Materials Science and Engineering C 49 (2015) 93–100
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Microstructures, mechanical and corrosion properties and biocompatibility of as extruded Mg–Mn–Zn–Nd alloys for biomedical applications Ying-Long Zhou a,⁎, Yuncang Li b, Dong-Mei Luo c, Yunfei Ding b, Peter Hodgson b a b c
Department of Mechatronics Engineering, Foshan University, Foshan 528000, Guangdong, China Institute for Frontier Materials, Deakin University, Victoria 3217, Australia Department of Civil Engineering, Foshan University, Foshan 528000, Guangdong, China
a r t i c l e
i n f o
Article history: Received 24 March 2014 Received in revised form 22 October 2014 Accepted 17 December 2014 Available online 18 December 2014 Keywords: Mg–Mn–Zn–Nd alloy Microstructure Mechanical property Corrosion behavior Extrusion
a b s t r a c t Extruded Mg–1Mn–2Zn–xNd alloys (x = 0.5, 1.0, 1.5 mass %) have been developed for their potential use as biomaterials. The extrusion on the alloys was performed at temperature of 623 K with an extrusion ratio of 14.7 under an average extrusion speed of 4 mm/s. The microstructure, mechanical property, corrosion behavior and biocompatibility of the extruded Mg–Mn–Zn–Nd alloys have been investigated in this study. The microstructure was examined using X-ray diffraction analysis and optical microscopy. The mechanical properties were determined from uniaxial tensile and compressive tests. The corrosion behavior was investigated using electrochemical measurement. The biocompatibility was evaluated using osteoblast-like SaOS2 cells. The experimental results indicate that all extruded Mg–1Mn–2Zn–xNd alloys are composed of both α phase of Mg and a compound of Mg7Zn3 with very fine microstructures, and show good ductility and much higher mechanical strength than that of cast pure Mg and natural bone. The tensile strength and elongation of the extruded alloys increase with an increase in neodymium content. Their compressive strength does not change significantly with an increase in neodymium content. The extruded alloys show good biocompatibility and much higher corrosion resistance than that of cast pure Mg. The extruded Mg–1Mn–2Zn–1.0Nd alloy shows a great potential for biomedical applications due to the combination of enhanced mechanical properties, high corrosion resistance and good biocompatibility. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, Mg alloys have attracted increasing attention as biodegradable implant materials due to their good biocompatibility, low density and elastic modulus which are closer to those of human bone as compared with the other metallic biomaterials (stainless steel, titanium and its alloys), and much higher mechanical strength than the biodegradable polymers. The main disadvantages of current Mg alloys, however, are that they corrode too rapidly in physiological environment and lose mechanical integrity before the tissue has healed, and that the hydrogen pockets produced in corrosion process retard the tissue healing [1–3]. Therefore, it is desirable to improve the corrosion rate of Mg alloys so that they corrode at an appropriate rate and hydrogen pockets can be absorbed by the surrounding tissue without negative effect. Element alloying is one of the most effective measures to improve the corrosion resistance of Mg alloys. For bio-Mg alloys, non-toxic alloying elements are essential, so calcium (Ca), zinc (Zn), manganese (Mn) and low toxicity rare earth (RE) elements at a low concentration ⁎ Corresponding author. E-mail address: [email protected] (Y.-L. Zhou).
http://dx.doi.org/10.1016/j.msec.2014.12.057 0928-4931/© 2014 Elsevier B.V. All rights reserved.
are suggested as alloying elements for biomedical applications in previous investigations [1,3]. Accordingly, many bio-Mg alloys, such as Mg–Ca [4–6], Mg–Zn [7], Mg–Zn–Ca [8], Mg–Zn–Mn [9,10] and Mg–Zn–Mn–Ca alloys [11,12], have been developed for biomedical applications. Those alloys provide much better strength and corrosion resistance than pure Mg. However, bio-Mg alloys must possess certain mechanical and biodegradable properties to serve as suitable stent materials (specifically in cardiovascular applications) [13]: (i) appropriate mechanical characteristics at body temperature, such as sufficient ductility (a uniform strain optimally ≥ 15%), and reasonable strength (ultimate tensile strength (UTS) ≥ 250 MPa); (ii) a moderate and homogeneous degradation performance (biodegradable period of 12–24 months); (iii) good biocompatibility. Based on those requirements, the above bio-Mg alloys cannot be regarded as ideal biodegradable materials for biomedical applications. For example, the tensile strength and elongation of Mg–Ca alloys are low for the load-bearing applications and the corrosion resistance also needs to be improved furthermore [4–6,12]; the bending strength of Mg– Zn alloy deteriorates rapidly at the initial corrosion stage from 625 MPa to 390 MPa with about 6% loss of weight during degradation [7]; the Mg–Zn–Ca alloys have inadequate mechanical properties
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(UTS b 185 MPa) [8]; and the Mg–Zn–Mn–Ca alloys have good corrosion resistance but inadequate mechanical properties (UTS b 190 MPa) [11,12]. Therefore, optimization of alloying element and composition of bio-Mg alloys to obtain new biodegradable Mg alloys with appropriate mechanical properties, corrosion resistance, and biocompatibility is very crucial for biomedical applications. Ca element has been proven to not be an effective element to increase the strength and corrosion resistance of Mg alloys [4–6,12]. Mn element has the function of refining grain size and improving the tensile strength of magnesium alloy [14]. Zn element is next to aluminum in strengthening effectiveness as an alloying element in magnesium [9,15] and adding Zn can improve both the tensile strength and the corrosion resistance of Mg alloys [16]. In addition, RE elements can purify Mg alloys and accordingly improve their corrosion behavior and mechanical properties [17]. Neodymium (Nd) is one of the light RE elements with maximum solubility in solid Mg of 3.6% at eutectic temperature [18]. Recent research indicates that a small amount of Nd does not exhibit cell toxicity [19]. Therefore, Mg–Zn–Mn alloys with a low concentration of Nd are expected to be good candidates for biomedical application due to their promising good mechanical properties, corrosion resistance and biocompatibility. Few reports on Mg–Zn–Mn–Nd alloys are available at present. In this study, Mg–1Mn–2Zn–xNd alloys (x = 0.5, 1.0, 1.5; mass%, hereafter) have been designed on the basis of the above consideration and their various properties have been investigated to check their potential use in biomedical applications. Since the previous investigations [20–24] suggest that the extrusion can greatly refine the microstructures of Mg alloys, resulting in a remarkable improvement of the mechanical and corrosion properties, the extrusion was also performed on the designed Mg–1Mn–2Zn–xNd alloys to further improve their properties. 2. Experimental 2.1. Material preparation The designed Mg–1Mn–2Zn–xNd alloys (x = 0.5, 1.0, 1.5%) were fabricated from high purity Mg (99.99%), high purity Zn (99.99%), Mg–10Mn (99.98%), and Mg–25Nd (99.97%) master alloys. High purity Mg and Zn were first placed in a high-purity graphite crucible, and the master alloys were then added when the temperature reached 1053 K. After melting at 1023–1053 K for 30 min, the melt was cast into a steel mold at approximate 1003 K. The cast ingots were then homogenized at 673 K for 24 h with furnace cooling. The extrusion was performed at a temperature of 623 K with an extrusion ratio of 14.7 and an extrusion diameter of 12 mm under an average extrusion speed of 4 mm/s. The chemical compositions of the designed alloys were analyzed by X-ray fluorescence spectrometric method, as listed in Table 1. 2.2. Microstructural characterization The phase constitution of the alloys was determined using X-ray diffraction (XRD) analysis. For metallographic observation, the specimens were examined by optical microscopy (OM) after they were ground with SiC emery papers of up to 3000 grit, polished with 0.5 μm diamond powder, and then etched with a solution of 1.5 g picric acid, 10 ml acetic acid, 10 ml distilled water add 25 ml ethanol. Table 1 Chemical compositions of the alloys (mass %). Alloy code
Mn
Zn
Nd
Ni
Fe
Al
Mg
Mg–1Mn–2Zn–0.5Nd Mg–1Mn–2Zn–1.0Nd Mg–1Mn–2Zn–1.5Nd
1.29 1.23 1.21
2.42 2.31 2.2
0.28 0.81 1.21
0.01 0.02 0.02
0.02 0.02 0.03
– 0.03 0.03
Bal. Bal. Bal.
