journal of the mechanical behavior of biomedical materials 29 (2014) 375–384
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Research Paper
Preparation, mechanical and degradation properties of Mg–Y-based microwire Qiuming Penga,n, Hui Fua, Junling Panga, Jinghuai Zhangb, Wenlong Xiaoc a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China b Key Laboratory of Superlight Materials & Surface Technology, Harbin Engineering University, Harbin 150001, PR China c School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China
art i cle i nfo
ab st rac t
Article history:
Mg–Y-based microwire which offers high strength in combination of low degradation rate
Received 14 June 2013
has been prepared for the first time by a modified melt extraction technique. A circular
Received in revised form
Mg–Y-based microwire is achieved with an extraction rate of 40 m/s, which is composed of
7 September 2013
Mg matrix and an amorphous phase, and exhibits higher basal texture than that of as-cast
Accepted 12 September 2013
sample. The outstanding tensile strength accompanying with an acceptable elongation is
Available online 12 October 2013
obtained with an extraction rate of 40 m/s. The improved strength is mainly attributed to
Keywords:
high solid solution strengthening, fine grain and the presence of an amorphous phase.
Mg biomaterials
In addition, the reduction of secondary phase and homogenous microstructure after melt
Microwire
extraction eliminate both pitting corrosion and micro-galvanic corrosion. A low degrada-
Microstructure
tion rate of 0.366 mm/y is attained in a simulated body fluid, which is less than 1/10 of that
Mechanical properties
of as-cast sample. These excellent mechanical properties and low degradation rate provide
Degradation properties
some prerequisites to develop bio-Mg implants. It reveals that this modified extraction technique is one of effective approaches to prepare microwire, which can be directly used for Mg-based stent self-assembly. & 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
The stent thrombosis and in-stent re-stenosis are still two bottlenecks in the applications of stents (Movahed et al., 2006). Two possible approaches have been proposed to deal with these shortcomings: the drug eluting and degradable stents (Waksman, 2006). Unfortunately, the following clinical tests indicated that the drug eluting stents are still facing the potential risk of late thrombosis (Hermawan et al., 2010). On the contrary, the implantation of degradable stents probably achieves the ideal restoration process of arterial n
Corresponding author. Tel.: þ86 335 8057047; fax: þ86 335 8074545. E-mail address: [email protected] (Q. Peng).
1751-6161/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2013.09.015
vessel, in which the implantation provides suitable mechanical support within a reasonable period for remodeling the artery and eliminates the long-term rejection reaction (Schranz et al., 2006). Thereafter, the development of degradation bio-Mg stents has been received worldwide attention increasingly. On the one hand, Mg alloys were found to be more suitable for load-bearing applications in contrast to ceramics or polymeric materials because of their good combination of mechanical strength and fracture toughness (Serre et al., 1993). On the other hand, the non-toxic oxides or hydroxides which formed during the degradation process
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journal of the mechanical behavior of biomedical materials 29 (2014) 375 –384
can enhance the activity of osteoblast and decrease the amount of osteoclast (Janning et al., 2010). Therefore, how to prepare high performance Mg stents becomes a critical issue in the field of bio-Mg implants. Mg stents are normally prepared by the strategy of other permanent metallic implants (Purnama et al., 2010). In brief, the as-cast ingot is manufactured to an empty pipe by extrusion technique or cold drawing method firstly, and then the pipe is machined into required dimension by a laser technique. However, this conventional process is not appropriate to bio-Mg stents. First, it is very difficult to prepare a uniform and concentric pipe owing to weak deformability of Mg alloys at low temperatures (Agnew et al., 2001; Bohlen et al., 2007). In addition, it is prone to generate an oxide film with a thickness of 50–100 μm during the machining process (Zeng et al., 2007), hence, the post-treatment, such as electrochemical corrosion process (Liu et al., 2011), is necessary to obtain a clean surface, resulting in the formation of inhomogeneous stents. As a result, leaving out of cost consideration, the conventional approach is not suitable to prepare high performance bio-Mg stents. In the present work, a modified melt extraction technique is proposed to prepare Mg-based microwires. A homogenous microwire is prepared from Mg melt with a high cooling rate, which possesses high mechanical properties in combination of good anti-corrosion properties. Hereafter, a bio-Mg stent can be achieved immediately by means of weaving technique according to the application requirements. The yttrium (Y) element has been recently reported as an effective alloying addition to prepare Mg-based biomaterial (Hanzi et al., 2010). Of particular importance is that the mechanical properties and anti-corrosion properties of Mg alloys can be improved by adding Y alloying element (Wu et al., 2012; Zhang et al., 2008). More attractively, the toxic range of Y is wide and its metabolic pathway is clear. For example, the Y-containing compound has been used for cancer treatment (Berman, 1980). Thus, Mg–Y based system is regarded as a potential one to manufacture bio-Mg stent. The objectives of this study are to clarify the effect of an extraction rate on the microwire configuration, and to investigate the mechnical and degradation properties of the microwire as compared with the as-cast samples. These results will provide some prerequisites to develop uniform bio-Mg stents in the future.
2.
Materials and methods
2.1.
Materials preparation
High purity Mg–7Y–0.2Zn alloy ingot (wt%, hereafter all compositions is given by wt%) was prepared by zone solidification method (Peng et al., 2010). The alloy was melt in a tantalum crucible under the protection by a mixture of CO2 and SF6. After holding at 700 1C for 1 h, the alloy was cast into a tantalum mould preheated at 620 1C. The filled tantalum mould was then held at 670 1C for 1 h under the protective gas. After that, the tantalum crucible with the melt was immersed into the cooling water at 30 mm/s. When the bottom of crucible touched the water, it stopped for 2 s. As soon as the liquid level of inside melt was alignment with the height of outside water, the solidification process was
terminated. The as-cast ingot with 80 mm in diameter and 180 mm in length was obtained. The ingot was machined into the samples with 10 mm in diameter and 20 mm in length for melt extraction. The microwires were fabricated by a modified melt extraction method. The representative schematic diagram is shown in Fig. 1 (Peng et al., 2013a). The rates of copper wheel were 20 m/s, 30 m/s and 40 m/s, respectively. In the meantime, the as-cast specimen was used for comparison.
2.2.
Microstructural characterization
The microstructural investigations were performed using scanning electron microscopy (SEM) The microwire for SEM observation were adhered into the conducting resin directly. The calorimetric response of the as-cast alloy and the mirowire with 40 m/s were measured using differential scanning calorimetry (DSC). A heating rate of 6 1C /min was employed under argon purge at 35 ml/min. The phase compositions were identified by X-ray diffraction with Cu Kα radiation at a scan rate of 21/min.
