Effects of Mg Content and Pulsed Magnetic Field Treatment on Microstructure and Properties of As-Cast Biodegradable Zn-3Cu Alloy

Abstract

The microstructure, mechanical properties, corrosion behavior, cytocompatibility, and antibacterial properties of biodegradable Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys with or without pulsed magnetic field treatment during casting were systematically investigated. Mg addition induced the formation of fine Mg₂Zn₁₁ precipitated along the matrix grain boundaries. With the increase in Mg content, the precipitation of the Mg₂Zn₁₁ phase increased, and the grain size became finer. Pulsed magnetic field treatment exacerbated the occurrence of this phenomenon. Under the combined action of the Mg₂Zn₁₁ phase and refined grain size, Zn3Cu0.5Mg alloy with pulsed magnetic field treatment had the best strength–ductility match (σ_UTS = 181.46 ± 1.06 MPa, δ = 3.95 ± 0.07%), moderate corrosion rate (i_corr = 5.69 ± 3.96 μA/cm²), positive cytocompatibility, and antibacterial properties. This study indicated that Zn3Cu0.5Mg alloy with pulsed magnetic field treatment had the greater potential to further improve its properties through subsequent conventional metal-forming processing and severe plastic deformation techniques to meet clinical requirements, compared to existing as-cast Zn alloys.

1. Introduction

Magnesium (Mg)-, iron (Fe)-, and zinc (Zn)-based alloys are considered biodegradable metallic materials for medical applications. However, the rapid degradation of Mg-based alloys in physiological environments can lead to a premature loss of mechanical integrity of the implant [1]. Fe-based alloys with degradation rates significantly below clinical requirements (complete bioabsorption within 12–24 months) may generate fragmentary embolism, as noted with other non-biodegradable implants [2]. Zn has a standard corrosion potential of 0.76 V, which is intermediate between that of Fe (0.44 V) and Mg (2.37 V) [3]. Therefore, in recent years, owing to moderate corrosion rates, the development of Zn-based biodegradable alloys has emerged as a research hotspot in biomedicine. Notably, Zn-Cu alloys have attracted more attention recently for their excellent ductility and strength enhanced by Cu solid solution, grain refinement, and dispersed CuZn₅ phase in the η-Zn matrix [4,5,6]. Tang et al. [7] reported that the elongation of as-extruded Zn-4Cu alloy reached 50.6 ± 2.8%, and the ultimate tensile strength reached 270 ± 0.5 MPa. Zn-Cu alloys exhibit moderate in vitro degradation rates (0.02–0.11 mm/year) [8,9]. Additionally, the alloying element Cu is an essential trace element for humans, and Zn-Cu alloys have been reported to stimulate endothelial cell proliferation, enhance angiogenesis, and possess antibacterial properties [7,10]. However, the mechanical properties of Zn-Cu alloys still fail to meet clinical strength standards (ultimate tensile strength (σ_UTS) > 300 MPa, yield strength (σ_YS) > 200 MPa [9]).

Mg has a superior strengthening ability for Zn alloys [11,12,13,14]. Pachla et al. [15] reported that the mechanical properties of Zn-xMg (x = 0, 0.5, 1, 1.5 wt.%) alloys continued to improve as the Mg content increased. Su et al. [16] incorporated various amounts (0, 0.5, 1, 2 wt.%) of Mg element into Zn-5Cu alloy. Compared to Zn-5Cu alloy, the tensile strength of Zn-5Cu-2Mg alloy increased by 35% to 216 MPa. The Mg₂Zn₁₁ phase precipitates when the content of Mg is below 1.5 wt.% [13]. Dispersing the Mg₂Zn₁₁ phase effectively impedes dislocation motion and, thus, improves the strength of Zn-Mg alloys [14]. Additionally, the effect of Mg addition on the corrosion behavior of Zn alloys has been studied by many scholars [17,18]. Jin et al. [17] reported that adding Mg improved the biocompatibility and degrading performance of Zn alloys. Ye et al. [11] demonstrated that the corrosion rate of Zn-Mg alloys varied due to the competition between the positive effect of grain refinement resulting from the addition of Mg and the negative effect of Mg₂Zn₁₁ volume fraction increment. Furthermore, Mg is an essential element for humans, which can not only prevent acute thrombosis and regulate vascular dilation [19] but also stimulate bone growth and lower the risk of osteoporosis [20]. Therefore, adding Mg to Zn-Cu alloys is expected to optimize their mechanical properties, corrosion behavior, and biocompatibility.

