Science Press
In the past few years, magnesium and its alloys have attracted extensive attention as biodegradable materials due to their excellent biocompatibilityle, elastic modulus close to human bone and degradability in physiological environment. Magnesium can avoid the stress shielding effect and promote the healing of bone for bone implants[1-3]. It can also reduce the risk of restenosis by self-degradation after a period of positive remodeling with stent support and drug treatment in the stenosis vessels for cardiovascular stent application[4,5]. It should be noted that the metal ions released after the degradation of magnesium alloy can cause biological reactions in surrounding tissues. Inappropriate elements may produce cytotoxicity and even cause the death of cells in nearby tissues[6,7]. At present, the research of industrial magnesium alloy system is mainly focused on Mg-RE and Mg-Al magnesium alloys, but studies have found that Al is cytotoxicity and associated with Alzheimer's disease. The toxi-city of many rare earth elements is unclear[8,9]. Nutrient element alloying is one of the latest and most effective methods to improve mechanical properties of biological magnesium alloy from the perspective of biocompatibility. It is better to choose the elements that exist in the human body, especially the essential elements in the human body[10,11]. Based on this design principle, a series of binary alloys have been developed, including Mg-Ca[12], Mg-Zn[13,14], Mg-Sr[15,16], Mg-Mn[17], Mg-Sn[18,19], Mg-Li[20], etc.
Among the as-studied alloy systems, Mg-Sr alloys have been considered as a potential biodegradable materials used in orthopedic surgery mainly due to the effect of Sr on stimulating bone formation and decrease of bone resorption. Strontium is a trace metal found in the human body and 99% of it is found in bones. An adult male has 140 mg of strontium and the mean daily intake is 2 mg. Because strontium has a stimulating effect on bone formation, oral strontium salt can be used to treat osteoporotic patients with increasing bone mass and reducing the incidence of fracture[21,22]. Zhao et al[23] conducted immersion and electro-chemical tests and found that the extruded Mg-0.5Sr alloy has good corrosion resistance and no cytotoxicity to cells through cytotoxicity tests. Gu et al[21] researched that the rolled Mg-2Sr alloy has grade I cytotoxicity and increased ALP activity through in vitro cell experiments. The results of animal experiments showed that the rolled Mg-2Sr alloy can promote bone mineralization and new bone formation around the implant without causing any obvious adverse reactions. Bornapour et al[24] found that the implantation of Mg-0.5Sr alloy in animal blood vessels does not cause thrombosis within three weeks. Moreover, Sr-substituted HA layer forms on the surface of Mg-0.5Sr alloy during SBF biological corrosion and it improves the corrosion resistance of magnesium alloy under physiological conditions and can promote the growth, proliferation and healing of cells around the bone implant. It is expected that Mg-Sr has good biocompatibility and can be used as a bone implant material.
The Mg-Sr binary alloys perform favorable biological compatibility, but their mechanical properties and corrosion resistance are difficult to meet the design requirements. The previous research has shown that adding a small amount of Zn has a significant effect on refining the microstructure of Mg alloy, which is beneficial to improve the mechanical properties and corrosion resistance of Mg alloy. Moreover, the implants of magnesium alloy with Zn element do not produce harmful corrosion products during the degradation. As an essential trace element, Zn is involved in the development and growth of human somatic cells, gene expression, immune system, nervous system and other physiological processes[14,25]. Zhang[26] studied the effect of solid solution precipitation strengthening of Zn element on Mg-6Zn alloy, which significantly refined the grain and improved the yield strength and ultimate tensile strength of the alloy. Song et al[27] researched that the mechanical properties of Mg-0.5wt% Ca alloy can be improved obviously by adding a small amount of Zn element. Brar et al[28] studied the effect of different Zn content on the mechanical properties and corrosion resistance of Mg-0.5Sr and found that the addition of Zn can improve the mechanical properties and ultimate strength of the alloy. Cai et al[14] found that in binary Mg-Zn alloy, the strength of the alloy and the corrosion resistance increase with the increase in Zn content. Chen et al[29] researched that adding 1wt% Zn into Mg-2Sr alloy can significantly improve corrosion resistance and biocompatibility of the alloy.
At present, it is rarely reported that the mechanical properties and corrosion resistance of Mg-2Sr can be adjusted by adding different contents of other elements. In this work, the properties of Mg-2Sr alloys with different contents of Zn were investigated.