2.3. Mechanical property test Following ASTM E8-04 [25], the tensile samples (25 mm in gauge length, 6.35 mm in width, and 5 mm in thickness) were machined from the extruded bars along the longitudinal direction. The tensile tests were conducted at a crosshead speed of 1 mm/min at room temperature in air using an Instron type machine (Instron 5900, Instron Corporation, USA). According to ASTM E9-09 [26], the compressive samples (27 mm in gauge length, 11 mm in diameter) were machined from the extruded bars along the longitudinal direction. The compressive tests were conducted using the same Instron machine at a crosshead speed of 0.0025 mm/s at room temperature. The ultimate tensile strength (UTS), 0.2% yield strength (YS), elongation to failure (Ef), compressive strength (CS), and compressive yield strength (CYS) were obtained based on the average of three test samples. 2.4. Electrochemical measurement Electrochemical impedance spectroscopy (EIS) and polarization curves of the alloys were measured in the simulative body fluid (SBF) at 37 ± 0.5 °C. The SBF was prepared through dissolving the following chemicals in 1 L deionized water sequentially: NaCl (5.403 g), NaHCO3 (0.504 g), Na2CO3 (0.426 g), KCl (0.225 g), K2HPO4·3H2O (0.230 g), MgCl2·6H2O (0.311 g), CaCl2 (0.293 g) and Na2SO4 (0.072 g). The solution was buffered to pH 7.40 with HEPES and 1 M NaOH at 37 °C [27]. For both measurements, disc samples with a diameter of 10 mm and thickness of 2 mm were used for the electrochemical tests. All the samples were connected with a copper wire and then mounted in epoxy resin as the working electrode. Before the electrochemical test, the mounted samples were successively polished with 240, 600 and 1200-grit sand papers, then carefully degreased with acetone, ethanol and rinsed with distilled water, finally dried in a stream of warm air. The open circuit potential and the potentiodynamic polarization curve of samples soaked in SBF were measured by electrochemical station (1470E Multichannel Potentiostat, Solartron, UK) equipped with Multistate software. A three-electrode cell system with a saturated calomel electrode (SCE), a 1.5 × 1.5 cm2 platinum electrode and the sample mounted in epoxy resin as reference electrode, count electrode, and the working electrode, respectively was used in this study. Before the measurement, the working electrode was immersed in test solution for stability. The corrosion density current (icorr) was calculated using CView software. EIS measurement had begun after stabilization of the open circuit potential for an hour. The polarization curve measurement was subsequently conducted with a scan rate of 1 mV/s after the EIS measurement. The polarization started from a cathodic potential of − 50 mV relative to the open circuit potential and stopped at an anodic potential where the anodic current increased dramatically. 2.5. Cytotoxicity test Osteoblast-like cells (SaOS2) are a human osteosarcoma cell line with osteoblast properties [28] and were used to evaluate the biocompatibility of Mg–1Mn–2Zn–xNd alloys using the indirect contact method according to ISO 10993-5 [29,30]. Briefly, Mg alloys were immersed in cell culture media with a surface area to media ratio of 0.6 cm2/ml in a humidified atmosphere with 5% CO2 at 37 °C for 72 h. The supernatant fluid was withdrawn and filter-sterilized with a 0.22-μm filter (Falcon, BD Biosciences, San Jose, CA, USA) to obtain the extracts of Mg alloys. The control groups involved the use of cell culture medium as negative controls. SaOS2 cells were mixed with media at a cell density of 5 × 104 cells/ml. A mixture of 200 μl was transferred into a well of 48-well cell culture plate. The cell culture plate was placed in an incubator at a humidified atmosphere with 5% CO2 at 37°C for 24 h to allow cells attachment. The media in each well was then sucked out and an extract of 200 μl was refilled in each well. After further incubation of the cells for 24 h, the 48-well cell culture plates, MTS assay, a
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colorimetric assay [30], was used to measure the cell number in each well through absorbance reading at 490 nm using a spectrophotometer (GENios Pro, Tecan, Mannedorf, Switzerland). The cell viability ratio (CVR) for every Mg alloy was defined as the ratio of the live cell number in extraction media to that in the control, which can be calculated using the equation: CVR = absorbance reading in Mg alloy extract / absorbance reading in control. 3. Results and discussion 3.1. Microstructural characteristics The XRD patterns of extruded Mg–1Mn–2Zn–xNd alloys are shown in Fig. 1 (Extruded-1, Extruded-2, and Extruded-3 were used to represent the extruded alloys with 0.5 Nd, 1.0 Nd, and 1.5 Nd, respectively, hereafter). The strongest peak for all the extruded alloys is (100) while that of Mg (α) phase is (101) according to the standard XRD patterns of Mg (powder), which is due to the obvious texture caused by the extrusion process. As compared with the XRD peaks of as cast Mg, it can be observed that all the extruded Mg alloys are composed of both α phase of Mg and a compound of Mg7Zn3. Although 2% Zn can fully dissolve in α Mg matrix by the Mg–Zn binary phase diagram [18], the solubility of Zn in α Mg decreases and the compound may produce when the other alloying elements are added. This is related to the formation of compound Mg7Zn3 in the Mg–1Mn–2Zn, Mg–1Mn–3Zn alloys [9] and the Mg–1Mn–2Zn–xNd alloys in this study. It also can be seen that the peak values of Mg7Zn3 slightly increase with addition of Nd content, which suggests the increase of volume fraction of Mg7Zn3 with Nd content. Since the solubility of alloying elements in α Mg at room temperature is limited due to the supersaturation, more Mg7Zn3 precipitates naturally with the increase in Nd content. Figs. 2 and 3 show the microstructures of cross-sections and longitudinal sections of the extruded Mg–1Mn–2Zn–xNd alloys, respectively. It can be observed that the extruded Mg alloys exhibit very fine microstructures, which is attributed to the phenomenon of recrystallization during the hot-extrusion. In the cross-section microstructures (Fig. 2), a typical equiaxed microstructure is observed in all the alloys. In the longitudinal section microstructures (Fig. 3), a similar equiaxed grain structure and a few unrecrystallized bands indicated by red arrows
Fig. 2. OM microstructures of the cross-sections of extruded Mg–1Mn–2Zn–xNd alloys.
along the longitudinal direction are observed, which is related to the above texture detected by XRD analysis. The particulate of Mg7Zn3 is invisible in Figs. 2 and 3, which is related to their very fine structure and the limited resolution of OM. 3.2. Mechanical properties
Fig. 1. XRD patterns of extruded Mg–1Mn–2Zn–xNd alloys and cast pure Mg.
The mechanical properties of the extruded Mg–1Mn–2Zn–xNd alloys are shown in Fig. 4. It can be noticed that the extruded Mg– 1Mn–2Zn–xNd alloys exhibit much higher mechanical strength than as cast pure Mg, which is due to the solid solution strengthening by alloying elements and the working hardening by hot extrusion.
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Fig. 4. (a) Tensile properties and (b) compressive properties of extruded Mg–1Mn–2Zn– xNd alloys.
increase with the increasing of Nd content, which is associated with their very fine microstructures as shown in Figs. 2 and 3 since fine grain structure can improve both the strength and ductility of metallic materials at the same time (fine grain strengthening theory). Their compressive strength, however, slightly decreases with the increase in Nd content. This is possibly related to that the precipitation of Mg7Zn3, which increases with the increase in Nd content, destroys the continuity of microstructures of the Mg alloys and becomes a crack source. During
Fig. 3. OM microstructures of the longitudinal sections of extruded Mg–1Mn–2Zn–xNd alloys. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
As compared with the extruded Mg–1Mn–2Zn alloy [9], the extruded Mg–1Mn–2Zn–xNd alloys show slightly higher tensile strength, which suggests that Nd is not a highly effective element to increase the strength of Mg–Mn–Zn alloys. It is normal that the extruded Mg–1Mn–2Zn–xNd alloys have smaller elongation than the Mg–1Mn–2Zn alloy since it is consistent with the ordinarily observed strength–ductility relationship, i.e., when the yield strength increases, the ductility accordingly decreases. However, there is a big difference in elongation between the Mg–1Mn–2Zn–xNd alloys and the Mg–1Mn–2Zn alloy. The reason for this big difference is unclear and further investigation is necessary. The tensile strength and elongation of extruded Mg–1Mn–2Zn–xNd alloys
Fig. 5. Hydrogen evolution of the extruded alloys and cast pure Mg with immersion time.
