- Email: [email protected]
Degradation, stress corrosion cracking behavior and cytocompatibility of high strain rate rolled Mg-Zn-Sr alloys
Materials Letters 260 (2020) 126920
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Degradation, stress corrosion cracking behavior and cytocompatibility of high strain rate rolled Mg-Zn-Sr alloys Zonglin Yu, Jihua Chen ⇑, Hongge Yan ⇑, Weijun Xia, Bin Su, Xiaole Gong, Hui Guo School of Materials Science and Engineering, Hunan University, 410082, China
a r t i c l e
i n f o
Article history: Received 6 July 2019 Received in revised form 15 October 2019 Accepted 30 October 2019 Available online 31 October 2019 Keywords: Magnesium alloys Corrosion Microstructure Stress corrosion cracking Biomaterials
a b s t r a c t The major obstacle of Mg alloys to the clinical applications is their rapid degradation behavior. In the present study, high strain rate rolling (HSRR) and Sr micro-alloying are conducted to improve bio-corrosion & stress corrosion cracking (SCC) resistance, reduce SCC susceptibility and enhance cytocompatibility of Mg-4Zn alloy. The effects on bio-corrosion & SCC resistance improvement are more pronounced with the higher Sr content (0.2%), attributable to grain refinement, the enhanced microstructural homogeneity and the more compact corrosion film. Sr addition can promote the cell growth at the early stage and improve the cell adhesion ability. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Mg alloys are the attractive biodegradable materials, which can avoid the secondary removal after surgery and effectively reduce the cost and the infection risk of patients. Unfortunately, they always corrode too quickly in Cl- containing solutions, resulting in fast loss of mechanical integrity [1]. Moreover, Mg alloy implants suffer mechanical loading, making them sudden fracture before healing [2]. Therefore, the bio-corrosion and SCC behaviors should be investigated to ensure their safety. The Mg-Zn-Sr alloy is a promising biomaterial since Sr can accelerate the growth of osteoblasts [3]. Furthermore, Sr can refine grains and improve strength and corrosion resistance of Mg alloys [4]. On one side, grain refinement enhances SCC resistance when the grain size is in micron level [5]. On the other side, it causes hydrogen embrittlement and results in poor SCC properties [6]. However, the as-cast Mg-4Zn-0.6Zr-xSr alloys exhibit higher SCC susceptibility with the higher Sr content (0.4–1.6%) [7]. So far, the bio-corrosion and SCC behaviors of wrought Mg-Zn-Sr alloys have not been reported yet. HSRR can significantly modify the microstructure and reduce the degradation rate of Mg-4Zn alloy [8]. However, its SCC behavior and cytocompatibility remain unclear. It is reported that 0.2%Sr addition brings about the greatest bio-corrosion improvement in as-cast Mg-5Zn-xSr alloys (x 1.0 wt%) through grain refinement ⇑ Corresponding author. E-mail address: [email protected] (J. Chen). https://doi.org/10.1016/j.matlet.2019.126920 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
[9]. Therefore, minor Sr (below 0.2%) is added to further refine grains and improve microstructural homogeneity, and its effects on bio-corrosion, SCC properties and cytocompatibility of asHSRRed Mg-4Zn alloy are investigated to offer a better understanding of the Mg-Zn-Sr alloy. 2. Experimental Mg-4Zn-xSr (x = 0, 0.1, 0.2) alloys were prepared by HSRR with a reduction of 80% in one pass at 9.1 s1 [8]. See the melting details and the HSRR parameters in the literatures [9–10]. Microstructure was examined on a MM-6 microscope (Leitz). Polished and preweighed samples were exposed to Hank’s solution at 37 °C for 7 days and weight loss rate was calculated. Electrochemical polarization (EP) curves and electrochemical impedance spectroscopy (EIS) were measured by a CHI 760D potentiostat. Slow strain rate tensile (SSRT) testing was performed on the RGM-6050 machine (REGER) at 6.67 10-7 s1. SCC susceptibility index (ISCC) was calculated according to Equation (1) [7]. UTSin Hank0 s and UTSin air are ultimate tensile strength in Hank’s solution and in air, respectively. ein Hank0 s and ein air are elongation to rupture in Hank’s solution and in air, respectively. Fracture was examined on a FEI QUANTA 200 scanning electron microscopy (SEM). The proliferation data of MG63 cells incubated in pure Mg, Mg-Zn and Mg-Zn-Sr extractions for 24 h, 72 h and 120 h were collected with the average cell viability of pure Mg for 24 h of 100% as the control and cell adhesion was examined after culturing for 24 h.