The cast Mg-2Sr-xZn ingots used in this study were prepared by vacuum melting method. The pure Mg (99.99%), Mg-20%Sr master alloy and pure Zn (99.99%) were melted in a high purity graphite crucible under a vacuum condition. After heating at 700 °C for 30 min, the melt was poured into a steel mold preheated to 280 °C. The actual chemical compositions of three cast alloys were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-MS, Optima 2000DV), as shown in Table 1.
Table 1
Actual chemical compositions of as-cast Mg-2Sr-xZn magnesium alloys (wt%)
| Alloy | Mg-2Sr | Mg-2Sr-Zn | Mg-2Sr-2Zn | Mg-2Sr-4Zn |
|---|
|
Mg |
Bal. |
Bal. |
Bal. |
Bal. |
|
Sr |
1.89 |
1.88 |
1.91 |
1.87 |
|
Zn |
- |
0.93 |
1.88 |
3.86 |
|
Al |
0.012 |
0.01 |
0.014 |
0.011 |
|
Cu |
≤0.001 |
≤0.001 |
≤0.001 |
≤0.001 |
|
Fe |
0.0004 |
0.0003 |
0.0003 |
0.0004 |
|
Ni |
≤0.0001 |
≤0.0001 |
≤0.0001 |
≤0.0001 |
The sample with a dimension of 8 mm was used for microstructure observation and composition analysis, which was first cut from the ingot by wire cutting, then coarsely ground with SiC grinding to a grit of 2000#, and then fine-ground with SiC grinding to a grit of 4000#, and finally polished with SiC polishing fluid. Then the sample was etched with a mixed solution of picric acid (0.5 g), acetic acid (5 mL), ethanol (25 mL) and water (10 mL) to obtain a clear grain boundary. The microstructure was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Grain size was measured using the linear intercept method described in ASTM standard E112-12.
The tensile test at room temperature was conducted on the universal experimental machine with INSTRON5982, and the tensile direction was consistent with the casting direction. The standard for the test was GB/T228.1-2010 and three samples were tested for each state and the mean value was used to evaluate the tensile mechanism. The fracture morphology of samples was observed using a scanning electron microscope (SEM, JMS-6460).
The electrochemical performance tests of Mg-2Sr-xZn samples were carried out in simulated body fluid (SBF) with a pH of 7.4 by the ZENNIUMTM6 electrochemical analyzer. The SBF was composed of 8.035 g/L NaCl, 0.355 g/L NaHCO3, 0.225 g/L KCl, 0.231 g/L K2HPO4·3H2O, 0.311 g/L MgCl2·6H2O, 39 mL/L (1 mol/L) HCl, 0.292 g/L CaCl2, 0.072 g/L Na2SO4, and 6.118 g/L (CH2OH)3CNH2. The electro-chemical tests adopted three electrode equipment, with the control or treated samples as a working electrode, the saturated calomel electrode as a reference electrode and the platinum plate electrode as a counter electrode. The area of the samples exposed to SBF was about 1 cm2. Both the potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) tests were conducted at a stable open-circuit potential. The scan rate of the potentiodynamic polarization test was 2 mV/s. EIS was carried out at a sinusoidal potential of 5 mV in the frequency range from 10-2 Hz to 105 Hz. The tests results and corrosion mechanism were fitted by the software Zsimpwin 3.60. Each test was conducted three times. After the tests, the samples were dried at room temperature.
The immersion tests were also carried out in SBF solution at 37 °C according to ASTM-G31-72 standard. The ratio of sample area to SBF solution was 1 cm2:20 mL. After immersion for 10 d, the samples were removed from SBF solution, gently rinsed with distilled water and dried in air. The corrosion surface morphology and chemical composition of corrosion products were measured by scanning electron microscope (SEM, JMS-6460).
2.1 Microstructure characterization
Fig.1 shows the SEM images of Mg-2Sr-xZn magnesium alloy with different contents of Zn element. It can be seen that the grain homogenization of the alloy is obvious through the addition of Zn element. With the increase in Zn content, the grains of Mg-2Sr-xZn alloy are significantly refined. Compared with Mg-2Sr magnesium alloy, the Mg-2Sr-2Zn has the most obvious grain refinement effect among all the magnesium alloys, with an average grain size of 33 μm. The second phase at the grain boundary of Mg-2Sr magnesium alloy is coarse, but the second phase at the grain boundary is refined and evenly distributed by the addition of Zn element. However, with the further addition of Zn element, the grains of Mg-2Sr-4Zn grow up and the proportion of the second phase at grain boundary increases.
