I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. EXPERIMENTIII. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO...REFERENCESMg and its alloys have been extensively investigated in recent years as potential candidates for medical applications such as orthopedic and cardiovascular stents.1,21. M. P. Staiger, A. M. Pietak, J. Huadmai, and G. Dias, Biomaterials 27, 1728 (2006). https://doi.org/10.1016/j.biomaterials.2005.10.0032. B. O’Brien and W. Carroll, Acta Biomater. 5, 945 (2009). https://doi.org/10.1016/j.actbio.2008.11.012 Two different characteristics of Mg have become prominent in biomaterials science. The first one is their natural biocompatibility, biodegradability, and excellent mechanical properties very close to natural bone.3,43. F. Witte, Acta Biomater. 23, S28 (2015). https://doi.org/10.1016/j.actbio.2015.07.0174. Y. Zhang et al., Nat. Med. 22, 1160 (2016). https://doi.org/10.1038/nm.4162 Second, Mg alloys are able to degrade in vivo due to the presence of Cl− in the physiological environment, thus a secondary surgical operation is not required to remove the implant.55. C. Liu, Z. Ren, Y. Xu, S. Pang, X. Zhao, and Y. Zhao, Scanning 2018, 9216314 (2018). https://doi.org/10.1155/2018/9216314 In addition, pure Mg stimulates the growth of bone cells and accelerates the healing of bone tissue.66. C. Shuai, S. Li, S. Peng, P. Feng, Y. Lai, and C. Gao, Mater. Chem. Front. 3, 544 (2019). https://doi.org/10.1039/C8QM00507A However, mechanical integrity loss occurs in Mg-based alloys due to the high chloride environments of physiological systems and the high degradation rates that take place at pH levels between 7.4 and 7.6 (Ref. 77. M. B. Kannan and R. K. S. Raman, Biomaterials 29, 2306 (2008). https://doi.org/10.1016/j.biomaterials.2008.02.003). In the meantime, the rapid release of hydrogen gas bubbles can produce adverse effects, usually within the first week after surgery.88. G. E. J. Poinern, S. Brundavanam, and D. Fawcett, Am. J. Biomed. Eng. 2, 218 (2012). https://doi.org/10.5923/j.ajbe.20120206.02 Therefore, these rapidly produced bubbles of hydrogen gas can cause severe subcutaneous blisters, thereby delaying the healing of the implant site and even causing necrosis (tissue death) of tissues.99. G. Song and S. Song, Adv. Eng. Mater. 9, 298 (2007). https://doi.org/10.1002/adem.200600252 For this reason, it is quite important to control the decomposition rate of Mg alloys and thus the corrosion rate. The local corrosion rate is still one of the major obstacles in the field of medical application.1010. R. Zeng, W. Dietzel, F. Witte, N. Hort, and C. Blawert, Adv. Eng. Mater. 10, B3 (2008). https://doi.org/10.1002/adem.200800035 This poor corrosion resistance of Mg alloys in the biological environment can be increased by the addition of Zn. As a result of recent studies, it has been determined that Mg-Zn based alloys exhibit promising corrosion resistance.1111. Y. Chen, Z. Xu, C. Smith, and J. Sankar, Acta Biomater. 10, 4561 (2014). https://doi.org/10.1016/j.actbio.2014.07.005 Nevertheless, Nd as an alloying element, which has the same potential as Mg, is close to intermetallic phases or has the same electrochemical potential and can increase corrosion resistance by reducing the galvanic corrosion rate.1111. Y. Chen, Z. Xu, C. Smith, and J. Sankar, Acta Biomater. 10, 4561 (2014). https://doi.org/10.1016/j.actbio.2014.07.005 In addition, Zn is an important trace element that increases osteoblast (immature cell type that is the precursor of bone cell) phosphatase (an enzyme that is produced intensively in the skeletal system and gastrointestinal tract) in bones.1212. M. Nagata and B. Lönnerdal, J. Nutr. Biochem. 22, 172 (2011). https://doi.org/10.1016/j.jnutbio.2010.01.003 Another alloying element, Ca, is an important component of human bone and can accelerate the development of bone.1313. H. Li, D. Liu, Y. Zhao, F. Jin, and M. Chen, J. Mater. Eng. Perform. 25, 3890 (2016). https://doi.org/10.1007/s11665-016-2207-0 Mg-Ca alloy is not toxic and can degrade gradually as in vivo within 90 days by accelerating the formation of new bone.1414. X. Gu, Y. Zheng, Y. Cheng, S. Zhong, and T. Xi, Biomaterials 30, 484 (2009). https://doi.org/10.1016/j.biomaterials.2008.10.021Another major obstacle is inflammation. Because rapid degradation causes not only insufficient mechanical support but also accumulation of local alkaline ions after implantation, which may cause local inflammation.15,1615. Y. Xin, T. Hu, and P. K. Chu, Acta Biomater. 7, 1452 (2011). https://doi.org/10.1016/j.actbio.2010.12.00416. N. T. Kirkland, J. Lespagnol, N. Birbilis, and M. P. Staiger, Corros. Sci. 52, 287 (2010). https://doi.org/10.1016/j.corsci.2009.09.033 Ag and its compounds have been used safely in medicine for a long time because they are known to provide antibacterial activity.1717. M. Bosetti, A. Massè, E. Tobin, and M. Cannas, Biomaterials 23, 887 (2002). https://doi.org/10.1016/S0142-9612(01)00198-3 These retained thrombocyte amounts were found to be reduced in Mg-Ag binary alloys containing 1 wt. % Ag compared to pure Mg.1414. X. Gu, Y. Zheng, Y. Cheng, S. Zhong, and T. Xi, Biomaterials 30, 484 (2009). https://doi.org/10.1016/j.biomaterials.2008.10.021 In addition, even the addition of a small amount of Ag may be sufficient to increase the mechanical properties of Mg alloys.18,1918. Q. Wang, J. Chen, Z. Zhao, and S. He, Mater. Sci. Eng. A 528, 323 (2010). https://doi.org/10.1016/j.msea.2010.09.00419. Ş Açıkgöz, H. Şevik, and S. C. Kurnaz, J. Alloys Compd. 509, 7368 (2011). https://doi.org/10.1016/j.jallcom.2011.04.112