2.3.
Mechanical properties
The hardness test was carried out on Vickers hardness tester, the test load and dwelling time were 100 g and 15 s, respectively. Nine random spots were performed to calculate the average value. The rectangular specimens of as-cast state ingot (2 3 20 mm3) were used to measure tensile properties at room temperature with an initial strain rate of 1.7 10 3 s 1.
Fig. 1 – Schematic drawing of the modified melt extraction technique. The fabrication process is as follows: Mg molten alloy, heated in a quartz crucible, is pushed towards a highspeed rotating copper wheel and near circular wires were extracted instantaneously.
journal of the mechanical behavior of biomedical materials 29 (2014) 375 –384
The tensile properties of microwires were measured by a micro tensile testing machine under the same testing condition. The diameter of microwire which is meansured by SEM ranges from 30 to 50 μm and the length of microwire is 100 mm. The average results were calculated based on five samples. After tensile testing, the fracture surfaces were immediately observed by SEM.
2.4.
Electrochemical measurement
Electrochemical tests were carried out using a Bio-logic VSP potentiostat/frequency response analysis system to evaluate the electrochemical behaviors. Experiments were carried out in a three-electrode electrochemical cell, in which a saturated calomel electrode (SCE) as the reference electrode, a platinum mesh as counter electrode and the investigated specimen as the working electrode. The ion composition and preparation of simulated body fluid (SBF) were reported in previous result (Peng et al., 2012). In this study, the experiments were carried out in simulated body environment at ambient temperature. Briefly, the pH of SBF was 7.4. A SBF was poured into a tray and the whole samples after polishing are immersed in the simulated body fluid. During the whole test, the trays were located in an incubator. The temperature in the incubator was maintained at 37 1C. All experiments were repeated until three concurring replicates were obtained. Potentiodynamic polarization tests were performed at a scan rate of 0.5 mV/s. The polarization curves were used to estimate the corrosion and breakdown potentials (Ecorr, Ebp), and corrosion current density (icorr) at Ecorr by the Tafel extrapolation. In the Tafel extrapolation method, icorr is related to the average corrosion rate calculated by the equation (Pi ¼22.85icorr) (Song and StJohn, 2005). In the electrochemical impedance spectroscopy (EIS) measurements, the AC voltage signal amplitude was 10 mV (peak to zero). The EIS tests were carried out with the electrode in a steady state starting at an initial frequency of 100 kHz and stopping at a final frequency of 0.1 Hz. The number of points was 86. The samples were immersed in SBF solutions at different times, viz. 10 min, 20 min, 60 min and 180 min, to investigate the corrosion mechanism. All the measurements were taken immediately after the open circuit potential measurements were finished. The results were analyzed with
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the ZView2 software, from which the fitting parameters were determined.
3.
Results and discussion
3.1.
Microstructural characterization
The morphologies of the samples with different extraction rates are shown in Fig. 2, where the sample with 20 m/s (Fig. 2a) reveals a coarse surface. The width ranges from 30 to 57 μm. Meanwhile, a large number of grooves and fluctuations are detected on the surface. The morphology of the sample with 20 m/s is similar to the ribbon prepared by meltspinning sample (Yao et al., 2003). With increasing the extraction rate to 30 m/s, the surface becomes smooth and continuous in contrast to the sample by 20 m/s extraction rate, and the diameter is around 5275 μm. In addition, the double-layer stair-step morphology and some scratches are also observed. Note that the existence of these flaws (indicated by the arrows) results in a severe deterioration of their mechanical properties. When further increased the extraction rate to 40 m/s, the smooth and uniform microwire with a diameter of 4571 μm is achieved. The XRD patterns of the as-cast ingot and the microwires with different extraction rates are shown in Fig. 3. The as-cast
Fig. 3 – X-ray patterns of as-cast ingot and the microwire with different extraction rates.
Fig. 2 – Microstructures of samples with different extraction rates: (a) 20 m/s; (b) 30 m/s; (c) 40 m/s.
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ingot is mostly composed of Mg matrix and Mg24Y5 eutectic phase. With increasing the extraction rate, the volume fraction of secondary phase is decreased simultaneously. When the extraction rate reaches 40 m/s, only Mg peaks are observed in the microwire. It is worthy to note that the relative intensity of the peaks changes greatly. It indicates that the addition of yttrium can effectively reduce the basal texture, the strongest peak corresponds to (20–22)Mg pyramid plane in the as-cast specimen, which agrees well with the previous result (Agnew et al., 2001). Comparatively, the strongest peak in the microwire of 40 m/s is mainly related to (0 0 0 2)Mg basal plane, which reveals the texture is determined by not only the alloying elements but also the solidification cooling rate. The DSC curves of as-cast ingot and the microwires with different extraction rates are shown in Fig. 4. In the case of as-cast sample, a single peak (at 565710 1C) is observed during the heating and cooling processes. According to Mg–Y binary phase diagram, it can be confirmed this peak is related to Mg24Y5 eutectic phase. For the microwires of 20 and 30 m/s, some fluctuations are observed during the heating process, which are closely related to the variation of thermo. When the extraction rate reaches 40 m/s, this characteristic becomes more apparent. Namely, for the microwire of 40 m/s, a new exothermic peak at 282 1C is also observed in addition to the peak of Mg24Y5 eutectic phase, which is associated with the crystallization transformation. It is deduced that the amorphous phase was formed during the extraction process, and
it was changed to an ordered phase when heated to 282 1C (Huang et al., 2007; Shu et al., 2000). As mentioned above, the configuration was changed from the ribbon to circular microwire with increasing the extraction rate from 20 m/s to 40 m/s. At low extraction rates, the melt readily aggregates to form ball-shaped droplet owing to the surface tension role, which leads to form discontinuous and wide ribbon. With increasing the extraction rate, the extracted amount of melt per tangent length in the interface between the wheel and melt is decreased. Thus, the heat can be emitted rapidly. And then the melt is prone to form microwire with the help of surface tension role. This process is well confirmed in the melt-spun technique that a metallic glass can be prepared with increasing the wheel rate (Saida et al., 2000). Similarity, it is reasonable to believe that there exists a critical extraction rate to form a microwire since the sample possibly maintains the origin appearance if the extraction rate is over a certain value. This critical value of extraction rate will be further identified in the future.
3.2.