Fig.1 SEM images of as-cast Mg-2Sr-xZn magnesium alloy: (a) Mg-2Sr, (b) Mg-2Sr-1Zn, (c) Mg-2Sr-2Zn, and (d) Mg-2Sr-4Zn
Fig.2 shows the XRD patterns of Mg-2Sr-xZn magnesium alloys with different Zn contents. Most of the diffraction peaks with relatively strong intensities can be identified as the complete spectrum of α-Mg phase. The data show that all alloys contain α-Mg and Mg17Sr2 phases, but the MgZn phase only appears in Mg-2Sr-4Zn magnesium alloy. Compared with the case in Mg-2Sr-2Zn magnesium alloy, the formation of MgZn phase occurs at the grain boundary in Mg-2Sr-4Zn .
Fig.2 XRD patterns of as-cast Mg-2Sr-xZn alloys
Fig.3 shows the tensile test results of Mg-2Sr-xZn magne-sium alloy with different Zn contents at room temperature. With the increase in Zn content, the strength of Mg-2Sr-xZn magnesium alloy continuously increases, and the elongation shows a similar trend, but there is an obvious deterioration when the Zn content increases to 4wt%. This indicates that the mechanical properties of the alloys can be significantly affected by the addition of a small amount of Zn. The elongation of Mg-2Sr-2Zn alloy reaches the peak value, so that the comprehensive properties are improved most significantly. The YS, UTS and elongation increase from 119 MPa, 130 MPa and 3.2% to 181 MPa, 212 MPa and 8.1%, respectively. However, with the further addition of Zn element, the strength of Mg-2Sr-4Zn magnesium alloy reaches the maximum, but the ductility is worse than that of Mg-2Sr-2Zn magnesium alloy. Therefore, the experimental specimen with the addition of 2wt% Zn presents the best strength and ductility performance compared with other specimens.
Fig.3 Mechanical properties of as-cast Mg-2Sr-xZn alloys
Cleavage fracture, quasi-cleavage fracture and intergranular fracture are the main fracture modes of magnesium alloys. Fig.4 shows the SEM fracture morphologies of Mg-2Sr-xZn magnesium alloy tensile sample at room temperature. It can be seen that the fracture mode of all the alloys is mainly intergranular fracture, which is a typical brittle fracture. However, with the addition of Zn element, the grain size of the alloy is significantly reduced, and the crack requires more energy in the process of propagation, which is favorable to higher strength and plasticity. The fracture morphology of Mg-2Sr-Zn and Mg-2Sr-2Zn magnesium alloys shows a small number of cleavage planes, indicating that an appropriate amount of Zn is beneficial to improve the plasticity of the alloys. However, excessive Zn elements leads to deterioration of the plasticity of the alloy, and the fracture mode of Mg-2Sr-4Zn alloy is completely intergranular fracture. A large amount of the second phases are distributed on the fracture surface of all the samples, especially the Mg-2Sr-4Zn magnesium alloy.
Fig.4 SEM fracture morphologies of tensile specimen at room temperature of as-cast Mg-2Sr-xZn alloys: (a) Mg-2Sr, (b) Mg-2Sr-1Zn, (c) Mg-2Sr-2Zn, and (d) Mg-2Sr-4Zn
2.3 Experimental tests in SBF
The early degradation behavior of Mg-2Sr-xZn magnesium alloy was studied using a soaking experiment in SBF. The pH value of the solution was measured during immersion to reflect the early degradation of the alloy. Fig.5 shows the pH change curve of SBF solution during 144 h immersion. During the early soaking period of 0~36 h, the soaking pH value of different samples increases sharply from 7.4 to 8.3~8.88. Then the pH gradually increases during the subsequent immersion process, mainly due to the accumulation of early corrosion products, resulting in the formation of oxide film on the alloy surface. At the later soaking period between 96 h to 144 h, the pH value of the soaking sample increases slowly and tends to be stable. After soaking in SBF for 144 h, the minimum pH value of Mg-2Zn-2Sr alloy is 9.15, showing the best corrosion resistance.