In recent years, with the successful design of biodegradable Zn alloy compositions, researchers focused on further optimizing the microstructure and enhancing the mechanical properties of these alloys through subsequent conventional metal-forming processing, such as extrusion, drawing, rolling, forging, and severe plastic deformation techniques [4,6,7,10,15,16]. Additionally, bioactive coatings were prepared on the surface of biodegradable Zn alloys through a plasma electrolytic oxidation process, electrodeposition method, or microwave-assisted coating method [21,22] to regulate their corrosion rate and biocompatibility. However, the initial microstructure after solidification during casting, which also plays an important role in determining Zn alloys’ final properties due to structural inheritance, has garnered relatively limited attention. Therefore, employing specialized techniques to control the solidification structure of biodegradable Zn alloys during casting to improve their properties is of great significance.

Pulsed magnetic field treatment is a commonly used and effective method in the field of alloy solidification processing. It can achieve microstructural homogenization, reduce casting defects, and refine the alloy grains. Zhang et al. [23] found that pulsed magnetic field treatment refined the solidification microstructure of Mg-7Zn alloy and reduced the volume fraction of the second phase. The σ_UTS, σ_YS, and elongation of Mg-7Zn treated with a 300 V pulsed magnetic field were increased by 40.4%, 38.2%, and 48.3%, respectively, compared to those of the untreated alloy. Li et al. [24] studied the effects of the high-voltage pulsed magnetic field on the microstructure of the Zn-Ag alloys and indicated that it refined the primary ε-AgZn₃ dendrites and distributed them homogeneously in the specimens. Li et al. [25] applied a 6 kV pulsed magnetic field to the solidification process of Zn-xCu (x = 1.5, 2.5, 4.0 wt.%) alloys and discussed the structure of the alloys refined due to the convection and Joule heat induced by the pulsed magnetic field in the melt. However, studies on the effect of pulsed magnetic field treatment on biodegradable Zn alloys have primarily focused on microstructures, and their effects on properties have not yet been systematically evaluated. Therefore, this study systematically investigated the effects of Mg addition and pulsed magnetic field treatment on the microstructure, mechanical properties, corrosion behavior, cytocompatibility, and antibacterial property of Zn-3Cu-xMg (x = 0, 0.5, 1 wt%) alloys, aiming to develop an as-cast Zn alloy with improved properties for subsequent conventional metal-forming processing (extrusion, drawing, rolling, or forging) and severe plastic deformation techniques.

2. Materials and Methods

Commercially pure Zn (99.99%), Cu (99.99%), and Mg (99.96%) were used to prepare the as-cast Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys. According to the composition of the Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys, the dried pure metals were placed into a refractory crucible and completely melted through electrical resistance furnace (SG2-5-10, Shanghai real research Electric Furnace Co., Ltd., Shanghai, China). Then, the molten alloy was held at 620 °C for 30 min. During this period, the molten alloy was continuously stirred, and impurities were promptly removed. Subsequently, the molten alloy was poured into a cylindrical graphite mold positioned in the pulsed magnetic field device, as shown in Figure 1. Immediately after pouring, the pulsed magnetic field device was activated until the metal solidified completely. To prevent oxidation of the alloy, the entire process of melting, pouring, and solidification was carried out under a protective gas atmosphere (CO₂: 99.5%, SF₆: 0.5%).