Mechanical properties
The microhardness of different samples at room temperature is listed in Table 1. The value of as-cast ingot is 68 HV. Compared with as-cast sample, the microwire exhibits higher microhardness. Moreover, the microhardness increased with the increase of extraction rate. For instance, the value
Fig. 4 – DSC curves of samples; (a) as-cast ingot; (b) the microwire with an extraction rate of 20 m/s; (c) the microwire with an extraction rate of 30 m/s; (d) the microwire with an extraction rate of 40 m/s.
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Table 1 – Mechanical properties of Mg–Y based alloys under at room temperature. Sample
State
HV
YS (MPa)
UTS (MPa)
ε (%)
Microwire
20 m/s 30 m/s 40 m/s As-cast Backward extrusion
8575 9274 9875 6874 8474
310720 360719 495720 14879 14972
442721 465725 541720 18978 246712
4.670.4 3.570.3 1.970.5 7.570.3 18.171.8
As-cast ingot Mg–8Y–0.5Zn (Peng et al., 2013b)
Fig. 5 – Typical tensile curves of as-cast ingot and the microwire with an extraction rate of 40 m/s at room temperature.
is 98 HV in the sample by 40 m/s, which is 1.15 times higher than that of the one by 20 m/s. The representative tensile curves of as-cast ingot and the microwires with differemt extraction rates are shown in Fig. 5. The tensile properties of the microwires with different extraction rates are summarized in Table 1. Both the yield strength (YS) and ultimate tensile strength (UTS) of the microwires are improved as compared with the as-cast ingot. And it is found that the YS and UTS are increased gradually with increasing the extraction rate. Namely, both YS and UTS are greatly improved with increasing the cooling rate. Notably, the values of YS and UTS in the microwire with 40 m/s are 495 MPa and 541 MPa, respectively, which are about three times higher than those of as-cast ingot. The values of backward extruded (BE) Mg–8Y–0.5Zn alloy are also listed in Table 1. It can be seen that the strengths of the microwire by 40 m/s are further higher than those of BE-Mg– 8Y–0.5Zn sample. The UTS of BE-Mg–8Y–0.5Zn alloy is 246 MPa, which is only 45% of that of the microwire by 40 m/s. The high tensile strength provides a prerequisite to design high performance bio-Mg stent by providing a high strength-volume ratio. It can be seen that the elongation (ε) is reduced with increasing the extraction rate. The ε ranges from 4.6 to 1.9%. However, it should be mentioned that the microwire can be used to weave the stents without any other manufacture process. Based on the previous weaving results (Lee et al., 2009), the elongation for the prepared microwires is sufficient to meet the requirement of weaving the stents.
The typical fracture surfaces of as-cast ingot and the microwires with differemt extraction rates are shown in Fig. 6. In the case of as-cast specimen, a large amount of cleavage planes, river-shaped morphology and the microcrack (denoted as an arrow) are observed on the surface. Under high magnification (Fig. 6b), some deep dimples are observed in the intragranular. It is confirmed that the fracture is dominated by quasi-cleavage fracture mode. For the microwire of 20 m/s, the configuration near to facture surface is deformed greatly (Fig. 6c). Some cleavages and a crack are observed under high magnification (Fig. 6d). In the cases of the microwires of 30 m/s and 40 m/s (Fig. 6e–h), the edge of fractured surface of microwire is in order. The microstructure near to the facture keeps intact, and some small dimples are found on the fracture surface under high magnification, showing a typical brittle fracture characteristic, which agrees well with the tensile test results. The improved mechanical strength is mainly ascribed to the following three reasons. First, based on the Inoue theory (Inoue, 2000), the cooling rate of extraction rate with 40 m/s is around 1 104 K/s, which is 1000 times higher than that of ascast technique. With increasing the cooling rate, the solubility of Y in Mg matrix is increased. The Y atoms replace Mg atoms to form a random substitutional solid solution. The stresses roughen the slip plane, making it harder for dislocation movement. It improves the resistance opposing the motion of a dislocation. Hence, it increases the strength correspondingly. Second, the high strength is mainly attributed to fine microstructure. A dislocation passing into the near grains or dendrites of different orientations has to change its direction of motion. The atomic disorder within a boundary region will result in a discontinuity of slip planes from one to the other. As a result, the tensile strength is improved as the grain size decreases (Friedman and Chrzan, 1998). Finally, the improved strength is partially attributed to the existence of an amorphous phase. It is a well-known fact that an amorphous phase includes short period ordered structure, in which the origination and movement of dislocations are effectively prohibited. And then the strength is enhanced significantly. These similar results are reported in other melt-spun Mg–Zn–Ca alloy (Zberg et al., 2009).
3.3.
Degradation properties
The polarization curves of different alloys measured after immersing for 2 h are shown in Fig. 7. The detail data deduced from the Tafel curves are listed in Table 2. The microwires display more positive potentials than that of ascast ingot. Meanwhile, a stable pitting platform is observed in
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Fig. 6 – Morphologies of fractured samples after tensile test at room temperature: (a) and (b) as cast ingot; (c) and (d) the microwire with an extraction rate of 20 m/s; (e) and (f) the microwire with an extraction rate of 30 m/s; (g) and (h) the microwire with an extraction rate of 40 m/s.
an anode polarization branch. For instance, the Ecorr in as-cast ingot is 1649 mV (vs. SCE), and it changes to 1587 mV (vs. SCE) in the microwire by 30 m/s. In addition, the Ebp values of microwires are shifted toward positive direction in contrast to that of as-cast sample. The icorr is reduced from 0.227 mA cm 2 in as-cast sample to 0.016 in microwire with 40 m/s.
Correspondingly, the average corrosion rate of microwire is 0.366 mm/y, which is lower than 1/10 of that of as-cast ingot. More importantly, the value is also lower than a critical value of degradation implants (0.5 mm/y) (Purnama et al., 2010). Fig. 8 shows the SEM graphs of both as-cast ingot and microwire with 40 m/s after immersing in a SBF for 12 h. In
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Fig. 7 – Typical Tafel curves of different samples under different states.