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the compressive test, when the compressive stress increases, the crack source is more sensitive to the compressive stress than the tensile stress. Further study is necessary to explore the detailed reason for this phenomenon. Those results show that the extruded Mg–1Mn–2Zn–xNd alloys provide good ductility (elongation to failure ≥ 14%) and much higher mechanical strength (UTS N 285 MPa and CS N 395 MPa) than natural bone (UTS: 90–140 MPa [31] and CS: 100–230 MPa [12]), thus they show great potential for biomedical applications according to the requirement of mechanical properties of bio-Mg alloys [13]. 3.3. Corrosion property 3.3.1. Immersion corrosion behavior of extruded alloys In the physiological environment, Mg and its alloys are relatively reactive metals. They degrade to release H2 gas and form corrosion products such as Mg(OH)2 and calcium phosphates. For use as biomaterials, they should maintain a suitable degradation rate and mechanical integrity before the tissue healing. Fig. 5 shows the hydrogen evolution tendency of extruded alloys and pure Mg (as control) during immersion in SBF solution. It can be observed that hydrogen gases increase with immersion time in SBF for all the extruded alloys and pure Mg, and the extruded alloys continuously produce less hydrogen gas than cast pure Mg. Extruded-2 alloy shows the lowest hydrogen evolution rate among the extruded alloys and cast pure Mg. The corrosion rate Ph mm/y was determined by the hydrogen evolution rate Vh ml/(cm2 day) using the equation [32,33]. Ph ¼ 2:279Vh:
ð1Þ
In this study, the samples were immersed in SBF for 185 h. The hydrogen production rate and corrosion rate for the extruded alloys and cast pure Mg are summarized in Table 2. It can be seen that Extruded2 exhibits the lowest corrosion rate among the samples. 3.3.2. Electrochemical corrosion behavior of the extruded alloys Fig. 6 shows the electrochemical polarization curves for the specimens after immersion in SBF solution. Both cathodic and anodic polarization curves present the same trend for the alloys and pure Mg. The corrosion potential (Ecorr), corrosion current density (icorr), cathodic Tafel slope (βc), anodic Tafel slope (βa), and corrosion rate of the extruded alloys and cast pure Mg, which were extracted from the polarization curves (Fig. 6), are listed in Table 3. In addition to the above electrochemical parameters, the polarization resistance (RP) was calculated according to the following equation [34]. Rp ¼
βa βc 2:3ðβa þ βc Þicorr
ð2Þ
It can be seen that the extruded alloys exhibit more positive corrosion potential, much smaller current densities and lower corrosion rate than cast pure Mg, which indicates that the extruded alloys exhibit much better corrosion resistance than cast pure Mg. The extruded alloys have close RP values, however, much larger than that of cast pure Mg. There is no significant difference in the electrochemical parameters among the extruded alloys, which indicates that addition of Nd content Table 2 Hydrogen evolution and corrosion rates of Mg–1Mn–2Zn–xNd alloys and pure Mg in SBF. Samples
H2 production rate ml/(cm2 day)
Corrosion ratea mm/year
Pure Mg Extruded-1 Extruded-2 Extruded-3
2.57 2.37 2.16 2.47
5.86 5.40 4.93 5.64
a
Corrosion rate was calculated from H2 production rate.
Fig. 6. Polarization curves in SBF of the extruded alloys and cast pure Mg.
does not effectively alter the corrosion resistance of extruded Mg–Zn– Mn–Nd alloys. For better understanding, the electrochemical parameters of as cast Mg–Zn–Mn–Nd alloys are also provided in Table 3. It can be observed that the cast alloys exhibit more positive corrosion potential, smaller current density and lower corrosion rate than those of cast pure Mg, respectively. However, the corrosion potential, current density, and corrosion rate of the cast alloys are close to one another. This indicates that addition of Nd content does not effectively influence the corrosion resistance of cast Mg–Zn–Mn–Nd alloys. Elements, Zn, Mn, and Nd, can purify Mg alloys by elimination of the harmful corrosive effects from the impurity elements and accordingly enhance corrosion resistance of the Mg alloys [17,35]. This results in better corrosion resistance of as cast Mg–Zn–Mn–Nd alloys than cast pure Mg. It can also be seen that the extruded alloys exhibit a much smaller current density and lower corrosion rate than those of the cast alloys, respectively, which indicates that the extrusion process can greatly enhance the corrosion resistance of Mg–Zn–Mn–Nd alloys. Previous studies show that fine grain structure can create more grain boundaries that act as corrosion barriers and accordingly enhance the corrosion resistance of Mg alloys [20–24,30,36,37]. This helps to understand why the extruded alloys have much better corrosion resistance than the cast alloys. Therefore, the as extruded Mg–Zn–Mn–Nd alloys exhibit much better corrosion resistance than cast pure Mg due to both the effects of alloying elements (Zn, Nd, and Mn) and the extrusion process. Fig. 7 shows the EIS spectra for the extruded Mg alloys and as-cast Mg in SBF. As can be seen from Fig. 7(a), Extruded-1, Extruded-2 and as-cast Mg have similar degradation behaviors during the immersion in SBF after 1 h. Extruded-1, Extruded-2 and as-cast pure Mg contain one high frequency capacitance loop, one medium frequency capacitance
Table 3 Electrochemical parameters of Mg–1Mn–2Zn–xNd alloys and pure Mg. Sample code
Ecorr (V)
icorr (μA/cm2)
βc (mV/dec)
βa (mV/dec)
Rp (kΩ cm2)
Corrosion rate (mm/year)
Extruded-1 Extruded-2 Extruded-3 Pure Mg Cast-1 Cast-2 Cast-3
−1.469 −1.489 −1.495 −1.915 −1.546 −1.502 −1.534
50.16 26.69 33.48 177.8 112.22 107.61 79.43
146.7 81.1 90.9 89.4 102.8 144.2 139.2
115.1 64.2 84.5 88.7 93.6 114.4 123.2
0.55 0.58 0.56 0.11 0.19 0.26 0.35
1.46 0.78 0.97 3.86 3.26 3.12 2.31
Note: Cast-1, Cast-2, and Cast-3 were used to represent the cast Mg–1Mn–2Zn–xNd alloys with 0.5, 1.0, and 1.5 Nd, respectively.
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cast Mg. In addition, the charge transfer resistance for the extruded Mg alloys and as-cast Mg reduces in the order of Extruded-3 N Extruded-2 N Extruded-1 N as-cast Mg according to the intersection of the capacitance loops with the x-axis. In the case of Bode plots |Z| vs. frequency, as-cast Mg exhibits the lowest impedance values from the high frequency to low frequency. Extruded Mg alloys show an increase of impedance in the medium frequency, however, the impedance values reduce at the low frequency regions for all the extruded Mg alloys. Compared to Extruded-1 and Extruded-2, Extruded-3 presents a sharp decrease of impedance in the low frequency, indicating the severe pitting. The slope of plots for the extruded Mg alloys in the region of frequency from 100 Hz to 10 Hz is close to −1, and the plots in the high frequency from 100 kHz to 100 Hz exhibit a slight incline, which suggests the presence of capacitor. As for the Bode plots of phase (Z) degree vs. frequency, two wave crests are visible for Extruded-1, Extruded-2 alloys and ascast Mg, indicating the presence of two capacitance loops. Whereas, one wave trough can be seen for Extruded-3 at the low frequency regions from 10 Hz to 0.1 Hz, suggesting the boosted pitting. In order to further analyze the degradation behaviors of the extruded alloys, the EIS spectra were analyzed and the fitting results of the EIS spectra are presented in Table 4. In addition, the equivalent circuit of the EIS spectra and its model were established, as shown in Fig. 8(a) and (b), respectively. Fig. 8 illustrates that Rs represents the solution resistance [38]; CPE1 and CPE2 are the constant phase elements in parallel with the resistive elements, and each constant phase element has a capacitance (designated for example as C1.T and C2.T) and an associated phase angle (designated for example as n1 and n2) [39]. Rtp represents the charge transfer resistance associated with the micro-galvanic events, Rfp is the resistance in the film above the micro-galvanic events and L indicates the associated inductive element in parallel. Rtp and CPE1 are used to describe the first capacitance loop in high frequency [40]. In general, a higher value of Rtp indicates a lower degradation rate of Mg matrix. According to the fitting results in Table 4, the values of Rtp reduce in the order of Extruded-3 N Extruded-2 N Extruded1 N as-cast Mg, implying the increase of degradation rate. However, the higher value of L for Extruded-3 is an indication of boosted localized corrosion (pitting). The degradation of Mg in aqueous environment involves the electrochemical proceeds. At the initial stage of degradation, some sites are preferred for attack such as the matrix around particles or second phase because of the micro-galvanic effects. It can be seen from Fig. 7 that although each extruded alloy has the localized corrosion during the immersion in SBF, the surface layer on Extruded-1, Etruded-2 and as-cast Mg still has partial protection as shown in the medium frequency capacitance loop. However, Extruded-3 alloy indicates relatively severe pitting compared with the other extruded alloys as shown in the higher L and lower Rfp. With prolonged immersion time, Mg matrix close to the second phase or impurities may fall off the surface due to the galvanic effects, as shown in the model of Fig. 8(b), resulting in more defects on the surface and leading to breakages in the degradation layer, which could accelerate the pitting corrosion, and lead to the high overall degradation rate.