The pulsed magnetic field generates periodic pulsed forces within the melt, which combine with electromagnetic forces to form electromagnetic oscillations [26]. By controlling the pulsed voltage and frequency, the magnitude and period of the pulsed forces can be regulated. According to our previous study [27], the microstructure and mechanical properties of Zn-3Cu alloy were explored by setting different pulsed voltages and frequencies. It was demonstrated that when the pulsed voltage was 300 V and the frequency was 5 Hz, Zn-3Cu alloy had the finest and most homogeneous microstructure and the best mechanical properties. Based on this, in the solidification process of this study, an external pulsed magnetic field with a voltage of 300 V and a frequency of 5 Hz was directly applied. For the convenience of subsequent research description, the alloys without pulsed magnetic field treatment were named W-Zn3Cu, W-Zn3Cu0.5Mg, and W-Zn3Cu1Mg, respectively, and the alloys with pulsed magnetic field treatment were named P-Zn3Cu, P-Zn3Cu0.5Mg, and P-Zn3Cu1Mg, respectively.

For the microstructure observation, alloys were sandpapered to 5000 grit and then polished to a mirror finish. The polished section was etched by a solution of 1.0 g oxalic acid, 1.0 mL nitric acid, 1.0 mL acetic acid, and 150 mL distilled water. The elemental distribution on the surface of alloys was characterized using a scanning electron microscope (FEI Apreo, Thermo Fisher, Waltham, MA, USA) equipped with energy dispersive spectroscopy (EDS). Secondary electrons (SE) and backscattered electrons (BSE) were both used to characterize the surface morphology and phase distribution. In addition, the phase composition of alloys was detected by XRD techniques. The XRD scanning range was from 20 to 80°, and the scanning rate was 10°/min.

Tensile tests were conducted on a universal material testing machine (WDW-200D, Shandong Wanchen Testing Machine Co., Ltd, Jinan, China) at a strain rate of 1 mm/min. The dimensions of tensile samples were designed according to GB/T 228.1-2021 [28], with a gauge of Φ5 × 25 mm.

The electrochemical corrosion investigation was carried out in a standard three-electrode electrochemical flask cell with an electrochemical workstation (Reference 600 TM, Gamry Instruments, Inc., Warminster, PA, USA). A saturated calomel electrode (SCE), a platinum sheet, and a sample having dimensions of Φ10 × 2 mm were utilized as the reference, counter, and working electrodes, respectively. The exposed area of the sample was approximately 0.636 cm² after sealing the edges of sample with silicone. The simulated body fluid (SBF) solution was used in this study, which consisted of NaCl (8.035 g/L), NaHCO₃ (0.355 g/L), KCl (0.225 g/L), K₂HPO₄·3H₂O (0.231 g/L), MgCl₂·6H₂O (0.311 g/L), 1mol/L HCl (39 mL/L), CaCl₂ (0.292 g/L), Na₂SO₄ (0.072 g/L), and finally, the pH value was adjusted to 7.40 using Tris and 1mol/L HCl. After the open circuit potential (OCP) test for 3600 s, the electrochemical impedance spectroscopy (EIS) tests were performed with a sinusoidal voltage signal of 10 mV and in a frequency range of 100 kHz to 0.1 Hz at E_OCP. The potentiodynamic polarization (PD) test was performed from −0.2 V_{vs OCP} to 0.3 V_{vs OCP} at a scanning rate of 0.5 mV/s. All the experiments were conducted thrice to obtain reproducible results.