2
Z. Yu et al. / Materials Letters 260 (2020) 126920
UTSin Hank s 1 þ ein Hank s UTSin airð1 þ ein airÞ 0
ISCC ¼ 1
0
ð1Þ
3. Results and discussion Almost complete dynamic recrystallization (DRX) is achieved at the surface layer in as-HSRRed alloys and the DRX degree decreases from surface to centre (Fig. 1). Sr addition can refine DRX grains and improve microstructural homogeneity. Deformation twin nucleation is the dominant DRX mechanism in as-HSRRed alloys [8]. Therefore, increasing twinning density enhances DRX. Sr addition can lower stacking fault energy (SFE) of Mg alloy [11] and the reduced SFE leads to a finer twin size and a higher twinning density [12], causing a finer DRX grain size. Moreover, Sr can promote dynamic precipitation in Mg-5Zn alloy during HSRR [13]. The Sr-containing alloy has finer grains since fine dispersed precipitates (mainly b01 , MgZn2) will pin grain boundaries and inhibit grain growth. Weight loss rate decreases with the higher Sr content and Mg4Zn-0.2Sr alloy presents the lowest value (0.32 mgcm2day1), about 27% lower than Mg-4Zn alloy (Table 1). The latter has the thickest corrosion film with many cracks (Fig. 1), indicating accelerated corrosion. The corrosion film has fewer cracks and becomes more compact by Sr addition, preventing corrosion effectively. EDS line scanning shows the corrosion film primarily consists of MgO and Mg(OH)2. Corrosion resistance of Mg alloys is affected by grain size, second phase, dislocation and twinning [14–16]. Grain refinement can improve corrosion resistance by the physical barrier effect of grain boundaries [16]. However, grain boundaries also act as crystal defects and facilitate corrosion [17]. The corrosion film on finegrained alloys is more compact, effectively inhibiting corrosion.
Table 1 Microstructure parameters and property data of as-HSRRed alloys. Alloys
Mg-4Zn
Mg-4Zn0.1Sr
Mg-4Zn0.2Sr
Average DRX grain size* (lm) (Square is as-cast state) DRX volume fraction at 1/4 depth (%) Weight loss rate (mgcm2day1) Ecorr (V) icorr (lA/cm2 ) RS (X cm2) Rct (X cm2) Rf (X cm2) RL (X cm2) ein air/% ein Hank’s/% UTS in air/MPa UTSin Hank’s/MPa ISCC* (Square is as-cast state)
4.0[80]
3.6[45]
3.3[57]
98 0.44 1.60 13.09 34.37 960 681.5 0.01 36.3 5.2 261 180 0.47 [0.22]
97 0.35 1.54 6.98 41.56 1601 1880 0.1951 34.9 5.4 263 190 0.44 [0.22]
99 0.32 1.51 6.14 40.26 1888 1912 463.5 33.5 7.2 268 195 0.41 [0.23]
Therefore, the bio-corrosion improvement by Sr addition is partly associated with grain refinement. Un-DRX regions with higher dislocation densities than DRX regions also play a role in corrosion of as-HSRRed alloys. The alloys with high dislocation & twinning densities always have poor corrosion resistance [15–16]. Nevertheless, DRX regions act as the preferential sites for pitting in bimodal structure due to the high grain boundary density [18]. Therefore, no DRX grain size but microstructural homogeneity is the most important factor on bio-corrosion improvement once the grain size is refined to a certain extent. Sr can improve microstructural homogeneity and enhance bio-corrosion resistance. EP curves have a similar shape (Fig. 2a), indicating the same corrosion mechanism. Ecorr shifts positively with the higher Sr content but Icorr decreases. Three well-defined loops and the equivalent
Fig. 1. OM images, SEM corrosion cross-sections and EDS line scanning of Mg-4Zn-xSr alloys (a) Sr-free (b) 0.1 (c) 0.2.