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the case of as-cast ingot, some grains are corroded. Some micro-cracks with a width of several micrometers are observed under high magnification (Fig. 8b), which are typical corrosion symbols of Mg alloys (Liu et al., 2009). The microcracks provide the main channels to release hydrogen, which undermine the corrosion product layer greatly. And then the solution medium readily touches the fresh surface and accelerates the corrosion process (Zainal Abidin et al., 2011). However, for the microwire sample, the corrosion morphology is very different from that of as-cast ingot. The microcrack and pitting corrosion are hardly observed on the surface. On the contrary, a thin and compact product layer is detected under high magnification (Fig. 8d and e). It demonstrates that the degradation process is very uniform. In order to investigate the different corrosion mechanism between as-cast ingot and microwires, electrochemical impedance spectroscopy was carried out. Fig. 9 displays the Nyquist diagrams of the as-cast ingot and microwire with
Table 2 – Electrochemical parameters of the alloys derived from polarization tests in an SBF solution. Sample
State
Ecorr (mV)
icorr (mA/cm2)
Ebp (mV)
Corrosion rate (mm/y)
Microwire
40 m/s 30 m/s 20 m/s –
1508 1509 1587 1649
0.016 0.031 0.188 0.227
347 1296 952 1392
0.366 0.708 4.296 5.187
As-cast ingot
Fig. 8 – Corrosion morphologies of different samples: (a) and (b) as cast ingot; (c) and (d) the microwire with an extraction rate of 40 m/s.
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40 m/s. It can be seen from Fig. 9a that the as-cast ingot is mainly composed of a capacitive loop in high frequency and an inductive loop in low frequency. The equivalent circuit model of Rs(Cdl Rct) (LRL) (Fig. 9b) is employed to analyze the EIS. Rs represents the solution resistance, Rct corresponds to the charge transfer resistance, i.e., the resistance to the electron transfer of the faradic process on the sample, in parallel to the double layer capacitance, Cdl. In addition, taking into account the inductive behavior, an inductor, L and a resistance, RL are introduced in to the equivalent circuit model. In general, the inductive reveals the occurrence of pitting corrosion (Blawert et al., 2010). Therefore, based on above SEM and EISs results, the corrosion process of as-cast ingot can be interpreted as following: at the beginning of corrosion process, the solution reacts with the alloys to form an electric double layer in the interface. However, due to the
existence of coarse secondary phase in the alloy, it is prone to produce Mg micro-galvanic corrosion or pitting corrosion (Lafront et al., 2005; Zhou et al., 2010). Thus, the released hydrogen destroys the product film, leading to the acceleration of corrosion. Comparatively, the EISs of microwire are composed of two capacitive loops in high and medium frequency ranges (Fig. 9c). The equivalent circuit model is Rs(Cdl Rct) (Cf Rf) (Fig. 9d), where the second capacitive loop is mainly associated with the diffusion of ions through the corrosion product layer or oxide film (Sudholz et al., 2011), which can be assigned with Cf (a film capacity ) and Rf (a film resistance). With increasing the extraction rate, the cooling rate is also increased correspondingly. Thus, the solubility of Y in Mg matrix is enhanced, resulting in the reduced amount of secondary phase. Therefore, a compact oxide film readily
Fig. 9 – The Nyquist diagrams for different immersion time and equivalvent circuit models; (a) and (b) as-cast ingot; (c) and (d) the microwire with an extraction rate of 40 m/s.
Table 3 – The partial resistances (Ohm) of the equivalent elements in equivalent circuit for different alloys. Alloys
Time (s)
Rs
Rct
Rf
RL
Microwire (40 m/s)
10 30 60 180 10 30 60 180
5867 6457 5642 6452 3499 3624 4280 4405
13779 14774 16317 24492 5968 10595 12532 20110
10632 10579 10117 17715 – – – –
– – – – 1494 2217 2276 6112
As-cast ingot
journal of the mechanical behavior of biomedical materials 29 (2014) 375 –384
generates during the corrosion process (Fig. 8d and e). Additionally, the high cooling rate gives rise to form more homogenous, even amorphous structures (Fig. 4b). Both Ecorr and Ebp are shifted to positive direction. As a consequence, the corrosion properties are improved. In addition, the equivalent circuit parameters are summarized in Table 3. It can be seen from the Rct values that the microwire exhibits higher anti-corrosion properties than the as-cast ingot. Moreover, the values are increased greatly with retarding the immersion time, which indicates that the electric double layer becomes thicker and thicker. In addition, the increased Rf values dependence of the immersion time in Table 3 suggest that the formed oxide film in the interface between the solution medium and the matrix is very compact and stable, which accounts for the low degradation rate in a SBF.
4.
Conclusions
A modified extraction method was successfully performed to prepare Mg based microwires. Based on the microstructures, mechanical properties and degradation performances of Mg–Y based microwires, the following conclusions can be drawn.
A uniform Mg–Y based microwire with a diameter of
45 μm has been prepared firstly by a modified melt extraction method with an extraction rate of 40 m/s. The yield strength and ultimate tensile strength of the Mg–Y based microwire by 40 m/s extraction rate at room temperature are 495 MPa and 541 MPa, respectively, which are 3.34 and 2.86 times higher than those of as-cast ingot, respectively. The improved strength is related to high solid solution strengthening, fine grain and the formation of an amorphous phase. The Mg–Y based microwire exhibits outstanding anticorrosion in a SBF, which is mainly associated with the elimination of pitting corrosion, together with the formation of a compact oxide film in the degradation process.
Acknowledgments This research is supported by National Natural Science Foundation of China (51101142 and 50821001), New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0690), Science Foundation for the Excellent Youth Scholars of Hebei Province (Y2012019) and Science Supporting Project of Hebei Province (13961002D).
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Schranz, D., Zartner, P., Michel-Behnke, I., Akintürk, H., 2006. Bioabsorbable metal stents for percutaneous treatment of critical recoarctation of the aorta in a newborn. Catheter Cardiovasc. Interv. 67, 671–673. Serre, C.M., Papillard, M., Chavassieux, P., Boivin, G., 1993. In vitro induction of a calcifying matrix by biomaterials constituted of collagen and/or hydroxyapatite: an ultrastructural comparison of three types of biomaterials. Biomaterials 14, 97–106. Shu, K.Y., Lei, Y.Q., Yang, X.G., Zhang, S.K., Lü, G.L., Zhang, H., Wang, Q.D., 2000. Micro-crystalline C14 Laves phase in melt-spun AB2 type Zr-based alloy. J. Alloys Compd. 311, 288–291. Song, G., StJohn, D.H., 2005. Corrosion of magnesium alloys in commercial engine coolants. Mater. Corros. 56, 15–23. Sudholz, A.D., Kirkland, N.T., Buchheit, R.G., Birbilis, N., 2011. Electrochemical properties of intermetallic phases and common impurity elements in magnesium alloys. Electrochem. Solid-State Lett. 14, C5–C7. Waksman, R.O.N., 2006. Update on bioabsorbable stents: from bench to clinical. J. Interv. Cardiol. 19, 414–421.