Fig.5 pH change curve of SBF solution during 144 h immersion of as-cast Mg-2Sr-xZn magnesium alloys
Fig.6 shows the dynamic potential polarization curve of Mg-2Sr-xZn magnesium alloy with different Zn contents in Hank's solution. Table 2 lists the corrosion potential and corrosion current density of the dynamic potential polarization curve of the as-cast Mg-2Sr-xZn alloy as well as the related corrosion rate. By adding an appropriate amount of zinc, the corrosion potential (Ecorr) of Mg-2Sr-xZn magnesium alloy increases obviously, but excessive zinc leads to the decrease in corrosion potential. The corrosion current density (icorr) and related corrosion rate (Ri) of alloys are in the order of Mg-2Sr-2Zn<Mg-2Sr-Zn<Mg-2Sr-4Zn<Mg-2Sr.
Fig.6 Dynamic potential polarization curves of as-cast Mg-2Sr-xZn magnesium alloys
Table 2
Values measured from mass loss and the polarization curves for as-cast Mg-2Sr-xZn magnesium alloys in SBF
| Alloy | Ecorr/V | icorr/μA·cm-2 | Ri/mm·a-1 |
|---|
|
Mg-2Sr |
-1.612 |
242±10.3 |
5.53±0.05 |
|
Mg-2Sr-Zn |
-1.561 |
113±6.2 |
2.55±0.03 |
|
Mg-2Sr-2Zn |
-1.533 |
81±5.6 |
1.83±0.02 |
|
Mg-2Sr-4Zn |
-1.582 |
200±6.5 |
4.58±0.03 |
Fig.7a shows the electrochemical impedance spectroscopy (EIS) spectra of Mg-2Sr-xZn magnesium alloys measured at open circuit potential. The EIS spectra of the four magnesium alloys are all composed of two rings: the capacitance ring in the high frequency region (HF) and the capacitance ring in the middle frequency region (MF). According to the same shape and different radii of impedance spectrum, it can be analyzed that the four magnesium alloys have the same corrosion mechanism and different corrosion rates. The Mg-2Sr-2Zn magnesium alloy has the largest impedance radius, indicating the best corrosion resistance.
Fig.7 Nyquist plots for as-cast Mg-2Sr-xZn magnesium alloys after soaking in SBF for 1 h (a) and corresponding equivalent circuit (b)
Fig.8 shows the mass loss rate of Mg-2Sr-xZn magnesium alloy with different Zn contents soaked in Hank's solution for 240 h. It can be seen that the addition of Zn alloy contributes to the improvement of the corrosion resistance of Mg-2Sr-xZn, but when the addition content of Zn reaches a certain level, excessive Zn will lead to the decrease in the corrosion resistance. The corrosion rate of Mg-2Sr-xZn alloy calculated by mass loss method decreases from 6.63 mm/a to 2.21 mm/a with the increase in Zn content from 0wt% to 2wt%, and increases to 4.88 mm/a with the increase of Zn content to 4wt%. These results are consistent with pH tests, indicating that Mg-2Sr-2Zn magnesium alloy has the best corrosion resistance in all soaking samples.
Fig.8 Mass loss rate of as-cast Mg-2Sr-xZn alloy soaked in Hank's solution for 240 h
Fig.9 shows the surface morphologies of Mg-2Sr-xZn magnesium alloys observed by SEM after soaking in SBF for 240 h. It can be seen that the surface of all samples is covered with corrosion product film after immersion. The oxide film on the surface of Mg-2Sr magnesium alloy is loose that is more likely to cause more serious local corrosion in the subsequent immersion process. With the addition of Zn element, the oxide film on the surface of Mg-2Sr-xZn alloys becomes denser and shows better corrosion resistance. The oxide film generated on the surface of alloy is relatively dense, which can better impede the possibility of subsequent serious corrosion and ensure the integrity of the sample. However, with the increase in Zn element to 4wt%, more cracks occur on the surface of Mg-2Sr-4Zn, resulting in poor density.