Z. Yu et al. / Materials Letters 260 (2020) 126920
3
Fig. 2. EP (a), EIS (b) curves and equivalent circuit (c) of as-HSRRed alloys in Hank’s solution at 37 °C.
circuit model are presented in Fig. 2b and c. Rs, Rct, Rf and RL represent the solution resistance, the electron transfer resistance of asHSRRed alloy, the corrosion film resistance and the inductance resistance, respectively. Rct increases with the higher Sr content, confirming the beneficial effect of Sr on bio-corrosion. Rf shows the barrier effect of corrosion film, in agreement with Fig. 1. Strength and plasticity of as-HSRRed alloys increase with the higher Sr content (Fig. 3 and Table 1). The Mg-4Zn-0.2Sr alloy shows UTSin air of 268 MPa, ein air of 33.5%, UTSin Hank0 s of 195 MPa and ein Hank0 s of 7.2%. The as-HSRRed alloy shows the lower ISCC with
the higher Sr content, indicating Sr can reduce SCC susceptibility. Inter-granular fracture is dominant in as-HSRRed alloys, different from as-cast alloys. Internal cracks become smaller with a higher Sr content (Fig. 3). SCC of Mg alloys is related to grain size, the protective corrosion film, second phase and microstructural homogeneity. Grain refinement can effectively improve SCC properties of Mg alloys [19]. Sr addition can refine grains and generate the more compact corrosion film, resulting in higher SCC resistance. Second phase also affects SCC of as-HSRRed alloy. Firstly, fine dispersed particles
Fig. 3. SSRT curves, SEM fracture images and the fracture cross-sections in Hank’s solution of as-HSRRed alloys.
4
Z. Yu et al. / Materials Letters 260 (2020) 126920
Fig. 4. MG63 proliferation data and SEM images of cell adhesion on as-HSRRed alloys.
can hinder dislocation movement and improve strength, enhancing SCC resistance more effectively than coarse particles. Secondly, galvanic corrosion occurs at the interface between second phase (mainly b01 , MgZn2) and a-Mg and corrosion pits act as crack sources, causing premature fracture of the alloy. Sr promotes dynamic precipitation during HSRR [13] and thus impairs SCC resistance. Therefore, Sr addition should be limited at a low level to avoid the coarse Sr-containing particles. The alloy with the higher Sr content has a more homogenous microstructure and a reduced grain size, resulting in higher SCC resistance and lower SCC susceptibility. The Mg-4Zn-0.1Sr extract exhibits an improved cell proliferation effect in comparison with pure Mg and Mg-4Zn alloy within 72 h and they the similar rates at 120 h (Fig. 4a), indicating Sr promotes the cell growth at the early stage. Many spreading cells are detected on Mg-4Zn-0.1Sr alloy and the filopodia between cells are obvious, indicating its good cell adhesion ability. 4. Conclusions The Mg-4Zn-xSr alloys exhibit higher corrosion & SCC resistance and lower SCC susceptibility with the higher Sr content (0.2%), attributable to grain refinement, the enhanced microstructural homogeneity and the more compact corrosion film. The as-HSRRed Mg-4Zn alloy with a low level of Sr provides excellent strength, good bio-corrosion resistance, low SCC susceptibility and excellent cytocompatibility for biomedical applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors are grateful to National Natural Science Foundation Project of China (51571089). References [1] P.K. Bowen, A. Drelich, J. Drelich, J. Goldman, J. Biomed. Mater. Res. A. 103 (2015) 341–349. [2] L. Choudhary, R.K.S. Raman, Acta Biomater. 8 (2012) 916–923. [3] S.G. Dahl, P. Allain, P.J. Marie, Y. Mauras, G. Boivin, P. Ammann, Bone 28 (2001) 446–453. [4] H.S. Brar, J. Wong, M.V. Manuel, J. Mech. Behav. Biomed. Mater. 7 (2012) 87– 95. [5] T. Asabe, M. Rifai, M. Yuasa, et al., Inter. J. Corros. 