Wu, Q., Zhu, S., Wang, L., Liu, Q., Yue, G., Wang, J., Guan, S., 2012. The microstructure and properties of cyclic extrusion compression treated Mg–Zn–Y–Nd alloy for vascular stent application. J. Mech. Behav. Biomed. Mater. 8, 1–7. Yao, H.B., Li, Y., Wee, A.T.S., 2003. Passivity behavior of melt-spun Mg–Y alloys. Electrochim. Acta 48, 4197–4204. Zainal Abidin, N.I., Martin, D., Atrens, A., 2011. Corrosion of high purity Mg, AZ91, ZE41 and Mg2Zn0.2Mn in Hanks solution at room temperature. Corros. Sci. 53, 862–872. Zberg, B., Uggowitzer, P.J., Löffler, J.F., 2009. MgZnCa glasses without clinically observable hydrogen evolution for biodegradable implants. Nat. Mater. 8, 887–891. Zeng, L., Yang, S., Zhang, W., Guo, Y., Yan, C., 2007. Preparation and characterization of a double-layer coating on magnesium alloy AZ91D. Electrochim. Acta 55, 3376–3383. Zhang, E., He, W., Du, H., Yang, K., 2008. Microstructure, mechanical properties and corrosion properties of Mg–Zn–Y alloys with low Zn content. Mater. Sci. Eng., A 488, 102–111. Zhou, W., Shen, T., Aung, N.N., 2010. Effect of heat treatment on corrosion behaviour of magnesium alloy AZ91D in simulated body fluid. Corros. Sci. 52, 1035–1041.
Available online at www.sciencedirect.com
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Research Paper
Preparation, mechanical and degradation properties of Mg–Y-based microwire Qiuming Penga,n, Hui Fua, Junling Panga, Jinghuai Zhangb, Wenlong Xiaoc a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China b Key Laboratory of Superlight Materials & Surface Technology, Harbin Engineering University, Harbin 150001, PR China c School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China
art i cle i nfo
ab st rac t
Article history:
Mg–Y-based microwire which offers high strength in combination of low degradation rate
Received 14 June 2013
has been prepared for the first time by a modified melt extraction technique. A circular
Received in revised form
Mg–Y-based microwire is achieved with an extraction rate of 40 m/s, which is composed of
7 September 2013
Mg matrix and an amorphous phase, and exhibits higher basal texture than that of as-cast
Accepted 12 September 2013
sample. The outstanding tensile strength accompanying with an acceptable elongation is
Available online 12 October 2013
obtained with an extraction rate of 40 m/s. The improved strength is mainly attributed to
Keywords:
high solid solution strengthening, fine grain and the presence of an amorphous phase.
Mg biomaterials
In addition, the reduction of secondary phase and homogenous microstructure after melt
Microwire
extraction eliminate both pitting corrosion and micro-galvanic corrosion. A low degrada-
Microstructure
tion rate of 0.366 mm/y is attained in a simulated body fluid, which is less than 1/10 of that
Mechanical properties
of as-cast sample. These excellent mechanical properties and low degradation rate provide
Degradation properties
some prerequisites to develop bio-Mg implants. It reveals that this modified extraction technique is one of effective approaches to prepare microwire, which can be directly used for Mg-based stent self-assembly. & 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
The stent thrombosis and in-stent re-stenosis are still two bottlenecks in the applications of stents (Movahed et al., 2006). Two possible approaches have been proposed to deal with these shortcomings: the drug eluting and degradable stents (Waksman, 2006). Unfortunately, the following clinical tests indicated that the drug eluting stents are still facing the potential risk of late thrombosis (Hermawan et al., 2010). On the contrary, the implantation of degradable stents probably achieves the ideal restoration process of arterial n
Corresponding author. Tel.: þ86 335 8057047; fax: þ86 335 8074545. E-mail address: [email protected] (Q. Peng).
1751-6161/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2013.09.015
vessel, in which the implantation provides suitable mechanical support within a reasonable period for remodeling the artery and eliminates the long-term rejection reaction (Schranz et al., 2006). Thereafter, the development of degradation bio-Mg stents has been received worldwide attention increasingly. On the one hand, Mg alloys were found to be more suitable for load-bearing applications in contrast to ceramics or polymeric materials because of their good combination of mechanical strength and fracture toughness (Serre et al., 1993). On the other hand, the non-toxic oxides or hydroxides which formed during the degradation process
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journal of the mechanical behavior of biomedical materials 29 (2014) 375 –384
can enhance the activity of osteoblast and decrease the amount of osteoclast (Janning et al., 2010). Therefore, how to prepare high performance Mg stents becomes a critical issue in the field of bio-Mg implants. Mg stents are normally prepared by the strategy of other permanent metallic implants (Purnama et al., 2010). In brief, the as-cast ingot is manufactured to an empty pipe by extrusion technique or cold drawing method firstly, and then the pipe is machined into required dimension by a laser technique. However, this conventional process is not appropriate to bio-Mg stents. First, it is very difficult to prepare a uniform and concentric pipe owing to weak deformability of Mg alloys at low temperatures (Agnew et al., 2001; Bohlen et al., 2007). In addition, it is prone to generate an oxide film with a thickness of 50–100 μm during the machining process (Zeng et al., 2007), hence, the post-treatment, such as electrochemical corrosion process (Liu et al., 2011), is necessary to obtain a clean surface, resulting in the formation of inhomogeneous stents. As a result, leaving out of cost consideration, the conventional approach is not suitable to prepare high performance bio-Mg stents. In the present work, a modified melt extraction technique is proposed to prepare Mg-based microwires. A homogenous microwire is prepared from Mg melt with a high cooling rate, which possesses high mechanical properties in combination of good anti-corrosion properties. Hereafter, a bio-Mg stent can be achieved immediately by means of weaving technique according to the application requirements. The yttrium (Y) element has been recently reported as an effective alloying addition to prepare Mg-based biomaterial (Hanzi et al., 2010). Of particular importance is that the mechanical properties and anti-corrosion properties of Mg alloys can be improved by adding Y alloying element (Wu et al., 2012; Zhang et al., 2008). More attractively, the toxic range of Y is wide and its metabolic pathway is clear. For example, the Y-containing compound has been used for cancer treatment (Berman, 1980). Thus, Mg–Y based system is regarded as a potential one to manufacture bio-Mg stent. The objectives of this study are to clarify the effect of an extraction rate on the microwire configuration, and to investigate the mechnical and degradation properties of the microwire as compared with the as-cast samples. These results will provide some prerequisites to develop uniform bio-Mg stents in the future.
2.
Materials and methods
2.1.