Fig.9 SEM surface morphologies of as-cast Mg-2Sr-xZn magnesium alloy after soaking in SBF for 240 h: (a) Mg-2Sr, (b) Mg-2Sr-1Zn, (c) Mg-2Sr-2Zn, and (d) Mg-2Sr-4Zn
The microstructure, mechanical properties and corrosion behavior of Mg-2Sr alloys added with different Zn contents (0wt%~4wt%) were investigated. As can be seen from Fig.1, the addition of an appropriate amount of Zn element is beneficial to grain refinement and distribution homogeni-zation of the second phase. Due to the low solid solubility of Sr solute in α-Mg matrix (about 0.11wt%), Sr mainly exists in the form of Mg17Sr2 in the second phase[30,31]. The solid solubility of Zn solute in α-Mg matrix is relatively high (about 1.6wt%), and the amount of Zn element mainly exists in the form of solid solution, while the excess Zn element mainly exists in the form of the second phase of MgZn[14,32]. It can be determined from the element distribution diagram in Fig.10 that most element Sr is distributed at grain boundaries in the Mg-2Sr-2Zn magnesium alloy, while element Zn is partially distributed in grains. Although the volume fraction of secondary phase is generally reduced by a lower percentage of alloying elements, the Zn/Sr atomic ratio can play a critical role in mechanical and corrosion properties of the Mg-Zn-Sr alloys. Li et al[33] also suggested that the formation of the MgxZnySrz is strongly related to the Zn content in the Mg-Zn-Sr alloys. The reason why the addition of Zn leads to the decrease in grain size is that the segregation of Zn will lead to the formation of a dense component undercooling zone in the diffusion layer at the front of the solid-liquid interface. When the undercooling degree is large enough, the balance of the interface will be destroyed and dendrites will be generated. With the increase in Zn content, the potential nucleation particles in the supercooled region of the component are activated, which promotes the nucleation of α-Mg matrix, and eventually leads to grain size refinement[34].
Fig.10 SEM image of Mg-2Sr-2Zn magnesium alloy (a) and corresponding EDS mappings of Mg (b), Sr (c), and Zn (d)
The addition of Zn is beneficial to grain refinement, which can improve the uniformity of deformation and reduce the stress concentration. According to the Hall-Petch formula, grain refinement provides more grain boundaries, leading to an increase in strength[35]. A proper amount of Zn atoms dissolved into Mg matrix also produces solid solution strengthening effect. The increase in strength can be attributed to grain refining and precipitation strengthening effect of Zn element. Refining grain improves deformation compatibility and plasticity. In addition, the casting defects, relative density, strength and plasticity of Mg-Zn alloy are improved by grain refinement[36]. When Zn is added further, the precipitation of MgZn greatly increases the possibility of crack source and leads to the decrease in alloy plasticity[37].
It has been reported that the addition of Zn to magnesium can improve the corrosion potential of Mg, which is consistent with the results in this study[38]. As a cathode, the secondary phase is easy to form a micro couple with Mg matrix, which accelerates the corrosion. The synergistic effect of grain refinement and secondary phase formation results in the change of corrosion resistance of the alloys with different Zn contents[10,37]. With the increase in Zn content to more than 4wt%, the number of MgZn intermetallic phases in the matrix increases rapidly, forming a continuous network structure and more anode-cathode sites. Therefore, more galvanic corrosion may occur at these locations, resulting in rapid corrosion[21].
In this study, the impedance spectra of all magnesium alloys are composed of two rings: capacitance ring in the high frequency region and capacitance ring in the middle frequency region, as shown in Fig.7a. Generally speaking, the high frequency capacitor circuit is attributed to the relaxation process of electrochemical reaction impedance corresponding to the dissolution of Mg and electric double layer capacitance (Cdl) between the magnesium surface dissolution and the interface of the corrosive medium, while the medium frequency capacitor circuit is related to the formation of oxide film during the corrosion process[14,39]. The Nyquist plots of Mg-2Sr-xZn alloys can be interpreted using the equivalent circuit shown in Fig.7b. According to the shape of impedance spectrum, the electrode reaction process related to the selection of high frequency capacitor circuit can be described by a parallel circuit consisting of capacitor CPE1 and resistor Rt. The intermediate frequency capacitor circuit is described by the parallel circuit composed of capacitor CPE2 and resistor Rf. Rt is inversely proportional to the exchange current density, that is, the larger the Rt, the slower the corrosion rate. According to Nyquist figure, Rt from low to high is in the order of Mg-2Sr (385 cm2), Mg-2Sr-4Zn (615 cm2), Mg-2Sr-1Zn (986 cm2) and Mg-2Sr-2Zn (1352 cm2).