7 (2017) 1–8. [6] L.X. Chen, Y.Y. Shen, X.J. Wang, X. Zhao, H. Liu, W. Li, Mater 11 (4) (2018) 551– 568. [7] L.X. Chen, Y.H. Bin, W.Q. Zou, X.J. Wang, W. Li, J. Mech. Behav. Biomed. Mater. 66 (2017) 187–200. [8] Z.Y. Zou, J.H. Chen, H.G. Yan, B. Su, X.L. Gong, J. Mater. Eng. Perform. 25 (2016) 1974–1985. [9] M.X. Cheng, J.H. Chen, H.G. Yan, B. Su, Z.H. Yu, W.J. Xia, X.L. Gong, J. Alloy. Compd. 691 (2017) 95–102. [10] S.Q. Zhu, H.G. Yan, J.H. Chen, Y.Z. Wu, J.Z. Liu, J. Tian, Scripta Mater. 63 (2010) 985–988. [11] M. Yuasa, N. Miyazawa, M. Hayashi, M. Mabuchi, Y. Chino, Acta Mater. 83 (2015) 294–303. [12] Y. Zhang, N.R. Tao, K. Lu, Scripta Mater. 60 (4) (2009) 211–213. [13] T. Ding, H.G. Yan, J.H. Chen, W.J. Xia, B. Su, Z.L. Yu, T. Nonferr, Metals Soc. 29 (8) (2019) 1631–1640. [14] M.C. Zhao, M. Liu, G.L. Song, A. Atrens, Corros. Sci. 50 (2008) 1939–1953. [15] G.B. Hamu, D. Eliezer, L. Wagner, J. Alloy. Compd. 468 (2009) 222–229. [16] N.N. Aung, W. Zhou, Corros. Sci. 52 (2008) 589–594. [17] G.L. Song, Z. Xu, Electrochim. Acta 55 (2010) 4148–4161. [18] H. Guo, J.H. Chen, H.G. Yan, W.J. Xia, B. Su, X.L. Gong, Z.L. Yu, Mater. Res. Express. 6 (8) (2019) 65c5. [19] O. Hakimi, E. Aghion, J. Goldman, Mater. Sci. Eng. C 51 (2015) 226–232.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Degradation, stress corrosion cracking behavior and cytocompatibility of high strain rate rolled Mg-Zn-Sr alloys Zonglin Yu, Jihua Chen ⇑, Hongge Yan ⇑, Weijun Xia, Bin Su, Xiaole Gong, Hui Guo School of Materials Science and Engineering, Hunan University, 410082, China
a r t i c l e
i n f o
Article history: Received 6 July 2019 Received in revised form 15 October 2019 Accepted 30 October 2019 Available online 31 October 2019 Keywords: Magnesium alloys Corrosion Microstructure Stress corrosion cracking Biomaterials
a b s t r a c t The major obstacle of Mg alloys to the clinical applications is their rapid degradation behavior. In the present study, high strain rate rolling (HSRR) and Sr micro-alloying are conducted to improve bio-corrosion & stress corrosion cracking (SCC) resistance, reduce SCC susceptibility and enhance cytocompatibility of Mg-4Zn alloy. The effects on bio-corrosion & SCC resistance improvement are more pronounced with the higher Sr content (0.2%), attributable to grain refinement, the enhanced microstructural homogeneity and the more compact corrosion film. Sr addition can promote the cell growth at the early stage and improve the cell adhesion ability. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Mg alloys are the attractive biodegradable materials, which can avoid the secondary removal after surgery and effectively reduce the cost and the infection risk of patients. Unfortunately, they always corrode too quickly in Cl- containing solutions, resulting in fast loss of mechanical integrity [1]. Moreover, Mg alloy implants suffer mechanical loading, making them sudden fracture before healing [2]. Therefore, the bio-corrosion and SCC behaviors should be investigated to ensure their safety. The Mg-Zn-Sr alloy is a promising biomaterial since Sr can accelerate the growth of osteoblasts [3]. Furthermore, Sr can refine grains and improve strength and corrosion resistance of Mg alloys [4]. On one side, grain refinement enhances SCC resistance when the grain size is in micron level [5]. On the other side, it causes hydrogen embrittlement and results in poor SCC properties [6]. However, the as-cast Mg-4Zn-0.6Zr-xSr alloys exhibit higher SCC susceptibility with the higher Sr content (0.4–1.6%) [7]. So far, the bio-corrosion and SCC behaviors of wrought Mg-Zn-Sr alloys have not been reported yet. HSRR can significantly modify the microstructure and reduce the degradation rate of Mg-4Zn alloy [8]. However, its SCC behavior and cytocompatibility remain unclear. It is reported that 0.2%Sr addition brings about the greatest bio-corrosion improvement in as-cast Mg-5Zn-xSr alloys (x 1.0 wt%) through grain refinement ⇑ Corresponding author. E-mail address: [email protected] (J. Chen). https://doi.org/10.1016/j.matlet.2019.126920 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