Fig. 7. EIS spectra of the extruded Mg alloys and cast pure Mg in SBF for 1 h: (a) Nyquist plots, (b) Bode plots of |Z| vs. frequency and (c) Bode plots of phase (Z) degree vs. frequency.
and one low frequency inductive loop. While, Extruded-3 inhibits the degradation with one high to medium frequency capacitance loop and one medium to low frequency inductive loop. The diameter of the inductive loop for Extruded-3 is larger than those for the other alloys and as-
3.3.3. Biodegradation rates of the extruded alloys In this study, the hydrogen evolution, polarization and EIS have been used to evaluate the biodegradation rate of the extruded alloys and ascast pure Mg. The hydrogen evolution and polarization curves indicate that Extruded-2 exhibits the lowest degradation rate among the extruded alloys and as-cast pure Mg. However, the biodegradation rate derived from the hydrogen evolution is much higher than that of the polarization curves. Unlike the polarization and EIS, hydrogen evolution is unaffected by the formation of corrosion products (surface layer), and it shows the average degradation rate of Mg alloys against the immersion time. In general, samples are immersed in SBF for at least 12 h or even longer for hydrogen evolution tests. During the immersion, hydrogen evolution is significantly influenced by the cathodic reaction [41]. Hence, galvanic effects between second phase or impurities and Mg
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Table 4 Fitting results of the EIS spectra. Specimen
Extruded-1 Extruded-2 Extruded-3 As-cast Mg
Rs
Rtp
C1.T
(Ω cm2)
(Ω cm2)
(10−6F cm−2)
29.43 30.27 26.28 29.24
149.60 154.60 172.40 45.86
62.77 73.49 85.26 198.60
matrix greatly affect the hydrogen evolution rate during the immersion time. It should be considered that EIS provides only partial information regarding the corrosion kinetics, and it cannot exhibit the influence of the microstructural features such as second phase, impurities and grain size on the corrosion potential which links to the second phase and Mg matrix [42]. Moreover, EIS also cannot directly imply an accurate corrosion rate. For some chemical reactive Mg alloys in SBF, a low frequency drift is especially apparent attributing to the relatively rapid degradation rate. This rapid degradation makes low frequency measurements difficult because the active reactions occur continuously during the EIS tests [39]. Polarization tests for Mg alloys allow a single scan and it can take as little as 5 min to complete. It should be noted that in actual measurement, most Mg alloys degraded uniformly. The priority condition for polarization tests is that Mg samples have the general corrosion. Subsequently, polarization cannot show an absolute degradation rate or average degradation rate, but it is an indication of degradation at a select point in time using the calculated current density [43]. In this study, the extrude Mg alloys and as-cast Mg were immersed in SBF for
n1
C2.T
n2
(10−6 F m−2) 0.8194 0.8159 0.8590 0.7469
484.16 940.51 805.42 776.67
0.1871 0.1937 0.1036 0.1855
Rfp
L
(Ω cm2)
(H cm−2)
30.29 37.44 28.51 30.98
15.73 15.59 37.79 27.68
more than 180 h. The results of hydrogen evolution provide the average degradation rate, which was greatly affected by the galvanic effects between the second phase and Mg matrix during the long immersion tests. Unlike the hydrogen evolution, polarization and EIS simply unravel the changes of extruded Mg alloys in short time (around 10 min in this study). The surfaces of Mg alloys in the short time may not change significantly even after 1 h of stabilization, which leads to a relatively lower degradation rate as compared to that of hydrogen evolution. The EIS result indicates that Extruded-3 has the higher charge transfer resistance, however, it has a relatively boosted pitting corrosion. The aggressive galvanic effects could greatly change the surface with the prolonged immersion time. Generally, the degradation is related to the ion release to the physically environment. The high degradation may produce an over burden of metallic ions for the human body, and even leads to early failure due to the loss of mechanical integrity. According to the results of hydrogen evolution and polarization tests, Extrude-2 exhibits a lower degradation rate compared with the other alloys and as-cast Mg, which indicates that released ion concentration from Extrude-2 is lower than that from the others. The Mg alloy implant after implantation would directly come in contact with the organics or tissues. The degradation of Mg alloys is a reaction between metals and a physiological environment such as proteins, cations and anions. In many cases, the biocompatibility of Mg alloys is determined by released ion of the alloying elements. The lower released ion concentration leads to better biocompatibility. Extruded-2 possesses better bio-corrosion resistance to SBF compared with the other alloys and as-cast Mg, which indicates the lowest released ion concentration from this alloy, considering it to be a promising candidature of implant materials. 3.4. In vitro cytotoxicity Fig. 9 shows the CVR of extruded alloys, defined as the ratio of the live cell number in extraction media of the extruded Mg alloys to that of in cell culture media without any Mg alloys (the control). It is noticeable that the extruded alloys display similar high cell growth factors (85, 96.1, and 86.5% for Extruded-1, Extruded-2, and Extruded-3, respectively) and Extruded-2 has the highest CVR value among the alloys.
Fig. 8. (a) Equivalent circuit of the EIS spectra; (b) corrosion model of extrude Mg alloys in SBF.
Fig. 9. Cell viability ratios (CVR) of the extruded alloys.