When the immersion corrosion of Mg-2Sr-xZn alloy occurs in SBF, the microelectrical corrosion occurs between the Mg matrix and the second phase. The anodic reaction converts Mg to Mg2+ ions and the cathodic reaction converts H2O to OH-, resulting in Mg(OH)2 accumulated on the surface of the sample and gradually forming a protective corrosion film[40,41]. The following reactions give the corrosion reactions of magnesium in Cl--containing solution[10]:
Mg(s)+2H2O→Mg(OH)2+ H2↑
Mg(s)+2Cl-(aq)→MgCl2
Mg(OH)2+2Cl-→MgCl2
The addition of different contents of Zn will lead to different degradation rates of magnesium alloys. Magnesium alloys with better corrosion resistance have slower corrosion at the initial stage of degradation, and less H2 is precipitated on the surface, which is more conducive to the formation of Mg(OH)2[42,43]. The thick and complete corrosion product layer reduces the reaction rate of magnesium alloy in SBF solution, thus improving its corrosion resistance[44]. Zn(OH) 2 is produced during the addition of Zn element, which can improve the compactness of the corrosion products layers[45,46]. Therefore, different alloys have different pH values and H2 formation amounts. Although corrosion layer has a certain protective effect on alloy corrosion, chloridion ions in SBF will continue to react with the matrix through the corrosion product layer, and the pH value of the solution will continue to increase[16].
The corrosion rate of magnesium alloys is closely related to the microstructure, and the grain size and grain boundary precipitates are important factors affecting the corrosion rate[28,47]. Grain refinement contributes to the formation of passivation layer on the surface of magnesium alloy to improve the corrosion resistance of magnesium alloy, but on the other hand, excessive second phase strengthens the galvanic corrosion between magnesium matrix and the second phase, thus reducing the corrosion resistance of the alloy[47]. Cai et al[14] believed that the refinement of the second phase of magnesium alloy will also improve its corrosion resistance and reduce the galvanic corrosion between the matrix and the second phase. However, the second phase MgZn is formed at the grain boundary by adding excess Zn element, resulting in increased microgalvanic corrosion to α-Mg matrix, thus redu-cing the corrosion resistance of Mg-2Sr-xZn magnesium alloy and accelerating its corrosion rate.
1) The microstructure, mechanical properties and corrosion resistance of Mg-2Sr-xZn (x=0, 1, 2, 4, wt%) alloys are studied, and the content of Zn in the alloy leads to different outcomes.
2) The Mg-2Sr-2Zn magnesium alloy performs excellent property. An appropriate amount of Zn (2wt%) helps to refine the grain and plays a role in solid solution enhancement without producing additional second phase. This method is beneficial to improve the plasticity and corrosion resistance of the Mg-2Sr-xZn magnesium alloys.
3) Excessive Zn element will lead to the formation of MgZn phase, which deteriorates the plasticity of the alloy and intensifies the microgalvanic corrosion between the matrix and the second phase, resulting in the accelerated corrosion rate of the Mg-2Sr-xZn magnesium alloys.