In this study, considering the antibacterial properties of Ag element, Q, QZ, QE, and QX coded alloys were produced by adding 3% Ag and 0.5% Zn, Nd, and Ca alloying elements by weight into the Mg alloy. The influence of Ag as well as other alloying elements on the microstructure, corrosion, and wear resistance of the alloy was studied to develop a new biodegradable Mg alloy.

II. EXPERIMENT

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENT <<III. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO...REFERENCES

Compositions of the alloys prepared by 99.9 wt. % pure Mg, Ag, and Zn metals and Mg—20 wt. % Ca and Mg—30 wt. % Nd elements in the master alloy form, respectively, were provided by BDM Bilginoglu company, 4D Machine, and Technology, and Nanography Nano Technology Inc. companies.

An atmosphere controlled induction furnace was used for melting the alloys. After melting, the casting temperature of the alloys and the mold temperature were determined, respectively, as 750 and 250 °C with temperature-controlled heating plates. The casting process was carried out with the help of a permanent mold (CO2 + 1 vol. % SF6) under gas protection.

The chemical compositions of the alloys were determined by the XRF (x-ray fluorescence) method by using the Rigaku ZSX Primus II brand device, and the analysis results are given in Table I.

TABLE I. Compositions by weight in the investigated Mg-Ag alloys.