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It can be concluded that all the alloys are compatible, Nd content at a low concentration does not significantly influence the biocompatibility of extruded alloys, and Extruded-2 exhibits better biocompatibility than the other alloys. Those results are in good agreement with the corrosion behavior of extruded alloys since the biocompatibility of the biodegradable magnesium alloys can be affected by the corrosion resistance significantly and better corrosion resistance is expected to achieve better biocompatibility [44,45]. 4. Conclusions The microstructure, mechanical and corrosion properties and biocompatibility of extruded Mg–1Mn–2Zn–xNd alloys were investigated in this study. The conclusions are summarized as follows. All the extruded Mg–1Mn–2Zn–xNd alloys are composed of both α phase of Mg and a compound of Mg7Zn3, and they exhibit very fine microstructures, better ductility, and much higher mechanical strength as compared with cast pure Mg and natural bone. The tensile strength and elongation of the extruded alloys increase with the increase in Nd content. Their compressive strength slightly declines with Nd content. The extruded alloys have much better corrosion resistance than pure Mg and good biocompatibility. The extruded Mg–1Mn–2Zn–1.0Nd alloy shows a great potential for biomedical applications due to the combination of enhanced mechanical properties, high corrosion resistance and good biocompatibility. Acknowledgments The work was financially supported by the Special Funding for Science and Technology of Foshan City, Guangdong Province, China. (Grant No. 2011AA100101). Special gratitude is expressed to Professor Lei Wang from Northeastern University, China for his kind help during the hot-extrusion process. References [1] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728. [2] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, H. Windhagen, Biomaterials 26 (2005) 3557. [3] G. Song, Corros. Sci. 49 (2007) 1696. [4] Z. Li, X. Gu, S. Lou, Y. Zheng, Biomaterials 29 (2008) 1329. [5] W.-C. Kim, J.-G. Kim, J.-Y. Lee, H.-K. Seok, Mater. Lett. 62 (2008) 4146. [6] H.R. Bakhsheshi-Rad, M.H. Idris, M.R. Abdul-Kadir, S. Farahany, Mater. Des. 33 (2012) 88. [7] S. Zhang, X. Zhang, C. Zhao, J. Li, Y. Song, C. Xie, H. Tao, Y. Zhang, Y. He, Y. Jiang, Y. Bian, Acta Biomater. 6 (2010) 626. [8] B. Zhang, Y. Hou, X. Wang, Y. Wang, L. Geng, Mater. Sci. Eng. C 31 (2011) 1667.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Microstructures, mechanical and corrosion properties and biocompatibility of as extruded Mg–Mn–Zn–Nd alloys for biomedical applications Ying-Long Zhou a,⁎, Yuncang Li b, Dong-Mei Luo c, Yunfei Ding b, Peter Hodgson b a b c
Department of Mechatronics Engineering, Foshan University, Foshan 528000, Guangdong, China Institute for Frontier Materials, Deakin University, Victoria 3217, Australia Department of Civil Engineering, Foshan University, Foshan 528000, Guangdong, China
a r t i c l e
i n f o
Article history: Received 24 March 2014 Received in revised form 22 October 2014 Accepted 17 December 2014 Available online 18 December 2014 Keywords: Mg–Mn–Zn–Nd alloy Microstructure Mechanical property Corrosion behavior Extrusion
a b s t r a c t Extruded Mg–1Mn–2Zn–xNd alloys (x = 0.5, 1.0, 1.5 mass %) have been developed for their potential use as biomaterials. The extrusion on the alloys was performed at temperature of 623 K with an extrusion ratio of 14.7 under an average extrusion speed of 4 mm/s. The microstructure, mechanical property, corrosion behavior and biocompatibility of the extruded Mg–Mn–Zn–Nd alloys have been investigated in this study. The microstructure was examined using X-ray diffraction analysis and optical microscopy. The mechanical properties were determined from uniaxial tensile and compressive tests. The corrosion behavior was investigated using electrochemical measurement. The biocompatibility was evaluated using osteoblast-like SaOS2 cells. The experimental results indicate that all extruded Mg–1Mn–2Zn–xNd alloys are composed of both α phase of Mg and a compound of Mg7Zn3 with very fine microstructures, and show good ductility and much higher mechanical strength than that of cast pure Mg and natural bone. The tensile strength and elongation of the extruded alloys increase with an increase in neodymium content. Their compressive strength does not change significantly with an increase in neodymium content. The extruded alloys show good biocompatibility and much higher corrosion resistance than that of cast pure Mg. The extruded Mg–1Mn–2Zn–1.0Nd alloy shows a great potential for biomedical applications due to the combination of enhanced mechanical properties, high corrosion resistance and good biocompatibility. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, Mg alloys have attracted increasing attention as biodegradable implant materials due to their good biocompatibility, low density and elastic modulus which are closer to those of human bone as compared with the other metallic biomaterials (stainless steel, titanium and its alloys), and much higher mechanical strength than the biodegradable polymers. The main disadvantages of current Mg alloys, however, are that they corrode too rapidly in physiological environment and lose mechanical integrity before the tissue has healed, and that the hydrogen pockets produced in corrosion process retard the tissue healing [1–3]. Therefore, it is desirable to improve the corrosion rate of Mg alloys so that they corrode at an appropriate rate and hydrogen pockets can be absorbed by the surrounding tissue without negative effect. Element alloying is one of the most effective measures to improve the corrosion resistance of Mg alloys. For bio-Mg alloys, non-toxic alloying elements are essential, so calcium (Ca), zinc (Zn), manganese (Mn) and low toxicity rare earth (RE) elements at a low concentration ⁎ Corresponding author. E-mail address: [email protected] (Y.-L. Zhou).
http://dx.doi.org/10.1016/j.msec.2014.12.057 0928-4931/© 2014 Elsevier B.V. All rights reserved.
are suggested as alloying elements for biomedical applications in previous investigations [1,3]. Accordingly, many bio-Mg alloys, such as Mg–Ca [4–6], Mg–Zn [7], Mg–Zn–Ca [8], Mg–Zn–Mn [9,10] and Mg–Zn–Mn–Ca alloys [11,12], have been developed for biomedical applications. Those alloys provide much better strength and corrosion resistance than pure Mg. However, bio-Mg alloys must possess certain mechanical and biodegradable properties to serve as suitable stent materials (specifically in cardiovascular applications) [13]: (i) appropriate mechanical characteristics at body temperature, such as sufficient ductility (a uniform strain optimally ≥ 15%), and reasonable strength (ultimate tensile strength (UTS) ≥ 250 MPa); (ii) a moderate and homogeneous degradation performance (biodegradable period of 12–24 months); (iii) good biocompatibility. Based on those requirements, the above bio-Mg alloys cannot be regarded as ideal biodegradable materials for biomedical applications. For example, the tensile strength and elongation of Mg–Ca alloys are low for the load-bearing applications and the corrosion resistance also needs to be improved furthermore [4–6,12]; the bending strength of Mg– Zn alloy deteriorates rapidly at the initial corrosion stage from 625 MPa to 390 MPa with about 6% loss of weight during degradation [7]; the Mg–Zn–Ca alloys have inadequate mechanical properties
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(UTS b 185 MPa) [8]; and the Mg–Zn–Mn–Ca alloys have good corrosion resistance but inadequate mechanical properties (UTS b 190 MPa) [11,12]. Therefore, optimization of alloying element and composition of bio-Mg alloys to obtain new biodegradable Mg alloys with appropriate mechanical properties, corrosion resistance, and biocompatibility is very crucial for biomedical applications. Ca element has been proven to not be an effective element to increase the strength and corrosion resistance of Mg alloys [4–6,12]. Mn element has the function of refining grain size and improving the tensile strength of magnesium alloy [14]. Zn element is next to aluminum in strengthening effectiveness as an alloying element in magnesium [9,15] and adding Zn can improve both the tensile strength and the corrosion resistance of Mg alloys [16]. In addition, RE elements can purify Mg alloys and accordingly improve their corrosion behavior and mechanical properties [17]. Neodymium (Nd) is one of the light RE elements with maximum solubility in solid Mg of 3.6% at eutectic temperature [18]. Recent research indicates that a small amount of Nd does not exhibit cell toxicity [19]. Therefore, Mg–Zn–Mn alloys with a low concentration of Nd are expected to be good candidates for biomedical application due to their promising good mechanical properties, corrosion resistance and biocompatibility. Few reports on Mg–Zn–Mn–Nd alloys are available at present. In this study, Mg–1Mn–2Zn–xNd alloys (x = 0.5, 1.0, 1.5; mass%, hereafter) have been designed on the basis of the above consideration and their various properties have been investigated to check their potential use in biomedical applications. Since the previous investigations [20–24] suggest that the extrusion can greatly refine the microstructures of Mg alloys, resulting in a remarkable improvement of the mechanical and corrosion properties, the extrusion was also performed on the designed Mg–1Mn–2Zn–xNd alloys to further improve their properties. 2. Experimental 2.1. Material preparation The designed Mg–1Mn–2Zn–xNd alloys (x = 0.5, 1.0, 1.5%) were fabricated from high purity Mg (99.99%), high purity Zn (99.99%), Mg–10Mn (99.98%), and Mg–25Nd (99.97%) master alloys. High purity Mg and Zn were first placed in a high-purity graphite crucible, and the master alloys were then added when the temperature reached 1053 K. After melting at 1023–1053 K for 30 min, the melt was cast into a steel mold at approximate 1003 K. The cast ingots were then homogenized at 673 K for 24 h with furnace cooling. The extrusion was performed at a temperature of 623 K with an extrusion ratio of 14.7 and an extrusion diameter of 12 mm under an average extrusion speed of 4 mm/s. The chemical compositions of the designed alloys were analyzed by X-ray fluorescence spectrometric method, as listed in Table 1. 2.2. Microstructural characterization The phase constitution of the alloys was determined using X-ray diffraction (XRD) analysis. For metallographic observation, the specimens were examined by optical microscopy (OM) after they were ground with SiC emery papers of up to 3000 grit, polished with 0.5 μm diamond powder, and then etched with a solution of 1.5 g picric acid, 10 ml acetic acid, 10 ml distilled water add 25 ml ethanol. Table 1 Chemical compositions of the alloys (mass %). Alloy code
Mn
Zn
Nd
Ni
Fe
Al
Mg
Mg–1Mn–2Zn–0.5Nd Mg–1Mn–2Zn–1.0Nd Mg–1Mn–2Zn–1.5Nd
1.29 1.23 1.21
2.42 2.31 2.2
0.28 0.81 1.21
0.01 0.02 0.02
0.02 0.02 0.03
– 0.03 0.03
Bal. Bal. Bal.