References
1 Liu Y, Zheng Y, Chen X H et al. Advanced Functional Mate- rials[J], 2019, 29(18): 1 805 402 [Baidu Scholar]
2 Liu J, Bian D, Zheng Y et al. Acta Biomaterialia[J], 2020, 102: 508 [Baidu Scholar]
3 Kraus T, Fischerauer S F, Hänzi A C et al. Acta Biomaterialia[J], 2012, 8(3): 1230 [Baidu Scholar]
4 Mei D, Lamaka S V, Lu X et al. Corrosion Science[J], 2020, 171: 108 722 [Baidu Scholar]
5 Gonzalez J, Hou R Q, Nidadavolu E P S et al. Bioactive Materials[J], 2018, 3(2): 174 [Baidu Scholar]
6 Sunil B R, Kumar T S S, Chakkingal U et al. Materials Science and Engineering C[J], 2016, 59: 356 [Baidu Scholar]
8 Xiong P, Jia Z, Li M et al. ACS Biomaterials Science & Engineering[J], 2018, 4(9): 3163 [Baidu Scholar]
9 Liu J, Zheng Y, Bi Y et al. Materials Letters[J], 2017, 205: 87 [Baidu Scholar]
10 Zhao C, Chen X, Pan F et al. Journal of Materials Science & Technology[J], 2019, 35(1): 142 [Baidu Scholar]
11 Dxab C, Fan Y, Yzb C et al. Bioactive Materials[J], 2021, 6(11):4186 [Baidu Scholar]
12 Li Z, Gu X, Lou S et al. Biomaterials[J], 2008, 29(10): 1329 [Baidu Scholar]
16 Zhao C, Pan F, Zhang L et al. Materials Science and Engineering C[J], 2017, 70: 1081 [Baidu Scholar]
17 Gu X, Zheng Y, Cheng Y et al. Biomaterials[J], 2009, 30(4): 484 [Baidu Scholar]
18 Dong C, Ren Y P, Yun G et al. Transactions of Nonferrous Metals Society of China[J], 2010, 20(7): 1321 [Baidu Scholar]
19 Zhao C, Pan F, Zhao S et al. Materials & Design[J], 2015, 70: 60 [Baidu Scholar]
21 Gu X N , Xie X H , Li N et al. Acta Biomaterialia[J], 2012, 8(6): 2360 [Baidu Scholar]
22 Wang Y, Tie D, Guan R et al. Journal of the Mechanical Behavior of Biomedical Materials[J], 2018, 77: 47 [Baidu Scholar]
23 Zhao C, Pan F, Zhang L et al. Materials Science and Engineering C[J], 2017, 70: 1081 [Baidu Scholar]
28 Brar H S, Wong J, Manuel M V. Journal of the Mechanical Behavior of Biomedical Materials[J], 2012, 7: 87 [Baidu Scholar]
30 Bornapour M, Celikin M, Cerruti M et al. Materials Science and Engineering C[J], 2014, 35(2): 267 [Baidu Scholar]
32 Kamoto H. Journal of Phase Equilibria[J], 1994, 15(1): 129 [Baidu Scholar]
33 Cipriano A F, Sallee A, Tayoba M et al. Acta Biomaterialia[J], 2017, 48: 499 [Baidu Scholar]
34 Romzi M A F, Alias J, Ramli M I M. Materials Today: Proceedings[J], 2022, 48: 1873 [Baidu Scholar]
35 Wen Y, Guan B, Xin Y et al. Scripta Materialia[J], 2022, 210: 114 451 [Baidu Scholar]
36 Yan H, Chen R S, Han E H. Materials Science and Engineering A[J], 2010, 527(15): 3317 [Baidu Scholar]
37 Zhang H J, Zhang D F, Ma C H et al. Materials Letters[J], 2013, 92: 45 [Baidu Scholar]
38 Meng X, Jiang Z, Zhu S et al. Journal of Alloys and Com- pounds[J], 2020, 838: 155 611 [Baidu Scholar]
39 Liu M, Uggowitzer P J, Nagasekhar A V et al. Corrosion Science[J], 2009, 51(3): 602 [Baidu Scholar]
40 Lai H, Li J, Li J et al. Journal of Materials Science: Materials in Medicine[J], 2018, 29(6): 1 [Baidu Scholar]
41 Li H, Peng Q, Li X et al. Materials & Design[J], 2014, 58: 43 [Baidu Scholar]
42 Wu Y, He G, Zhang Y et al. Scientific Reports[J], 2016, 6(1): 1 [Baidu Scholar]
43 Atrens A, Johnston S, Shi Z et al. Scripta Materialia[J], 2018, 154: 92 [Baidu Scholar]
44 Zander D, Zumdick N A. Corrosion Science[J], 2015, 93: 222 [Baidu Scholar]
45 Chen M X , Chen J H, Yan H G et al. Journal of Alloys and Compounds[J], 2017, 691: 95 [Baidu Scholar]
46 Gu X, Zheng Y, Zhong S et al. Biomaterials[J], 2010, 31(6): 1093 [Baidu Scholar]
47 Hamu G B, Eliezer D, Wagner L. Journal of Alloys and Compounds[J], 2009, 468(1-2): 222 [Baidu Scholar]