Materials preparation
High purity Mg–7Y–0.2Zn alloy ingot (wt%, hereafter all compositions is given by wt%) was prepared by zone solidification method (Peng et al., 2010). The alloy was melt in a tantalum crucible under the protection by a mixture of CO2 and SF6. After holding at 700 1C for 1 h, the alloy was cast into a tantalum mould preheated at 620 1C. The filled tantalum mould was then held at 670 1C for 1 h under the protective gas. After that, the tantalum crucible with the melt was immersed into the cooling water at 30 mm/s. When the bottom of crucible touched the water, it stopped for 2 s. As soon as the liquid level of inside melt was alignment with the height of outside water, the solidification process was
terminated. The as-cast ingot with 80 mm in diameter and 180 mm in length was obtained. The ingot was machined into the samples with 10 mm in diameter and 20 mm in length for melt extraction. The microwires were fabricated by a modified melt extraction method. The representative schematic diagram is shown in Fig. 1 (Peng et al., 2013a). The rates of copper wheel were 20 m/s, 30 m/s and 40 m/s, respectively. In the meantime, the as-cast specimen was used for comparison.
2.2.
Microstructural characterization
The microstructural investigations were performed using scanning electron microscopy (SEM) The microwire for SEM observation were adhered into the conducting resin directly. The calorimetric response of the as-cast alloy and the mirowire with 40 m/s were measured using differential scanning calorimetry (DSC). A heating rate of 6 1C /min was employed under argon purge at 35 ml/min. The phase compositions were identified by X-ray diffraction with Cu Kα radiation at a scan rate of 21/min.
2.3.
Mechanical properties
The hardness test was carried out on Vickers hardness tester, the test load and dwelling time were 100 g and 15 s, respectively. Nine random spots were performed to calculate the average value. The rectangular specimens of as-cast state ingot (2 3 20 mm3) were used to measure tensile properties at room temperature with an initial strain rate of 1.7 10 3 s 1.
Fig. 1 – Schematic drawing of the modified melt extraction technique. The fabrication process is as follows: Mg molten alloy, heated in a quartz crucible, is pushed towards a highspeed rotating copper wheel and near circular wires were extracted instantaneously.
journal of the mechanical behavior of biomedical materials 29 (2014) 375 –384
The tensile properties of microwires were measured by a micro tensile testing machine under the same testing condition. The diameter of microwire which is meansured by SEM ranges from 30 to 50 μm and the length of microwire is 100 mm. The average results were calculated based on five samples. After tensile testing, the fracture surfaces were immediately observed by SEM.
2.4.
Electrochemical measurement
Electrochemical tests were carried out using a Bio-logic VSP potentiostat/frequency response analysis system to evaluate the electrochemical behaviors. Experiments were carried out in a three-electrode electrochemical cell, in which a saturated calomel electrode (SCE) as the reference electrode, a platinum mesh as counter electrode and the investigated specimen as the working electrode. The ion composition and preparation of simulated body fluid (SBF) were reported in previous result (Peng et al., 2012). In this study, the experiments were carried out in simulated body environment at ambient temperature. Briefly, the pH of SBF was 7.4. A SBF was poured into a tray and the whole samples after polishing are immersed in the simulated body fluid. During the whole test, the trays were located in an incubator. The temperature in the incubator was maintained at 37 1C. All experiments were repeated until three concurring replicates were obtained. Potentiodynamic polarization tests were performed at a scan rate of 0.5 mV/s. The polarization curves were used to estimate the corrosion and breakdown potentials (Ecorr, Ebp), and corrosion current density (icorr) at Ecorr by the Tafel extrapolation. In the Tafel extrapolation method, icorr is related to the average corrosion rate calculated by the equation (Pi ¼22.85icorr) (Song and StJohn, 2005). In the electrochemical impedance spectroscopy (EIS) measurements, the AC voltage signal amplitude was 10 mV (peak to zero). The EIS tests were carried out with the electrode in a steady state starting at an initial frequency of 100 kHz and stopping at a final frequency of 0.1 Hz. The number of points was 86. The samples were immersed in SBF solutions at different times, viz. 10 min, 20 min, 60 min and 180 min, to investigate the corrosion mechanism. All the measurements were taken immediately after the open circuit potential measurements were finished. The results were analyzed with
377
the ZView2 software, from which the fitting parameters were determined.
3.
Results and discussion
3.1.
Microstructural characterization
The morphologies of the samples with different extraction rates are shown in Fig. 2, where the sample with 20 m/s (Fig. 2a) reveals a coarse surface. The width ranges from 30 to 57 μm. Meanwhile, a large number of grooves and fluctuations are detected on the surface. The morphology of the sample with 20 m/s is similar to the ribbon prepared by meltspinning sample (Yao et al., 2003). With increasing the extraction rate to 30 m/s, the surface becomes smooth and continuous in contrast to the sample by 20 m/s extraction rate, and the diameter is around 5275 μm. In addition, the double-layer stair-step morphology and some scratches are also observed. Note that the existence of these flaws (indicated by the arrows) results in a severe deterioration of their mechanical properties. When further increased the extraction rate to 40 m/s, the smooth and uniform microwire with a diameter of 4571 μm is achieved. The XRD patterns of the as-cast ingot and the microwires with different extraction rates are shown in Fig. 3. The as-cast
Fig. 3 – X-ray patterns of as-cast ingot and the microwire with different extraction rates.
Fig. 2 – Microstructures of samples with different extraction rates: (a) 20 m/s; (b) 30 m/s; (c) 40 m/s.