2.3. Mechanical property test Following ASTM E8-04 [25], the tensile samples (25 mm in gauge length, 6.35 mm in width, and 5 mm in thickness) were machined from the extruded bars along the longitudinal direction. The tensile tests were conducted at a crosshead speed of 1 mm/min at room temperature in air using an Instron type machine (Instron 5900, Instron Corporation, USA). According to ASTM E9-09 [26], the compressive samples (27 mm in gauge length, 11 mm in diameter) were machined from the extruded bars along the longitudinal direction. The compressive tests were conducted using the same Instron machine at a crosshead speed of 0.0025 mm/s at room temperature. The ultimate tensile strength (UTS), 0.2% yield strength (YS), elongation to failure (Ef), compressive strength (CS), and compressive yield strength (CYS) were obtained based on the average of three test samples. 2.4. Electrochemical measurement Electrochemical impedance spectroscopy (EIS) and polarization curves of the alloys were measured in the simulative body fluid (SBF) at 37 ± 0.5 °C. The SBF was prepared through dissolving the following chemicals in 1 L deionized water sequentially: NaCl (5.403 g), NaHCO3 (0.504 g), Na2CO3 (0.426 g), KCl (0.225 g), K2HPO4·3H2O (0.230 g), MgCl2·6H2O (0.311 g), CaCl2 (0.293 g) and Na2SO4 (0.072 g). The solution was buffered to pH 7.40 with HEPES and 1 M NaOH at 37 °C [27]. For both measurements, disc samples with a diameter of 10 mm and thickness of 2 mm were used for the electrochemical tests. All the samples were connected with a copper wire and then mounted in epoxy resin as the working electrode. Before the electrochemical test, the mounted samples were successively polished with 240, 600 and 1200-grit sand papers, then carefully degreased with acetone, ethanol and rinsed with distilled water, finally dried in a stream of warm air. The open circuit potential and the potentiodynamic polarization curve of samples soaked in SBF were measured by electrochemical station (1470E Multichannel Potentiostat, Solartron, UK) equipped with Multistate software. A three-electrode cell system with a saturated calomel electrode (SCE), a 1.5 × 1.5 cm2 platinum electrode and the sample mounted in epoxy resin as reference electrode, count electrode, and the working electrode, respectively was used in this study. Before the measurement, the working electrode was immersed in test solution for stability. The corrosion density current (icorr) was calculated using CView software. EIS measurement had begun after stabilization of the open circuit potential for an hour. The polarization curve measurement was subsequently conducted with a scan rate of 1 mV/s after the EIS measurement. The polarization started from a cathodic potential of − 50 mV relative to the open circuit potential and stopped at an anodic potential where the anodic current increased dramatically. 2.5. Cytotoxicity test Osteoblast-like cells (SaOS2) are a human osteosarcoma cell line with osteoblast properties [28] and were used to evaluate the biocompatibility of Mg–1Mn–2Zn–xNd alloys using the indirect contact method according to ISO 10993-5 [29,30]. Briefly, Mg alloys were immersed in cell culture media with a surface area to media ratio of 0.6 cm2/ml in a humidified atmosphere with 5% CO2 at 37 °C for 72 h. The supernatant fluid was withdrawn and filter-sterilized with a 0.22-μm filter (Falcon, BD Biosciences, San Jose, CA, USA) to obtain the extracts of Mg alloys. The control groups involved the use of cell culture medium as negative controls. SaOS2 cells were mixed with media at a cell density of 5 × 104 cells/ml. A mixture of 200 μl was transferred into a well of 48-well cell culture plate. The cell culture plate was placed in an incubator at a humidified atmosphere with 5% CO2 at 37°C for 24 h to allow cells attachment. The media in each well was then sucked out and an extract of 200 μl was refilled in each well. After further incubation of the cells for 24 h, the 48-well cell culture plates, MTS assay, a
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colorimetric assay [30], was used to measure the cell number in each well through absorbance reading at 490 nm using a spectrophotometer (GENios Pro, Tecan, Mannedorf, Switzerland). The cell viability ratio (CVR) for every Mg alloy was defined as the ratio of the live cell number in extraction media to that in the control, which can be calculated using the equation: CVR = absorbance reading in Mg alloy extract / absorbance reading in control. 3. Results and discussion 3.1. Microstructural characteristics The XRD patterns of extruded Mg–1Mn–2Zn–xNd alloys are shown in Fig. 1 (Extruded-1, Extruded-2, and Extruded-3 were used to represent the extruded alloys with 0.5 Nd, 1.0 Nd, and 1.5 Nd, respectively, hereafter). The strongest peak for all the extruded alloys is (100) while that of Mg (α) phase is (101) according to the standard XRD patterns of Mg (powder), which is due to the obvious texture caused by the extrusion process. As compared with the XRD peaks of as cast Mg, it can be observed that all the extruded Mg alloys are composed of both α phase of Mg and a compound of Mg7Zn3. Although 2% Zn can fully dissolve in α Mg matrix by the Mg–Zn binary phase diagram [18], the solubility of Zn in α Mg decreases and the compound may produce when the other alloying elements are added. This is related to the formation of compound Mg7Zn3 in the Mg–1Mn–2Zn, Mg–1Mn–3Zn alloys [9] and the Mg–1Mn–2Zn–xNd alloys in this study. It also can be seen that the peak values of Mg7Zn3 slightly increase with addition of Nd content, which suggests the increase of volume fraction of Mg7Zn3 with Nd content. Since the solubility of alloying elements in α Mg at room temperature is limited due to the supersaturation, more Mg7Zn3 precipitates naturally with the increase in Nd content. Figs. 2 and 3 show the microstructures of cross-sections and longitudinal sections of the extruded Mg–1Mn–2Zn–xNd alloys, respectively. It can be observed that the extruded Mg alloys exhibit very fine microstructures, which is attributed to the phenomenon of recrystallization during the hot-extrusion. In the cross-section microstructures (Fig. 2), a typical equiaxed microstructure is observed in all the alloys. In the longitudinal section microstructures (Fig. 3), a similar equiaxed grain structure and a few unrecrystallized bands indicated by red arrows
Fig. 2. OM microstructures of the cross-sections of extruded Mg–1Mn–2Zn–xNd alloys.
along the longitudinal direction are observed, which is related to the above texture detected by XRD analysis. The particulate of Mg7Zn3 is invisible in Figs. 2 and 3, which is related to their very fine structure and the limited resolution of OM. 3.2. Mechanical properties
Fig. 1. XRD patterns of extruded Mg–1Mn–2Zn–xNd alloys and cast pure Mg.
The mechanical properties of the extruded Mg–1Mn–2Zn–xNd alloys are shown in Fig. 4. It can be noticed that the extruded Mg– 1Mn–2Zn–xNd alloys exhibit much higher mechanical strength than as cast pure Mg, which is due to the solid solution strengthening by alloying elements and the working hardening by hot extrusion.
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Fig. 4. (a) Tensile properties and (b) compressive properties of extruded Mg–1Mn–2Zn– xNd alloys.
increase with the increasing of Nd content, which is associated with their very fine microstructures as shown in Figs. 2 and 3 since fine grain structure can improve both the strength and ductility of metallic materials at the same time (fine grain strengthening theory). Their compressive strength, however, slightly decreases with the increase in Nd content. This is possibly related to that the precipitation of Mg7Zn3, which increases with the increase in Nd content, destroys the continuity of microstructures of the Mg alloys and becomes a crack source. During
Fig. 3. OM microstructures of the longitudinal sections of extruded Mg–1Mn–2Zn–xNd alloys. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
As compared with the extruded Mg–1Mn–2Zn alloy [9], the extruded Mg–1Mn–2Zn–xNd alloys show slightly higher tensile strength, which suggests that Nd is not a highly effective element to increase the strength of Mg–Mn–Zn alloys. It is normal that the extruded Mg–1Mn–2Zn–xNd alloys have smaller elongation than the Mg–1Mn–2Zn alloy since it is consistent with the ordinarily observed strength–ductility relationship, i.e., when the yield strength increases, the ductility accordingly decreases. However, there is a big difference in elongation between the Mg–1Mn–2Zn–xNd alloys and the Mg–1Mn–2Zn alloy. The reason for this big difference is unclear and further investigation is necessary. The tensile strength and elongation of extruded Mg–1Mn–2Zn–xNd alloys
Fig. 5. Hydrogen evolution of the extruded alloys and cast pure Mg with immersion time.