[9]. Therefore, minor Sr (below 0.2%) is added to further refine grains and improve microstructural homogeneity, and its effects on bio-corrosion, SCC properties and cytocompatibility of asHSRRed Mg-4Zn alloy are investigated to offer a better understanding of the Mg-Zn-Sr alloy. 2. Experimental Mg-4Zn-xSr (x = 0, 0.1, 0.2) alloys were prepared by HSRR with a reduction of 80% in one pass at 9.1 s1 [8]. See the melting details and the HSRR parameters in the literatures [9–10]. Microstructure was examined on a MM-6 microscope (Leitz). Polished and preweighed samples were exposed to Hank’s solution at 37 °C for 7 days and weight loss rate was calculated. Electrochemical polarization (EP) curves and electrochemical impedance spectroscopy (EIS) were measured by a CHI 760D potentiostat. Slow strain rate tensile (SSRT) testing was performed on the RGM-6050 machine (REGER) at 6.67 10-7 s1. SCC susceptibility index (ISCC) was calculated according to Equation (1) [7]. UTSin Hank0 s and UTSin air are ultimate tensile strength in Hank’s solution and in air, respectively. ein Hank0 s and ein air are elongation to rupture in Hank’s solution and in air, respectively. Fracture was examined on a FEI QUANTA 200 scanning electron microscopy (SEM). The proliferation data of MG63 cells incubated in pure Mg, Mg-Zn and Mg-Zn-Sr extractions for 24 h, 72 h and 120 h were collected with the average cell viability of pure Mg for 24 h of 100% as the control and cell adhesion was examined after culturing for 24 h.
2
Z. Yu et al. / Materials Letters 260 (2020) 126920
UTSin Hank s 1 þ ein Hank s UTSin airð1 þ ein airÞ 0
ISCC ¼ 1
0
ð1Þ
3. Results and discussion Almost complete dynamic recrystallization (DRX) is achieved at the surface layer in as-HSRRed alloys and the DRX degree decreases from surface to centre (Fig. 1). Sr addition can refine DRX grains and improve microstructural homogeneity. Deformation twin nucleation is the dominant DRX mechanism in as-HSRRed alloys [8]. Therefore, increasing twinning density enhances DRX. Sr addition can lower stacking fault energy (SFE) of Mg alloy [11] and the reduced SFE leads to a finer twin size and a higher twinning density [12], causing a finer DRX grain size. Moreover, Sr can promote dynamic precipitation in Mg-5Zn alloy during HSRR [13]. The Sr-containing alloy has finer grains since fine dispersed precipitates (mainly b01 , MgZn2) will pin grain boundaries and inhibit grain growth. Weight loss rate decreases with the higher Sr content and Mg4Zn-0.2Sr alloy presents the lowest value (0.32 mgcm2day1), about 27% lower than Mg-4Zn alloy (Table 1). The latter has the thickest corrosion film with many cracks (Fig. 1), indicating accelerated corrosion. The corrosion film has fewer cracks and becomes more compact by Sr addition, preventing corrosion effectively. EDS line scanning shows the corrosion film primarily consists of MgO and Mg(OH)2. Corrosion resistance of Mg alloys is affected by grain size, second phase, dislocation and twinning [14–16]. Grain refinement can improve corrosion resistance by the physical barrier effect of grain boundaries [16]. However, grain boundaries also act as crystal defects and facilitate corrosion [17]. The corrosion film on finegrained alloys is more compact, effectively inhibiting corrosion.