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ingot is mostly composed of Mg matrix and Mg24Y5 eutectic phase. With increasing the extraction rate, the volume fraction of secondary phase is decreased simultaneously. When the extraction rate reaches 40 m/s, only Mg peaks are observed in the microwire. It is worthy to note that the relative intensity of the peaks changes greatly. It indicates that the addition of yttrium can effectively reduce the basal texture, the strongest peak corresponds to (20–22)Mg pyramid plane in the as-cast specimen, which agrees well with the previous result (Agnew et al., 2001). Comparatively, the strongest peak in the microwire of 40 m/s is mainly related to (0 0 0 2)Mg basal plane, which reveals the texture is determined by not only the alloying elements but also the solidification cooling rate. The DSC curves of as-cast ingot and the microwires with different extraction rates are shown in Fig. 4. In the case of as-cast sample, a single peak (at 565710 1C) is observed during the heating and cooling processes. According to Mg–Y binary phase diagram, it can be confirmed this peak is related to Mg24Y5 eutectic phase. For the microwires of 20 and 30 m/s, some fluctuations are observed during the heating process, which are closely related to the variation of thermo. When the extraction rate reaches 40 m/s, this characteristic becomes more apparent. Namely, for the microwire of 40 m/s, a new exothermic peak at 282 1C is also observed in addition to the peak of Mg24Y5 eutectic phase, which is associated with the crystallization transformation. It is deduced that the amorphous phase was formed during the extraction process, and
it was changed to an ordered phase when heated to 282 1C (Huang et al., 2007; Shu et al., 2000). As mentioned above, the configuration was changed from the ribbon to circular microwire with increasing the extraction rate from 20 m/s to 40 m/s. At low extraction rates, the melt readily aggregates to form ball-shaped droplet owing to the surface tension role, which leads to form discontinuous and wide ribbon. With increasing the extraction rate, the extracted amount of melt per tangent length in the interface between the wheel and melt is decreased. Thus, the heat can be emitted rapidly. And then the melt is prone to form microwire with the help of surface tension role. This process is well confirmed in the melt-spun technique that a metallic glass can be prepared with increasing the wheel rate (Saida et al., 2000). Similarity, it is reasonable to believe that there exists a critical extraction rate to form a microwire since the sample possibly maintains the origin appearance if the extraction rate is over a certain value. This critical value of extraction rate will be further identified in the future.
3.2.
Mechanical properties
The microhardness of different samples at room temperature is listed in Table 1. The value of as-cast ingot is 68 HV. Compared with as-cast sample, the microwire exhibits higher microhardness. Moreover, the microhardness increased with the increase of extraction rate. For instance, the value
Fig. 4 – DSC curves of samples; (a) as-cast ingot; (b) the microwire with an extraction rate of 20 m/s; (c) the microwire with an extraction rate of 30 m/s; (d) the microwire with an extraction rate of 40 m/s.
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Table 1 – Mechanical properties of Mg–Y based alloys under at room temperature. Sample
State
HV
YS (MPa)
UTS (MPa)
ε (%)
Microwire
20 m/s 30 m/s 40 m/s As-cast Backward extrusion
8575 9274 9875 6874 8474
310720 360719 495720 14879 14972
442721 465725 541720 18978 246712
4.670.4 3.570.3 1.970.5 7.570.3 18.171.8
As-cast ingot Mg–8Y–0.5Zn (Peng et al., 2013b)
Fig. 5 – Typical tensile curves of as-cast ingot and the microwire with an extraction rate of 40 m/s at room temperature.
is 98 HV in the sample by 40 m/s, which is 1.15 times higher than that of the one by 20 m/s. The representative tensile curves of as-cast ingot and the microwires with differemt extraction rates are shown in Fig. 5. The tensile properties of the microwires with different extraction rates are summarized in Table 1. Both the yield strength (YS) and ultimate tensile strength (UTS) of the microwires are improved as compared with the as-cast ingot. And it is found that the YS and UTS are increased gradually with increasing the extraction rate. Namely, both YS and UTS are greatly improved with increasing the cooling rate. Notably, the values of YS and UTS in the microwire with 40 m/s are 495 MPa and 541 MPa, respectively, which are about three times higher than those of as-cast ingot. The values of backward extruded (BE) Mg–8Y–0.5Zn alloy are also listed in Table 1. It can be seen that the strengths of the microwire by 40 m/s are further higher than those of BE-Mg– 8Y–0.5Zn sample. The UTS of BE-Mg–8Y–0.5Zn alloy is 246 MPa, which is only 45% of that of the microwire by 40 m/s. The high tensile strength provides a prerequisite to design high performance bio-Mg stent by providing a high strength-volume ratio. It can be seen that the elongation (ε) is reduced with increasing the extraction rate. The ε ranges from 4.6 to 1.9%. However, it should be mentioned that the microwire can be used to weave the stents without any other manufacture process. Based on the previous weaving results (Lee et al., 2009), the elongation for the prepared microwires is sufficient to meet the requirement of weaving the stents.
The typical fracture surfaces of as-cast ingot and the microwires with differemt extraction rates are shown in Fig. 6. In the case of as-cast specimen, a large amount of cleavage planes, river-shaped morphology and the microcrack (denoted as an arrow) are observed on the surface. Under high magnification (Fig. 6b), some deep dimples are observed in the intragranular. It is confirmed that the fracture is dominated by quasi-cleavage fracture mode. For the microwire of 20 m/s, the configuration near to facture surface is deformed greatly (Fig. 6c). Some cleavages and a crack are observed under high magnification (Fig. 6d). In the cases of the microwires of 30 m/s and 40 m/s (Fig. 6e–h), the edge of fractured surface of microwire is in order. The microstructure near to the facture keeps intact, and some small dimples are found on the fracture surface under high magnification, showing a typical brittle fracture characteristic, which agrees well with the tensile test results. The improved mechanical strength is mainly ascribed to the following three reasons. First, based on the Inoue theory (Inoue, 2000), the cooling rate of extraction rate with 40 m/s is around 1 104 K/s, which is 1000 times higher than that of ascast technique. With increasing the cooling rate, the solubility of Y in Mg matrix is increased. The Y atoms replace Mg atoms to form a random substitutional solid solution. The stresses roughen the slip plane, making it harder for dislocation movement. It improves the resistance opposing the motion of a dislocation. Hence, it increases the strength correspondingly. Second, the high strength is mainly attributed to fine microstructure. A dislocation passing into the near grains or dendrites of different orientations has to change its direction of motion. The atomic disorder within a boundary region will result in a discontinuity of slip planes from one to the other. As a result, the tensile strength is improved as the grain size decreases (Friedman and Chrzan, 1998). Finally, the improved strength is partially attributed to the existence of an amorphous phase. It is a well-known fact that an amorphous phase includes short period ordered structure, in which the origination and movement of dislocations are effectively prohibited. And then the strength is enhanced significantly. These similar results are reported in other melt-spun Mg–Zn–Ca alloy (Zberg et al., 2009).
3.3.
Degradation properties
The polarization curves of different alloys measured after immersing for 2 h are shown in Fig. 7. The detail data deduced from the Tafel curves are listed in Table 2. The microwires display more positive potentials than that of ascast ingot. Meanwhile, a stable pitting platform is observed in
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Fig. 6 – Morphologies of fractured samples after tensile test at room temperature: (a) and (b) as cast ingot; (c) and (d) the microwire with an extraction rate of 20 m/s; (e) and (f) the microwire with an extraction rate of 30 m/s; (g) and (h) the microwire with an extraction rate of 40 m/s.
an anode polarization branch. For instance, the Ecorr in as-cast ingot is 1649 mV (vs. SCE), and it changes to 1587 mV (vs. SCE) in the microwire by 30 m/s. In addition, the Ebp values of microwires are shifted toward positive direction in contrast to that of as-cast sample. The icorr is reduced from 0.227 mA cm 2 in as-cast sample to 0.016 in microwire with 40 m/s.
