro Corrosion Resistance
Considering the microstructure, it is possible to understand the role of Zn in the corrosion behavior of ZMJ alloys containing 0-4.5 wt% Zn. The addition of Zn changed three microstructural characteristics of the second phases: amount, type and VPD values. As the Zn content increased, the total area fraction of secondary particles increased from 1.73 to 3.69% (Fig. 3), and the amount of Zn solute also increased from 0.00 to 1.52 at.% ( Table 2). More importantly, the chemical composition of the secondary particles varied with the Zn content. Mg 17 Sr 2 was mostly observed in Z0-Z1 alloys and Mg 6 Zn 2 Sr was additionally detected with area fraction of 0.16% in Z1 alloy. On the other hand, Mg 6 Zn 2 Sr and MgZn were the dominant second phases in Z3 and Z5 alloys. Furthermore, the VPD values between the secondary particles and the α-Mg matrix increased in the order of Mg 17 Sr 2 < Mg 6 Zn 2 Sr < MgZn. This indicates that Zn was incorporated into the secondary particles, making them nobler (Figs. 7 and 9). Moreover, Fig. 11 reveals that the VPD of the second phases (Mg 17 Sr 2 and Mg 6 Zn 2 Sr) for the α-Mg matrix decreased as the Zn content in ZMJ alloys increased. Since Zn is a nobler element than Mg, the α-Mg matrix became more noble with increasing Zn solute. Thus, this reduced the VPD values between the second phases and the α-Mg matrix containing Zn. The corrosion morphologies of ZMJ alloys indicates that the secondary particles were the main cause of corrosion. They formed micro-galvanic coupling with the α-Mg matrix, which served as a preferential site for the initiation and propagation of localized corrosion (Fig. 9). Therefore, it is considered that the corrosion acceleration of Z3 and Z5 alloys results from the increase in the area fraction of Mg 6 Zn 2 Sr and MgZn. In contrast, in ZMJ alloys containing up to 1.0 wt% Zn (Z0-Z1 alloys), the addition of Zn slightly increased the amount of the second phases (Fig. 3), but improved the corrosion resistance (Fig. 8). Z0-Z1 alloys mostly contained Mg 17 Sr 2 , and only a small amount of Mg 6 Zn 2 Sr was observed in Z1 alloy. In other words, the corrosion behavior of Z0-Z1 alloys is dependent on Mg 17 Sr 2 . Therefore, for Z0-Z1 alloys, the decrease in CR with increasing Zn content is attributable to the decrease in VPD value between Mg 17 Sr 2 and the α-Mg matrix. This decrease in VPD can be attributed to the increase in Zn solute in the α-Mg matrix as well as the compositional change of the second phase due to Zn dissolved in Mg 17 Sr 2 . Summarizing the results so far, Z1 alloy exhibited the best anti-corrosion performance among five ZMJ alloys containing 0-4.5 wt% Zn.

Conclusions
This study investigates the microstructure, compressive mechanical properties and in vitro corrosion behavior of as-cast Mg-Zn-0.5Mn-0.5Sr alloys with Zn content of 0 to 5 wt%. Key findings are summarized as follows: (1) Increasing the Zn content in Mg-xZn-0.5Mn-0.5Sr alloy refined the grains from 215 ± 29 to 95 ± 3 µm and changed the secondary particles from Mg 17 Sr 2 to Mg 6 Zn 2 Sr and MgZn. As the Zn content increased, CYS of as-cast ZMJ alloys increased from 44 ± 0.87 to 67 ± 0.12 MPa due to grain boundary strengthening. (2) ZMJ alloys exhibited different corrosion behavior in PBS solution depending on the Zn content. The addition of 0 to 0.1 wt% Zn reduced CR from 0.71 ± 0.02 to 0.48 ± 0.15 mm/y and the lowest CR of 0.45 ± 0.04 mm/y was obtained by alloying 0.85 wt% Zn after immersion in PBS solution for 7 days. However, adding more Zn significantly increased CR up to 3.31 ± 0.18 mm/y. (3) The secondary particles were the main cause of corrosion of as-cast ZMJ alloys, and their corrosion resistance strongly depended on the characteristics of the second phases: amount, type and VPD values. The second phases formed micro-galvanic coupling with the α-Mg matrix, which served as a preferential site for the initiation and propagation of corrosion. Alloying more than 1.0 wt% Zn accelerated the pitting corrosion by increasing the area fraction of the Zn-related intermetallics, Mg 6 Zn 2 Sr and MgZn. In contrast, adding less than 1.0 wt% Zn reduced CR despite the formation of Mg 17 Sr 2 . This resulted from a decrease in VPD value between Mg 17 Sr 2 and the α-Mg matrix, which can be attributed to an increase in Zn solute in the α-Mg matrix as well as the compositional change of the second phase due to Zn dissolved in Mg 17 Sr 2 . (4) The optimum Zn content of Mg-xZn-0.5Mn-0.5Sr alloy can be determined as about 1 wt% by comprehensively considering the mechanical properties and in vitro corrosion behavior for designing biodegradable Mg alloys.