Human umbilical vein endothelial cells (HUVECs, KeyGEN Bio TECH, Nanjing, China) were used to assess the cytocompatibility of Zn-3Cu-xMg alloys. The experiment was designed with a negative control group, a positive control group, and experimental groups. Each group had five replicate wells in a 96-well plate. For the negative control group, 100 μL of medium was added to each well without culturing cells. For the positive control group, 100 μL of medium was added to each well to culture 5 × 10³ cells. For the experimental group, 100 μL of medium containing 10% extract of the alloy [7] was added to each well to culture 5 × 10³ cells. The medium used was RPMI 1640 medium (Hyclone, Logan, UT, USA) supplemented with 10% fetal bovine serum (Gibco, Langley, OK, USA), penicillin (80 U/mL), and streptomycin (0.08 mg/mL). The extract of the Zn-3Cu-xMg alloys was prepared in advance using a culture medium with an extraction ratio of alloy surface area to medium volume of 1.25 cm²/mL in a humidified atmosphere with 5% CO₂ at 37 °C for 3 d. The HUVECs were cultured for 1, 3, and 7 d. At each time point, wells were cleaned with PBS solution, and 100 μL of medium containing 10% CCK-8 solution was added to each well. After that, the cells were cultured at a temperature of 37 °C for 2 h. The absorbance was measured by a plate reader (MuLtiskan GO, Thermo Scientific, Waltham, MA, USA) at 450 nm. The relative growth rate (RGR) is calculated by Formula (1):

RGR (%) = (OD_EG − OD_NC)/(OD_PC − OD_NC) × 100% = (OD_Alloys/OD_Control) × 100%

(1)

where OD_Alloys is the optical density of experimental group (OD_EG) minus negative control group (OD_NC), and OD_Control is the optical density of positive control group (OD_PC) minus negative control group (OD_NC).

The antibacterial activity of Zn-3Cu-xMg alloys against Staphylococcus aureus (S. aureus, ATCC6538) was assessed by a plate count method according to ISO 10993-5:2009. The alloys were soaked in 1000 μL of bacterial suspension with a concentration of 10⁶ CFU/mL and incubated at a temperature of 37 °C with 90% relative humidity. After 24 h, alloys with adherent bacteria were transferred to 500 μL of fresh phosphate-buffered saline (PBS) and shaken mechanically by vortex for 1 min. Then, 100 μL of the solution, after vortexing, was spread onto a nutrient agar plate and incubated at 37 °C for 24 h. Thereafter, the number of bacterial colonies on the nutrient agar plate was counted, and the antibacterial rate was calculated. The antibacterial rate is calculated by the Formula (2):

Antibacterial rate (%) = (N_CK − N_alloys)/N_CK × 100%

(2)

where N_CK and N_alloys denote the mean numbers of bacterial colonies on control check and Zn-3Cu-xMg alloys, respectively.

All experiments were performed three times, and the data were expressed as mean ± standard deviation. Student t-test was used to analyze the significant difference. * p < 0.05, ** p < 0.01, and *** p < 0.001 were considered statistically significant.

3. Results and Discussion

3.1. Microstructure

Figure 2(a₁–c₁) show the SEM images of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys without pulsed magnetic field treatment, and Figure 2(a₂–c₂) show the corresponding BSEM images. For W-Zn3Cu, the matrix and about 50 μm dendritic second phase were observed, and after adding Mg, about 5 μm lamellar phases precipitated along the grain boundaries of the matrix. Based on the EDS (Figure 2d) and XRD (Figure 2e) patterns, it could be concluded that the matrix was $\mathsf{\eta }$-Zn, the dendritic second phase was CuZn₅, and the lamellar phase was Mg₂Zn₁₁. Based on the Zn-Cu binary phase diagram [29], a peritectic reaction Zn + L → CuZn₅ occurs at 425 °C. And a eutectic reaction L → Zn + Mg₂Zn₁₁ occurs at 364 °C based on the Zn-Mg binary phase diagram [30]. Consequently, during the solidification process, the CuZn₅ phase initially precipitated from the melt as the temperature decreased to approximately 425 °C, followed by the precipitation of Mg₂Zn₁₁ along the grain boundaries of the matrix at around 364 °C. The XRD (Figure 2e) patterns also show that higher intensity of the peaks of Mg₂Zn₁₁ was detected with increasing Mg content. The volume fraction of the phase was statistically analyzed, and the results are shown in Figure 2f. It clearly showed that the volume fraction of the lamellar Mg₂Zn₁₁ phase increased with increasing Mg content while that of the $\mathsf{\eta }$-Zn phase decreased. Meanwhile, the volume fraction of the CuZn₅ phase first decreased and then increased.