Table 1 Microstructure parameters and property data of as-HSRRed alloys. Alloys
Mg-4Zn
Mg-4Zn0.1Sr
Mg-4Zn0.2Sr
Average DRX grain size* (lm) (Square is as-cast state) DRX volume fraction at 1/4 depth (%) Weight loss rate (mgcm2day1) Ecorr (V) icorr (lA/cm2 ) RS (X cm2) Rct (X cm2) Rf (X cm2) RL (X cm2) ein air/% ein Hank’s/% UTS in air/MPa UTSin Hank’s/MPa ISCC* (Square is as-cast state)
4.0[80]
3.6[45]
3.3[57]
98 0.44 1.60 13.09 34.37 960 681.5 0.01 36.3 5.2 261 180 0.47 [0.22]
97 0.35 1.54 6.98 41.56 1601 1880 0.1951 34.9 5.4 263 190 0.44 [0.22]
99 0.32 1.51 6.14 40.26 1888 1912 463.5 33.5 7.2 268 195 0.41 [0.23]
Therefore, the bio-corrosion improvement by Sr addition is partly associated with grain refinement. Un-DRX regions with higher dislocation densities than DRX regions also play a role in corrosion of as-HSRRed alloys. The alloys with high dislocation & twinning densities always have poor corrosion resistance [15–16]. Nevertheless, DRX regions act as the preferential sites for pitting in bimodal structure due to the high grain boundary density [18]. Therefore, no DRX grain size but microstructural homogeneity is the most important factor on bio-corrosion improvement once the grain size is refined to a certain extent. Sr can improve microstructural homogeneity and enhance bio-corrosion resistance. EP curves have a similar shape (Fig. 2a), indicating the same corrosion mechanism. Ecorr shifts positively with the higher Sr content but Icorr decreases. Three well-defined loops and the equivalent
Fig. 1. OM images, SEM corrosion cross-sections and EDS line scanning of Mg-4Zn-xSr alloys (a) Sr-free (b) 0.1 (c) 0.2.
Z. Yu et al. / Materials Letters 260 (2020) 126920
3
Fig. 2. EP (a), EIS (b) curves and equivalent circuit (c) of as-HSRRed alloys in Hank’s solution at 37 °C.
circuit model are presented in Fig. 2b and c. Rs, Rct, Rf and RL represent the solution resistance, the electron transfer resistance of asHSRRed alloy, the corrosion film resistance and the inductance resistance, respectively. Rct increases with the higher Sr content, confirming the beneficial effect of Sr on bio-corrosion. Rf shows the barrier effect of corrosion film, in agreement with Fig. 1. Strength and plasticity of as-HSRRed alloys increase with the higher Sr content (Fig. 3 and Table 1). The Mg-4Zn-0.2Sr alloy shows UTSin air of 268 MPa, ein air of 33.5%, UTSin Hank0 s of 195 MPa and ein Hank0 s of 7.2%. The as-HSRRed alloy shows the lower ISCC with
the higher Sr content, indicating Sr can reduce SCC susceptibility. Inter-granular fracture is dominant in as-HSRRed alloys, different from as-cast alloys. Internal cracks become smaller with a higher Sr content (Fig. 3). SCC of Mg alloys is related to grain size, the protective corrosion film, second phase and microstructural homogeneity. Grain refinement can effectively improve SCC properties of Mg alloys [19]. Sr addition can refine grains and generate the more compact corrosion film, resulting in higher SCC resistance. Second phase also affects SCC of as-HSRRed alloy. Firstly, fine dispersed particles
Fig. 3. SSRT curves, SEM fracture images and the fracture cross-sections in Hank’s solution of as-HSRRed alloys.