Correspondingly, the average corrosion rate of microwire is 0.366 mm/y, which is lower than 1/10 of that of as-cast ingot. More importantly, the value is also lower than a critical value of degradation implants (0.5 mm/y) (Purnama et al., 2010). Fig. 8 shows the SEM graphs of both as-cast ingot and microwire with 40 m/s after immersing in a SBF for 12 h. In
journal of the mechanical behavior of biomedical materials 29 (2014) 375 –384
Fig. 7 – Typical Tafel curves of different samples under different states.
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the case of as-cast ingot, some grains are corroded. Some micro-cracks with a width of several micrometers are observed under high magnification (Fig. 8b), which are typical corrosion symbols of Mg alloys (Liu et al., 2009). The microcracks provide the main channels to release hydrogen, which undermine the corrosion product layer greatly. And then the solution medium readily touches the fresh surface and accelerates the corrosion process (Zainal Abidin et al., 2011). However, for the microwire sample, the corrosion morphology is very different from that of as-cast ingot. The microcrack and pitting corrosion are hardly observed on the surface. On the contrary, a thin and compact product layer is detected under high magnification (Fig. 8d and e). It demonstrates that the degradation process is very uniform. In order to investigate the different corrosion mechanism between as-cast ingot and microwires, electrochemical impedance spectroscopy was carried out. Fig. 9 displays the Nyquist diagrams of the as-cast ingot and microwire with
Table 2 – Electrochemical parameters of the alloys derived from polarization tests in an SBF solution. Sample
State
Ecorr (mV)
icorr (mA/cm2)
Ebp (mV)
Corrosion rate (mm/y)
Microwire
40 m/s 30 m/s 20 m/s –
1508 1509 1587 1649
0.016 0.031 0.188 0.227
347 1296 952 1392
0.366 0.708 4.296 5.187
As-cast ingot
Fig. 8 – Corrosion morphologies of different samples: (a) and (b) as cast ingot; (c) and (d) the microwire with an extraction rate of 40 m/s.
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40 m/s. It can be seen from Fig. 9a that the as-cast ingot is mainly composed of a capacitive loop in high frequency and an inductive loop in low frequency. The equivalent circuit model of Rs(Cdl Rct) (LRL) (Fig. 9b) is employed to analyze the EIS. Rs represents the solution resistance, Rct corresponds to the charge transfer resistance, i.e., the resistance to the electron transfer of the faradic process on the sample, in parallel to the double layer capacitance, Cdl. In addition, taking into account the inductive behavior, an inductor, L and a resistance, RL are introduced in to the equivalent circuit model. In general, the inductive reveals the occurrence of pitting corrosion (Blawert et al., 2010). Therefore, based on above SEM and EISs results, the corrosion process of as-cast ingot can be interpreted as following: at the beginning of corrosion process, the solution reacts with the alloys to form an electric double layer in the interface. However, due to the
existence of coarse secondary phase in the alloy, it is prone to produce Mg micro-galvanic corrosion or pitting corrosion (Lafront et al., 2005; Zhou et al., 2010). Thus, the released hydrogen destroys the product film, leading to the acceleration of corrosion. Comparatively, the EISs of microwire are composed of two capacitive loops in high and medium frequency ranges (Fig. 9c). The equivalent circuit model is Rs(Cdl Rct) (Cf Rf) (Fig. 9d), where the second capacitive loop is mainly associated with the diffusion of ions through the corrosion product layer or oxide film (Sudholz et al., 2011), which can be assigned with Cf (a film capacity ) and Rf (a film resistance). With increasing the extraction rate, the cooling rate is also increased correspondingly. Thus, the solubility of Y in Mg matrix is enhanced, resulting in the reduced amount of secondary phase. Therefore, a compact oxide film readily
Fig. 9 – The Nyquist diagrams for different immersion time and equivalvent circuit models; (a) and (b) as-cast ingot; (c) and (d) the microwire with an extraction rate of 40 m/s.
Table 3 – The partial resistances (Ohm) of the equivalent elements in equivalent circuit for different alloys. Alloys
Time (s)
Rs
Rct
Rf
RL
Microwire (40 m/s)
10 30 60 180 10 30 60 180
5867 6457 5642 6452 3499 3624 4280 4405
13779 14774 16317 24492 5968 10595 12532 20110
10632 10579 10117 17715 – – – –
– – – – 1494 2217 2276 6112
As-cast ingot
journal of the mechanical behavior of biomedical materials 29 (2014) 375 –384
generates during the corrosion process (Fig. 8d and e). Additionally, the high cooling rate gives rise to form more homogenous, even amorphous structures (Fig. 4b). Both Ecorr and Ebp are shifted to positive direction. As a consequence, the corrosion properties are improved. In addition, the equivalent circuit parameters are summarized in Table 3. It can be seen from the Rct values that the microwire exhibits higher anti-corrosion properties than the as-cast ingot. Moreover, the values are increased greatly with retarding the immersion time, which indicates that the electric double layer becomes thicker and thicker. In addition, the increased Rf values dependence of the immersion time in Table 3 suggest that the formed oxide film in the interface between the solution medium and the matrix is very compact and stable, which accounts for the low degradation rate in a SBF.
4.
Conclusions
A modified extraction method was successfully performed to prepare Mg based microwires. Based on the microstructures, mechanical properties and degradation performances of Mg–Y based microwires, the following conclusions can be drawn.
A uniform Mg–Y based microwire with a diameter of
45 μm has been prepared firstly by a modified melt extraction method with an extraction rate of 40 m/s. The yield strength and ultimate tensile strength of the Mg–Y based microwire by 40 m/s extraction rate at room temperature are 495 MPa and 541 MPa, respectively, which are 3.34 and 2.86 times higher than those of as-cast ingot, respectively. The improved strength is related to high solid solution strengthening, fine grain and the formation of an amorphous phase. The Mg–Y based microwire exhibits outstanding anticorrosion in a SBF, which is mainly associated with the elimination of pitting corrosion, together with the formation of a compact oxide film in the degradation process.
Acknowledgments This research is supported by National Natural Science Foundation of China (51101142 and 50821001), New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0690), Science Foundation for the Excellent Youth Scholars of Hebei Province (Y2012019) and Science Supporting Project of Hebei Province (13961002D).
r e f e r e n c e s
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