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the compressive test, when the compressive stress increases, the crack source is more sensitive to the compressive stress than the tensile stress. Further study is necessary to explore the detailed reason for this phenomenon. Those results show that the extruded Mg–1Mn–2Zn–xNd alloys provide good ductility (elongation to failure ≥ 14%) and much higher mechanical strength (UTS N 285 MPa and CS N 395 MPa) than natural bone (UTS: 90–140 MPa [31] and CS: 100–230 MPa [12]), thus they show great potential for biomedical applications according to the requirement of mechanical properties of bio-Mg alloys [13]. 3.3. Corrosion property 3.3.1. Immersion corrosion behavior of extruded alloys In the physiological environment, Mg and its alloys are relatively reactive metals. They degrade to release H2 gas and form corrosion products such as Mg(OH)2 and calcium phosphates. For use as biomaterials, they should maintain a suitable degradation rate and mechanical integrity before the tissue healing. Fig. 5 shows the hydrogen evolution tendency of extruded alloys and pure Mg (as control) during immersion in SBF solution. It can be observed that hydrogen gases increase with immersion time in SBF for all the extruded alloys and pure Mg, and the extruded alloys continuously produce less hydrogen gas than cast pure Mg. Extruded-2 alloy shows the lowest hydrogen evolution rate among the extruded alloys and cast pure Mg. The corrosion rate Ph mm/y was determined by the hydrogen evolution rate Vh ml/(cm2 day) using the equation [32,33]. Ph ¼ 2:279Vh:
ð1Þ
In this study, the samples were immersed in SBF for 185 h. The hydrogen production rate and corrosion rate for the extruded alloys and cast pure Mg are summarized in Table 2. It can be seen that Extruded2 exhibits the lowest corrosion rate among the samples. 3.3.2. Electrochemical corrosion behavior of the extruded alloys Fig. 6 shows the electrochemical polarization curves for the specimens after immersion in SBF solution. Both cathodic and anodic polarization curves present the same trend for the alloys and pure Mg. The corrosion potential (Ecorr), corrosion current density (icorr), cathodic Tafel slope (βc), anodic Tafel slope (βa), and corrosion rate of the extruded alloys and cast pure Mg, which were extracted from the polarization curves (Fig. 6), are listed in Table 3. In addition to the above electrochemical parameters, the polarization resistance (RP) was calculated according to the following equation [34]. Rp ¼
βa βc 2:3ðβa þ βc Þicorr
ð2Þ
It can be seen that the extruded alloys exhibit more positive corrosion potential, much smaller current densities and lower corrosion rate than cast pure Mg, which indicates that the extruded alloys exhibit much better corrosion resistance than cast pure Mg. The extruded alloys have close RP values, however, much larger than that of cast pure Mg. There is no significant difference in the electrochemical parameters among the extruded alloys, which indicates that addition of Nd content Table 2 Hydrogen evolution and corrosion rates of Mg–1Mn–2Zn–xNd alloys and pure Mg in SBF. Samples
H2 production rate ml/(cm2 day)
Corrosion ratea mm/year
Pure Mg Extruded-1 Extruded-2 Extruded-3
2.57 2.37 2.16 2.47
5.86 5.40 4.93 5.64
a
Corrosion rate was calculated from H2 production rate.
Fig. 6. Polarization curves in SBF of the extruded alloys and cast pure Mg.
does not effectively alter the corrosion resistance of extruded Mg–Zn– Mn–Nd alloys. For better understanding, the electrochemical parameters of as cast Mg–Zn–Mn–Nd alloys are also provided in Table 3. It can be observed that the cast alloys exhibit more positive corrosion potential, smaller current density and lower corrosion rate than those of cast pure Mg, respectively. However, the corrosion potential, current density, and corrosion rate of the cast alloys are close to one another. This indicates that addition of Nd content does not effectively influence the corrosion resistance of cast Mg–Zn–Mn–Nd alloys. Elements, Zn, Mn, and Nd, can purify Mg alloys by elimination of the harmful corrosive effects from the impurity elements and accordingly enhance corrosion resistance of the Mg alloys [17,35]. This results in better corrosion resistance of as cast Mg–Zn–Mn–Nd alloys than cast pure Mg. It can also be seen that the extruded alloys exhibit a much smaller current density and lower corrosion rate than those of the cast alloys, respectively, which indicates that the extrusion process can greatly enhance the corrosion resistance of Mg–Zn–Mn–Nd alloys. Previous studies show that fine grain structure can create more grain boundaries that act as corrosion barriers and accordingly enhance the corrosion resistance of Mg alloys [20–24,30,36,37]. This helps to understand why the extruded alloys have much better corrosion resistance than the cast alloys. Therefore, the as extruded Mg–Zn–Mn–Nd alloys exhibit much better corrosion resistance than cast pure Mg due to both the effects of alloying elements (Zn, Nd, and Mn) and the extrusion process. Fig. 7 shows the EIS spectra for the extruded Mg alloys and as-cast Mg in SBF. As can be seen from Fig. 7(a), Extruded-1, Extruded-2 and as-cast Mg have similar degradation behaviors during the immersion in SBF after 1 h. Extruded-1, Extruded-2 and as-cast pure Mg contain one high frequency capacitance loop, one medium frequency capacitance
Table 3 Electrochemical parameters of Mg–1Mn–2Zn–xNd alloys and pure Mg. Sample code
Ecorr (V)
icorr (μA/cm2)
βc (mV/dec)
βa (mV/dec)
Rp (kΩ cm2)
Corrosion rate (mm/year)
Extruded-1 Extruded-2 Extruded-3 Pure Mg Cast-1 Cast-2 Cast-3
−1.469 −1.489 −1.495 −1.915 −1.546 −1.502 −1.534
50.16 26.69 33.48 177.8 112.22 107.61 79.43
146.7 81.1 90.9 89.4 102.8 144.2 139.2
115.1 64.2 84.5 88.7 93.6 114.4 123.2
0.55 0.58 0.56 0.11 0.19 0.26 0.35
1.46 0.78 0.97 3.86 3.26 3.12 2.31
Note: Cast-1, Cast-2, and Cast-3 were used to represent the cast Mg–1Mn–2Zn–xNd alloys with 0.5, 1.0, and 1.5 Nd, respectively.
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cast Mg. In addition, the charge transfer resistance for the extruded Mg alloys and as-cast Mg reduces in the order of Extruded-3 N Extruded-2 N Extruded-1 N as-cast Mg according to the intersection of the capacitance loops with the x-axis. In the case of Bode plots |Z| vs. frequency, as-cast Mg exhibits the lowest impedance values from the high frequency to low frequency. Extruded Mg alloys show an increase of impedance in the medium frequency, however, the impedance values reduce at the low frequency regions for all the extruded Mg alloys. Compared to Extruded-1 and Extruded-2, Extruded-3 presents a sharp decrease of impedance in the low frequency, indicating the severe pitting. The slope of plots for the extruded Mg alloys in the region of frequency from 100 Hz to 10 Hz is close to −1, and the plots in the high frequency from 100 kHz to 100 Hz exhibit a slight incline, which suggests the presence of capacitor. As for the Bode plots of phase (Z) degree vs. frequency, two wave crests are visible for Extruded-1, Extruded-2 alloys and ascast Mg, indicating the presence of two capacitance loops. Whereas, one wave trough can be seen for Extruded-3 at the low frequency regions from 10 Hz to 0.1 Hz, suggesting the boosted pitting. In order to further analyze the degradation behaviors of the extruded alloys, the EIS spectra were analyzed and the fitting results of the EIS spectra are presented in Table 4. In addition, the equivalent circuit of the EIS spectra and its model were established, as shown in Fig. 8(a) and (b), respectively. Fig. 8 illustrates that Rs represents the solution resistance [38]; CPE1 and CPE2 are the constant phase elements in parallel with the resistive elements, and each constant phase element has a capacitance (designated for example as C1.T and C2.T) and an associated phase angle (designated for example as n1 and n2) [39]. Rtp represents the charge transfer resistance associated with the micro-galvanic events, Rfp is the resistance in the film above the micro-galvanic events and L indicates the associated inductive element in parallel. Rtp and CPE1 are used to describe the first capacitance loop in high frequency [40]. In general, a higher value of Rtp indicates a lower degradation rate of Mg matrix. According to the fitting results in Table 4, the values of Rtp reduce in the order of Extruded-3 N Extruded-2 N Extruded1 N as-cast Mg, implying the increase of degradation rate. However, the higher value of L for Extruded-3 is an indication of boosted localized corrosion (pitting). The degradation of Mg in aqueous environment involves the electrochemical proceeds. At the initial stage of degradation, some sites are preferred for attack such as the matrix around particles or second phase because of the micro-galvanic effects. It can be seen from Fig. 7 that although each extruded alloy has the localized corrosion during the immersion in SBF, the surface layer on Extruded-1, Etruded-2 and as-cast Mg still has partial protection as shown in the medium frequency capacitance loop. However, Extruded-3 alloy indicates relatively severe pitting compared with the other extruded alloys as shown in the higher L and lower Rfp. With prolonged immersion time, Mg matrix close to the second phase or impurities may fall off the surface due to the galvanic effects, as shown in the model of Fig. 8(b), resulting in more defects on the surface and leading to breakages in the degradation layer, which could accelerate the pitting corrosion, and lead to the high overall degradation rate.