4
Z. Yu et al. / Materials Letters 260 (2020) 126920
Fig. 4. MG63 proliferation data and SEM images of cell adhesion on as-HSRRed alloys.
can hinder dislocation movement and improve strength, enhancing SCC resistance more effectively than coarse particles. Secondly, galvanic corrosion occurs at the interface between second phase (mainly b01 , MgZn2) and a-Mg and corrosion pits act as crack sources, causing premature fracture of the alloy. Sr promotes dynamic precipitation during HSRR [13] and thus impairs SCC resistance. Therefore, Sr addition should be limited at a low level to avoid the coarse Sr-containing particles. The alloy with the higher Sr content has a more homogenous microstructure and a reduced grain size, resulting in higher SCC resistance and lower SCC susceptibility. The Mg-4Zn-0.1Sr extract exhibits an improved cell proliferation effect in comparison with pure Mg and Mg-4Zn alloy within 72 h and they the similar rates at 120 h (Fig. 4a), indicating Sr promotes the cell growth at the early stage. Many spreading cells are detected on Mg-4Zn-0.1Sr alloy and the filopodia between cells are obvious, indicating its good cell adhesion ability. 4. Conclusions The Mg-4Zn-xSr alloys exhibit higher corrosion & SCC resistance and lower SCC susceptibility with the higher Sr content (0.2%), attributable to grain refinement, the enhanced microstructural homogeneity and the more compact corrosion film. The as-HSRRed Mg-4Zn alloy with a low level of Sr provides excellent strength, good bio-corrosion resistance, low SCC susceptibility and excellent cytocompatibility for biomedical applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements The authors are grateful to National Natural Science Foundation Project of China (51571089). References [1] P.K. Bowen, A. Drelich, J. Drelich, J. Goldman, J. Biomed. Mater. Res. A. 103 (2015) 341–349. [2] L. Choudhary, R.K.S. Raman, Acta Biomater. 8 (2012) 916–923. [3] S.G. Dahl, P. Allain, P.J. Marie, Y. Mauras, G. Boivin, P. Ammann, Bone 28 (2001) 446–453. [4] H.S. Brar, J. Wong, M.V. Manuel, J. Mech. Behav. Biomed. Mater. 7 (2012) 87– 95. [5] T. Asabe, M. Rifai, M. Yuasa, et al., Inter. J. Corros. 7 (2017) 1–8. [6] L.X. Chen, Y.Y. Shen, X.J. Wang, X. Zhao, H. Liu, W. Li, Mater 11 (4) (2018) 551– 568. [7] L.X. Chen, Y.H. Bin, W.Q. Zou, X.J. Wang, W. Li, J. Mech. Behav. Biomed. Mater. 66 (2017) 187–200. [8] Z.Y. Zou, J.H. Chen, H.G. Yan, B. Su, X.L. Gong, J. Mater. Eng. Perform. 25 (2016) 1974–1985. [9] M.X. Cheng, J.H. Chen, H.G. Yan, B. Su, Z.H. Yu, W.J. Xia, X.L. Gong, J. Alloy. Compd. 691 (2017) 95–102. [10] S.Q. Zhu, H.G. Yan, J.H. Chen, Y.Z. Wu, J.Z. Liu, J. Tian, Scripta Mater. 63 (2010) 985–988. [11] M. Yuasa, N. Miyazawa, M. Hayashi, M. Mabuchi, Y. Chino, Acta Mater. 83 (2015) 294–303. [12] Y. Zhang, N.R. Tao, K. Lu, Scripta Mater. 60 (4) (2009) 211–213. [13] T. Ding, H.G. Yan, J.H. Chen, W.J. Xia, B. Su, Z.L. Yu, T. Nonferr, Metals Soc. 29 (8) (2019) 1631–1640. [14] M.C. Zhao, M. Liu, G.L. Song, A. Atrens, Corros. Sci. 50 (2008) 1939–1953. [15] G.B. Hamu, D. Eliezer, L. Wagner, J. Alloy. Compd. 468 (2009) 222–229. [16] N.N. Aung, W. Zhou, Corros. Sci. 52 (2008) 589–594. [17] G.L. Song, Z. Xu, Electrochim. Acta 55 (2010) 4148–4161. [18] H. Guo, J.H. Chen, H.G. Yan, W.J. Xia, B. Su, X.L. Gong, Z.L. Yu, Mater. Res. Express. 6 (8) (2019) 65c5. [19] O. Hakimi, E. Aghion, J. Goldman, Mater. Sci. Eng. C 51 (2015) 226–232.