Compared with Figure 2, the SEM, BSEM, EDS, and XRD results in Figure 3 all confirmed that applying a pulsed magnetic field during the solidification process, the phase composition of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys was the same as that of the alloys without pulsed magnetic field treatment. P-Zn3Cu consisted of the primary $\mathsf{\eta }$-Zn matrix and the dendritic CuZn₅ phase. Similarly, P-Zn3Cu0.5Mg and P-Zn3Cu1Mg consisted of the $\mathsf{\eta }$-Zn, CuZn₅ phase, and lamellar Mg₂Zn₁₁ phase. Statistical analysis of the phase volume fraction for Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys with pulsed magnetic field treatment was performed, with results compared to those of the alloys without pulsed magnetic field treatment (as indicated by the dashed line in Figure 3f). It showed that the pulsed magnetic field had a minimal effect on the volume fraction of $\mathsf{\eta }$-Zn and CuZn₅ phase for Zn-3Cu alloy. However, for Zn-3Cu-0.5Mg and Zn-3Cu-1Mg alloys, pulsed magnetic field treatment resulted in the reduced volume fraction of the CuZn₅ phase, while that of the Mg₂Zn₁₁ phase increased.

In addition, the SEM and BSEM images in Figure 2 and Figure 3 also indicated that the addition of Mg and pulsed magnetic field treatment affected the grain size of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys. Figure 4a–f show the optical microstructure (OM) images of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys with or without pulsed magnetic field treatment. The grain size of alloys was statistically analyzed and was shown in Figure 4g. As the Mg content increased, the grain size gradually decreased, and the pulse magnetic field treatment further refined the grain size. The refinement of grain size was mainly attributed to the addition of Mg, and pulsed magnetic field treatment increased the precipitation of the Mg₂Zn₁₁ phase along the grain boundaries (Figure 2 and Figure 3), which impeded the growth of the Zn matrix. And pulsed magnetic fields influenced the nucleation and growth processes of grains during solidification by increasing the number of nucleation sites [31], further refining grain size. Thus, P-Zn3Cu1Mg alloys had the smallest grain size of 59.90 ± 4.22 μm among all the Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys.

3.2. Mechanical Properties

Figure 5a shows the room temperature stress–strain curves of the Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys with or without pulsed magnetic field treatment. All alloys exhibited a rapid decline in the stress–strain curve after reaching the maximum stress, almost with no plastic deformation before fracture. The ultimate tensile strength (σ_UTS) and elongation (δ) are shown in Figure 5b,c. As the Mg content increased, σ_UTS gradually increased while δ decreased. The improvement of σ_UTS could be mainly attributed to the massive precipitation of the Mg₂Zn₁₁ phase (Figure 2f and Figure 3f), blocking the dislocation sliding [14] and strengthening the alloys. Massive precipitation of brittle Mg₂Zn₁₁ phase also led to the stress concentration, resulting in cracking, thus decreasing the ductility. It also showed that the pulsed magnetic field treatment increased both σ_UTS and δ of Zn-3Cu-xMg alloys. The improvements in strength and ductility were attributed to the grain refinement [4] after pulsed magnetic field treatment (Figure 4g). Therefore, under the combined action of the Mg₂Zn₁₁ phase and refined grain size, P-Zn3Cu0.5Mg alloy had the best strength–ductility match (σ_UTS = 181.46 ± 1.06 MPa, δ = 3.95 ± 0.07%). Compared with other types of as-cast Zn alloys [32] (Figure 5d), the P-Zn3Cu0.5Mg alloy had the greater potential to further improve its mechanical properties through subsequent conventional metal-forming processing such as extrusion, drawing, rolling, forging, and severe plastic deformation techniques to meet clinical requirements.