Fig. 7. EIS spectra of the extruded Mg alloys and cast pure Mg in SBF for 1 h: (a) Nyquist plots, (b) Bode plots of |Z| vs. frequency and (c) Bode plots of phase (Z) degree vs. frequency.
and one low frequency inductive loop. While, Extruded-3 inhibits the degradation with one high to medium frequency capacitance loop and one medium to low frequency inductive loop. The diameter of the inductive loop for Extruded-3 is larger than those for the other alloys and as-
3.3.3. Biodegradation rates of the extruded alloys In this study, the hydrogen evolution, polarization and EIS have been used to evaluate the biodegradation rate of the extruded alloys and ascast pure Mg. The hydrogen evolution and polarization curves indicate that Extruded-2 exhibits the lowest degradation rate among the extruded alloys and as-cast pure Mg. However, the biodegradation rate derived from the hydrogen evolution is much higher than that of the polarization curves. Unlike the polarization and EIS, hydrogen evolution is unaffected by the formation of corrosion products (surface layer), and it shows the average degradation rate of Mg alloys against the immersion time. In general, samples are immersed in SBF for at least 12 h or even longer for hydrogen evolution tests. During the immersion, hydrogen evolution is significantly influenced by the cathodic reaction [41]. Hence, galvanic effects between second phase or impurities and Mg
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Table 4 Fitting results of the EIS spectra. Specimen
Extruded-1 Extruded-2 Extruded-3 As-cast Mg
Rs
Rtp
C1.T
(Ω cm2)
(Ω cm2)
(10−6F cm−2)
29.43 30.27 26.28 29.24
149.60 154.60 172.40 45.86
62.77 73.49 85.26 198.60
matrix greatly affect the hydrogen evolution rate during the immersion time. It should be considered that EIS provides only partial information regarding the corrosion kinetics, and it cannot exhibit the influence of the microstructural features such as second phase, impurities and grain size on the corrosion potential which links to the second phase and Mg matrix [42]. Moreover, EIS also cannot directly imply an accurate corrosion rate. For some chemical reactive Mg alloys in SBF, a low frequency drift is especially apparent attributing to the relatively rapid degradation rate. This rapid degradation makes low frequency measurements difficult because the active reactions occur continuously during the EIS tests [39]. Polarization tests for Mg alloys allow a single scan and it can take as little as 5 min to complete. It should be noted that in actual measurement, most Mg alloys degraded uniformly. The priority condition for polarization tests is that Mg samples have the general corrosion. Subsequently, polarization cannot show an absolute degradation rate or average degradation rate, but it is an indication of degradation at a select point in time using the calculated current density [43]. In this study, the extrude Mg alloys and as-cast Mg were immersed in SBF for
n1
C2.T
n2
(10−6 F m−2) 0.8194 0.8159 0.8590 0.7469
484.16 940.51 805.42 776.67
0.1871 0.1937 0.1036 0.1855
Rfp
L
(Ω cm2)
(H cm−2)
30.29 37.44 28.51 30.98
15.73 15.59 37.79 27.68
more than 180 h. The results of hydrogen evolution provide the average degradation rate, which was greatly affected by the galvanic effects between the second phase and Mg matrix during the long immersion tests. Unlike the hydrogen evolution, polarization and EIS simply unravel the changes of extruded Mg alloys in short time (around 10 min in this study). The surfaces of Mg alloys in the short time may not change significantly even after 1 h of stabilization, which leads to a relatively lower degradation rate as compared to that of hydrogen evolution. The EIS result indicates that Extruded-3 has the higher charge transfer resistance, however, it has a relatively boosted pitting corrosion. The aggressive galvanic effects could greatly change the surface with the prolonged immersion time. Generally, the degradation is related to the ion release to the physically environment. The high degradation may produce an over burden of metallic ions for the human body, and even leads to early failure due to the loss of mechanical integrity. According to the results of hydrogen evolution and polarization tests, Extrude-2 exhibits a lower degradation rate compared with the other alloys and as-cast Mg, which indicates that released ion concentration from Extrude-2 is lower than that from the others. The Mg alloy implant after implantation would directly come in contact with the organics or tissues. The degradation of Mg alloys is a reaction between metals and a physiological environment such as proteins, cations and anions. In many cases, the biocompatibility of Mg alloys is determined by released ion of the alloying elements. The lower released ion concentration leads to better biocompatibility. Extruded-2 possesses better bio-corrosion resistance to SBF compared with the other alloys and as-cast Mg, which indicates the lowest released ion concentration from this alloy, considering it to be a promising candidature of implant materials. 3.4. In vitro cytotoxicity Fig. 9 shows the CVR of extruded alloys, defined as the ratio of the live cell number in extraction media of the extruded Mg alloys to that of in cell culture media without any Mg alloys (the control). It is noticeable that the extruded alloys display similar high cell growth factors (85, 96.1, and 86.5% for Extruded-1, Extruded-2, and Extruded-3, respectively) and Extruded-2 has the highest CVR value among the alloys.
Fig. 8. (a) Equivalent circuit of the EIS spectra; (b) corrosion model of extrude Mg alloys in SBF.
Fig. 9. Cell viability ratios (CVR) of the extruded alloys.
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It can be concluded that all the alloys are compatible, Nd content at a low concentration does not significantly influence the biocompatibility of extruded alloys, and Extruded-2 exhibits better biocompatibility than the other alloys. Those results are in good agreement with the corrosion behavior of extruded alloys since the biocompatibility of the biodegradable magnesium alloys can be affected by the corrosion resistance significantly and better corrosion resistance is expected to achieve better biocompatibility [44,45]. 4. Conclusions The microstructure, mechanical and corrosion properties and biocompatibility of extruded Mg–1Mn–2Zn–xNd alloys were investigated in this study. The conclusions are summarized as follows. All the extruded Mg–1Mn–2Zn–xNd alloys are composed of both α phase of Mg and a compound of Mg7Zn3, and they exhibit very fine microstructures, better ductility, and much higher mechanical strength as compared with cast pure Mg and natural bone. The tensile strength and elongation of the extruded alloys increase with the increase in Nd content. Their compressive strength slightly declines with Nd content. The extruded alloys have much better corrosion resistance than pure Mg and good biocompatibility. The extruded Mg–1Mn–2Zn–1.0Nd alloy shows a great potential for biomedical applications due to the combination of enhanced mechanical properties, high corrosion resistance and good biocompatibility. Acknowledgments The work was financially supported by the Special Funding for Science and Technology of Foshan City, Guangdong Province, China. (Grant No. 2011AA100101). Special gratitude is expressed to Professor Lei Wang from Northeastern University, China for his kind help during the hot-extrusion process. References [1] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728. [2] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, H. Windhagen, Biomaterials 26 (2005) 3557. [3] G. Song, Corros. Sci. 49 (2007) 1696. [4] Z. Li, X. Gu, S. Lou, Y. Zheng, Biomaterials 29 (2008) 1329. [5] W.-C. Kim, J.-G. Kim, J.-Y. Lee, H.-K. Seok, Mater. Lett. 62 (2008) 4146. [6] H.R. Bakhsheshi-Rad, M.H. Idris, M.R. Abdul-Kadir, S. Farahany, Mater. Des. 33 (2012) 88. [7] S. Zhang, X. Zhang, C. Zhao, J. Li, Y. Song, C. Xie, H. Tao, Y. Zhang, Y. He, Y. Jiang, Y. Bian, Acta Biomater. 6 (2010) 626. [8] B. Zhang, Y. Hou, X. Wang, Y. Wang, L. Geng, Mater. Sci. Eng. C 31 (2011) 1667.
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