AlloysAlloy codeElements (wt. %)AgZnNdCaMgMg-3AgQ3.35———BalanceMg-3Ag-0.5ZnQZ3.060.57——BalanceMg-3Ag-.5NdQE3.17—0.55—BalanceMg-3Ag-0.5CaQX3.24——0.55Balance

Homogenization (430 °C, 12 h) annealing was applied in order to eliminate defects such as small amounts of segregation, non-homogeneous grain size, and distribution that may occur in the as-cast parts after casting. The extrusion process, on the other hand, was carried out at a temperature of 300 °C approximately, at constant extrusion speeds of 1 mm/s, an extrusion ratio of 16:1, and 8 mm diameter extrusion products were obtained.

Microstructure investigations were carried out by Nikon Epiphot 200 brand optical microscope (OM) and Carl Zeiss Ultra Plus brand scanning electron microscopy (SEM). Before the microstructure characterizations, the samples were etched with the picric acid solution.

The tensile tests of the as-cast and extruded samples were carried out by a Zwick/Roell Z600 universal testing machine with a test speed of 1.67 × 10−3 s−1. At least five samples were used per alloy for the testing process. The hardness values of the samples were determined by the Vickers hardness (Vickers-Qness60A) test, and a load of 0.3 kg was used in the experiments.

The corrosion tests of the as-cast and extruded samples were carried out at ∼37 °C, in Hank’s fluid, which has a value of pH 7.4 and consists of the composition: NaCl 8.0 g/l, KCl 0.4 g/l, CaCl2 0.14 g/l, NaHCO3 0.35 g/l, C6H6O6 (glucose) 1.0 g/l, MgCl2⋅6H2O 0.1 g/l, MgSO4⋅7H2O 0.06 g/l, KH2PO4 0.06 g/l, Na2HPO4⋅12H2O 0.06 g/l.

Electrochemical measurements were performed at ∼37 °C, in Hank’s fluid pH 7.4, with a computer-controlled DC105 corrosion analyzed Gamry model PC4/300 mA potentiostat/galvanostat. The tests were repeated five times. Polarization curves were constructed starting from −0.25 V (open circuit potential vs Eoc) to +0.25 V (Eoc vs) at a scan rate of 1 mV s−1.

Immersion tests were performed at ∼37 °C in Hank’s fluid, which has a value of pH 7.4. The immersion test was carried out at 24-, 48-, and 72-h intervals. Immersion tests were repeated three times. After each test, the corrosion products of the samples were cleaned by immersion in 182 g l−1 aqueous chromic acid solution in an ultrasonic cleaner for 5 min. It was then washed with de-ionized water and placed in an ultrasonic bath in ethyl alcohol for 3 min.

The mean corrosion rate CRm was determined by using the following equation: CRm=8.76×104×ΔgA×t×ρ,(1)where Δg is the weight change (the difference before and after immersion) in g, A is the surface area of the sample in cm2, t is the immersion time in h, and ρ is the density in g/cm3.

After the immersion corrosion test, SEM images and EDX analyzes were taken from the surfaces of all samples.

The wear tests of the as-cast and extruded samples were carried out in Hank’s fluid, under a constant load of 20 N, at a constant speed of 0.1 m/s, at 100 m intervals, and up to 400 m in a reciprocal (back-and-forth) sliding wear test device. Mass losses of the investigated samples at each interval (100 m) were evaluated by weight measurement with a precision balance of 0.1 mg. After the experiments, SEM images and EDX analyzes were taken from the wear surfaces of all samples.