3.3. Corrosion Properties

The corrosion behavior of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys with or without pulsed magnetic field treatment in SBF solution was also evaluated systematically. Potentiodynamic polarization (PD) curves proved that all the alloys showed spontaneously passive behaviors in the SBF solution (Figure 6a). The corrosion current density (i_corr) is depicted in Figure 6b. It can be concluded that the value of icorr increased as the Mg content increased, and pulsed magnetic field treatment further increased icorr. The corrosion rate (CR) can be evaluated using ASTM G102-23 [33]:

CR = 3.27 × 10⁻³ × i_corr × EW/ρ

(3)

where EW is the equivalent weight (EW = 32.43 g/eq), and ρ is the density (ρ = 6.5 g/cm³) of the Zn-3Cu-xMg(x = 0, 0.5, 1 wt.%) alloys.

The CR value was calculated and shown in Figure 6c. It indicated that the addition of Mg and pulsed magnetic field treatment both accelerate the corrosion rate of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys. Precipitate phases influence the corrosion behavior of the material. The Mg₂Zn₁₁ phase and $\mathsf{\eta }$-Zn matrix phase could form a micro-battery cell due to the difference in their corrosion potentials [11,16], resulting in galvanic corrosion. In addition, the Mg₂Zn₁₁ phase precipitated along the grain boundary of the matrix had a low potential, causing the grain boundaries to be preferentially corroded during corrosion [34]. The addition of Mg and pulsed magnetic field treatment both promoted the precipitation of the Mg₂Zn₁₁ phase along grain boundaries (Figure 2f and Figure 3f) and increased the number of grain boundaries (Figure 4g) by refining grain size, thus escalating galvanic corrosion and grain boundary corrosion, accelerating the corrosion rate of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys.

Although the addition of Mg and pulsed magnetic field treatment accelerated the corrosion rate of Zn-3Cu alloy, these rates remained between 0.02 and 0.11 mm/year [8,9], and were lower than that of Mg alloys (0.42–4.83 mm/year) [35], retaining the characteristic of the moderate degradation rate of Zn alloys.

Figure 7 shows that as the Mg content increased, both high-frequency capacitive arc radius (related to the charge transfer) and low-frequency capacitive arc radius (related to the diffusion process) [36] decreased and were even smaller after pulsed magnetic field treatment. Smaller capacitive arc radii indicate poorer corrosion resistance. The R_s(CPE₁R₁)(CPE₂R₂) [36] model was used to analyze the EIS spectra, and the results are shown in

Table 1. Simulated EIS data for Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys.

Sample	Rs /Ω·cm2	CPE1 /μF·cm−2	n1	R1 /Ω·cm2	CPE2 /μF·cm−2	n2	R2 /Ω·cm2	χ2
W-Zn3Cu	11.54 ± 1.25	5.02 ± 0.36	0.87 ± 0.03	1219.00 ± 154.15	102.63 ± 12.02	0.73 ± 0.03	1571.50 ± 405.17	3.09 × 10−3
W-Zn3Cu0.5Mg	11.75 ± 1.21	5.25 ± 1.70	0.86 ± 0.03	1070.47 ± 495.62	228.54 ± 102.80	0.72 ± 0.05	1177.77 ± 539.01	1.37 × 10−3
W-Zn3Cu1Mg	11.64 ± 1.21	5.04 ± 1.50	0.83 ± 0.02	765.05 ± 332.27	210.71 ± 60.39	0.68 ± 0.02	976.00 ± 91.92	2.56 × 10−3
P-Zn3Cu	12.91 ± 1.09	122.82 ± 38.31	0.75 ± 0.03	1524.70 ± 496.79	4.73 ± 0.79	0.86 ± 0.01	1320.70 ± 447.63	1.97 × 10−3
P-Zn3Cu0.5Mg	11.81 ± 0.42	4.93 ± 0.75	0.79 ± 0.04	1326.63 ± 939.22	220.10 ± 45.61	0.72 ± 0.10	1132.57 ± 264.25	1.97 × 10−3
P-Zn3Cu1Mg	10.40 ± 2.64	179.28 ± 58.01	0.69 ± 0.13	1029.63 ± 610.66	6.13 ± 0.24	0.79 ± 0.14	883.73 ± 127.43	7.68 × 10−3

, where R_s represents the solution resistance, R₁ and CPE₁ represent the resistance and capacitance of the charge transfer layer, and R₂ and CPE₂ represent the resistance and capacitance of the corrosion product layer. It could be seen that both R₁ and R₂ values decreased in the order of Zn-3Cu > Zn-3Cu-0.5Mg > Zn-3Cu-1Mg, and the pulsed magnetic field treatment also decreased R₁ and R₂ values, indicating that adding Mg and pulsed magnetic field treatment made the charge transfer easier and the protective performance of corrosion product weaker, accelerating the corrosion rate of Zn-3Cu-xMg alloys.

3.4. Cytocompatibility and Antibacterial Activity

The deficiency in mechanical properties is one of the primary issues faced by Zn alloys at present. Considering the enhancement of strength and the acceleration of corrosion rate in Zn-3Cu-xMg alloys after pulsed magnetic field treatment, their responses to HUVECs and S. aureus were further investigated. Figure 8a,b show that Zn-3Cu-xMg alloys promoted cell proliferation and had no cytotoxicity compared with the control group. As the Mg content increased, the level of cell proliferation increased. Differences in cell viability can be attributed to the ion concentration of the extract, which is closely related to the alloy degradation process. The corrosion rate of Zn-3Cu-xMg alloy accelerated as the Mg content increased, releasing more Zn²⁺ and Mg²⁺ ions; thus, biocompatibility was improved. Li et al. [37] reported the in vivo corrosion rates of the Zn-1X (X = Mg, Ca, Sr) alloys as 0.17 mm /y, 0.19 mm/y, and 0.22 mm/y for Zn-1Mg, Zn-1Ca, and Zn-1Sr, respectively. And the radiographs of mice with Zn-1X alloy implants at 0, 1, 2, 3, 4, 6, and 8 weeks showed no inflammation and gas shadows around the implantation site, and all alloys exhibited no cytotoxicity in vivo. The corrosion rates of Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys were 0.06–0.12 mm/y (Figure 6c), lower than those of Zn-1X alloys. It could be concluded that the Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys should not cause cytotoxicity in vivo over extended periods. Figure 8c shows that the antibacterial rates of Zn-3Cu-xMg alloys were all up to 99.9%, owing to the combination of positively charged Zn²⁺, Cu²⁺, and Mg²⁺ ions with the negatively charged thiol, carboxyl, and amino groups on the bacterial membrane, leading to complete enzyme inactivation, protein destruction, and subsequent bacterial death [38].

4. Conclusions

This study investigated the effects of Mg addition and pulsed magnetic field treatment on the microstructure and properties of as-cast biodegradable Zn-3Cu-xMg (x = 0, 0.5, 1 wt.%) alloys. Mg addition induced the precipitation of fine Mg₂Zn₁₁ along matrix grain boundaries. With the increase in Mg content, the precipitation of the Mg₂Zn₁₁ phase increased, and the grain size became finer. Pulsed magnetic field treatment exacerbated the occurrence of this phenomenon. Under the combined action of Mg₂Zn₁₁ phase and refined grain size, Zn3Cu0.5Mg alloy with pulsed magnetic field treatment had the best strength–ductility match (σ_UTS = 181.46 ± 1.06 MPa, δ = 3.95 ± 0.07%), moderate corrosion rate (i_corr = 5.69 ± 3.96 μA/cm²), cytocompatibility and antibacterial property, which makes it a promising as-cast Zn alloy with improved properties for subsequent conventional metal-forming processing (extrusion, drawing, rolling, or forging) and severe plastic deformation techniques.