IV. SUMMARY AND CONCLUSIONS

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENTIII. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO... <<REFERENCESIt has been observed that the microstructures of QZ, QE, and QX alloys are generally finer-grained in comparison with Q alloy. Gungor and Incesu reported the effect of Zn and Ca contents on grain refinement of Mg-Zn-Ca alloys.3939. A. Gungor and A. Incesu, J. Magnes. Alloys 9, 241 (2021). https://doi.org/10.1016/j.jma.2020.09.009 It was determined that the yield strength and hardness of all as-cast and extruded alloys (QZ, QE, and QX) increased significantly compared to the as-cast and extruded Q alloys. The highest tensile strength was observed in QE alloy after extrusion. A similar situation was reported by Dvorsky et al. in their study. It was reported that Nd and Zn added (WE and WZ) extruded alloys exhibited the highest mechanical properties depending on their grain size properties.4040. D. Dvorský, J. Kubásek, I. Voňavková, and D. Vojtěch, Mater. Sci. Technol. 35, 520 (2019). https://doi.org/10.1080/02670836.2019.1570680 In addition, Cai et al. reported that the mechanical properties of Mg-Zn alloys improved with increasing Zn content (up to 5% wt.) but had an adverse effect when it was 7% (Ref. 4141. S. Cai, T. Lei, N. Li, and F. Feng, Mater. Sci. Eng. C 32, 2570 (2012). https://doi.org/10.1016/j.msec.2012.07.042). According to the results obtained from the potentiodynamic polarization tests, while the corrosion current densities increased in all extruded alloys, the corrosion potentials decreased. As a result of in vitro tests, it has been observed that the extruded alloys (QZ, QE, and QX) are highly corroded and rapidly degraded compared to their as-cast states. With the help of SEM analysis, it was seen that the as-cast surfaces were rougher, whereas the extruded surfaces were smoother. However, in Mg-Zn alloys, the corrosion resistance improved with increasing Zn content in the range of 1–5 wt. % was reported by Cai et al.4141. S. Cai, T. Lei, N. Li, and F. Feng, Mater. Sci. Eng. C 32, 2570 (2012). https://doi.org/10.1016/j.msec.2012.07.042 This situation is clearly observed in the as-cast QZ alloy. Zhang et al. reported that increasing Nd content (from 0% to 5%) and a number of days (up to seven days) weakened corrosion resistance in Mg-Nd alloys.4242. Y. Zhang et al., Acta Biomater. 121, 695 (2021). https://doi.org/10.1016/j.actbio.2020.11.050 After the corrosive wear test, the as-cast Q alloy exhibited the highest mass loss, while the as-cast QX alloy exhibited the lowest. In the extruded alloys, on the other hand, QZ exhibited the highest mass loss, while QX exhibited the lowest again. In the meantime, the lowest wear rate was observed in the as-cast and extruded QX alloys. However, Li et al. reported that with increasing Zn content (from 3 to 4 wt. %), both in dry sliding conditions and in the SBF fluid, the wear resistance of Mg-Zn-Ca alloy improved.1313. H. Li, D. Liu, Y. Zhao, F. Jin, and M. Chen, J. Mater. Eng. Perform. 25, 3890 (2016). https://doi.org/10.1007/s11665-016-2207-0 In another study, Yagi et al. reported that the highest specific wear rate among the different alloying elements (Ag, Al, Ca, Li, Mn, Y, and Zn) added Mg at 0.3 wt. % occurred in Mg-Ca alloy.4343. T. Yagi, T. Hirayama, T. Matsuoka, and H. Somekawa, Metall. Mater. Trans. A 48, 1366 (2017). https://doi.org/10.1007/s11661-016-3906-8 But Moussa et al., in their study, reported that the hardness of the alloy increased up to 0.3 wt. % of Ca added Mg-Si alloy, resulting in a decrease in mass loss and the highest wear resistance.4444. M. E. Moussa, M. A. Waly, and A. M. El-Sheikh, J. Magnes. Alloys 2, 230 (2014). https://doi.org/10.1016/j.jma.2014.09.005 In the Nd-containing QE alloy, it was observed that Nd addition did not promote the hardness increase much compared to the QZ and QX alloys so the QE alloy showed higher values in wear rates and mass losses. In the Nd-containing QE alloy, it was observed that Nd addition did not promote the hardness increase much compared to the QZ and QX alloys so the QE alloy showed higher values in wear rates and mass losses. According to all these results, it has been concluded that Mg-Ag alloys are generally promising biodegradable materials. It has been also concluded that all alloys (QZ, QE, and QX), which were manufactured specially by the effect of Q alloy and additional elements (Zn, Ca, and Nd), are important biomaterials that exhibit superior properties after microstructure, mechanical, and in vitro tests and can be used widely in the future.