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A novel Ni-free Zr-based bulk metallic glass with enhanced plasticity and good biocompatibility
Scripta Materialia 55 (2006) 605–608 www.actamat-journals.com
A novel Ni-free Zr-based bulk metallic glass with enhanced plasticity and good biocompatibility C.L. Qiu,a Q. Chen,a L. Liu,a,* K.C. Chan,b J.X. Zhou,c P.P. Chenc and S.M. Zhangc a
The State Key Lab of Die and Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, China b Department of Industrial and System Engineering, The Hong Kong Polytechnic University, Hong Kong, China c Advanced Biomaterials and Tissue Engineering Center, Huazhong University of Science and Technology, 430074 Wuhan, PR China Received 24 April 2006; revised 15 June 2006; accepted 16 June 2006 Available online 21 July 2006
A novel Ni-free bulk metallic glass (BMG) of Zr60Cu22.5Pd5Al7.5Nb5 was successfully prepared by copper mold casting. The BMG exhibited excellent mechanical properties, and superior corrosion resistance in an artificial body fluid and reasonably good biocompatibility, thus demonstrating its promise for biomedical application. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ni-free bulk metallic glass; Room-temperature plasticity; Corrosion resistance; Cell culture; Rapid solidification
Owing to their superior strength (2 Gpa), high elastic strain limit (2%), relatively low Young’s modulus (50–100 GPa), improved wear resistance and excellent corrosion resistance in artificial body fluids, Zr-based bulk metallic glasses (BMGs) are considered promising biomedical materials [1–9]. However, most Zr-based bulk metallic glasses developed up to date contain the element Ni [1–7], which is usually blamed for the occurrence of an allergy and has antiproliferative effects on cell cultures [10]. Recently, Jin et al. have developed a series of Ni-free Zr-based BMGs with the composition of (ZrxCu100 x)80(Fe40Al60)20 (where x = 62–81) [11], which show excellent glass forming ability and quite satisfactory biocompatibility [12]. However, little work has been done on the mechanical properties and corrosion resistance in vivo of this kind of BMG, which is also essential for the evaluation of a material for biomedical use. Qin et al. [13] did suggest that Ni in Zr55Cu30Al10Ni5 could be substituted by Pd because Ni and Pd belong to the same group in the periodic table. However, this substitution seems to deteriorate the glass forming ability of the BMG due to the formation of nanocrystals, which, in turn, causes the decrease in corrosion resistance in NaCl solution. In our previous works, we have prepared a few Zr-based BMGs with the structure * Corresponding author. Tel.: +86 27 87556894; fax: +86 27 87554405; e-mail: [email protected]
being modified by a small amount of Nb (e.g. Zr60Ni10Cu17.5Al7.5Nb5, Zr60Ni5Cu17.5Al7.5Pd5Nb5), and found that the addition of Nb can considerably enhance the plasticity and corrosion resistance of the base alloy in different artificial body fluids (ABFs) [7–9]. However, the BMGs also contained 10 at.% Ni, which as explained above is unfavorable for biomedical use. In this paper, we have developed a novel Ni-free, Zr-based BMG with the composition Zr60Nb5Cu22.5Pd5Al7.5. It will be shown that this BMG exhibits not only good biocompatibility, but also excellent mechanical properties and corrosion resistance in ABFs. An alloy ingot with the nominal composition Zr60Nb5Cu22.5Pd5Al7.5 was prepared by arc-melting a mixture of pure Zr, Nb, Cu, Pd and Al metals (purity>99.5%) under an argon atmosphere. From the master alloys, sample rods with a diameter of 3 mm and a length of 50 mm were produced by copper mould casting. The structure of the sample obtained was examined by X-ray diffraction (XRD, v’Pert PRO) with CuKa radiation and high resolution transmission electron microscopy (HRTEM, Jeol-2010). The thermal response of the BMG was examined using differential scanning calorimetry (DSC, Perkin–Elmer DSC-7) at a heating rate of 20 C/min under a constant flow of argon. A uniaxial quasi-static compression test was carried out at room temperature by using a MTS machine at a strain rate of 1 · 10 4s 1. Samples of length 6 mm and diameter 3 mm were cut from the rods for the test.
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.06.018
Intensity (a.u.)
Electrochemical polarization was conducted in a three-electrode cell using a platinum counter electrode and a saturated calomel reference electrode (SCE). The whole cell was kept at 37 C throughout the test. The electrolyte used in the present study is phosphate buffered solution (PBS, pH = 7.4) which is composed of 8 g/L NaCl, 0.2 g/L KCl, 0.14 g/L NaH2PO4, and 0.2 g/L KH2PO4. Prior to a corrosion test, the specimens were mechanically polished to mirror finish, then degreased in acetone, washed in distilled water and dried in air. The potentiodynamic polarization curves of the specimens were recorded at a potential sweep rate of 1 mV/s when the open-circuit potential became almost steady after immersion in artificial body fluid for at least 20 min. In order to characterize the composition and chemical states of the passive film formed after electrochemical treatment, the specimen was potentiodynamically polarized roughly to the pitting potential and then taken out immediately for X-ray photoelectron spectroscopy (XPS) measurements (Kratos XSAM 800, with Mg Ka excitation). XPS spectra were analyzed by a least-squares fit using XPSPEAK analytical software to obtain further information about chemical states. The potential cytotoxicity of the BMG was evaluated via a cell culture for one week followed by a 3-(4,5dimethylthiazol-2-yl-)-2,5-diphenyltetrazolium bromide (MTT) assay. In this process, NIH/3T3 cells (embryonic mouse fibroblast cell line) were first seeded at a density of 1.44 · 105cells/ml on the polished and sterilized alloy slices with a size of 5 mm · 3 mm · 1.5 mm in a 96-well plate, and then cultured in Dulbecco’s modified Eagle’s medium (DMEM; Gibco), supplemented with 10% fetal bovine serum (FBS) in a 37 C, humidified, 5%CO2/95% air environment. The culture medium was changed every three days. After seven days, the BMG specimens were taken out for MTT assay. Three specimens were tested simultaneously to get a statistically sound result. The cytotoxicity test of Ti–6Al–4V alloy was also performed under the same condition for comparison and a blank test (i.e., cells were cultured in wells without alloy samples) was carried out as a control. Figure 1 shows the XRD pattern of the Zr60Nb5Cu22.5Pd5Al7.5 rod prepared. A broad diffraction hump without any trace of crystalline peaks in the XRD pattern indicates that the as-cast sample is basically amorphous. The single phase amorphous structure of the sample was further verified by TEM. The DSC scan (as shown in the inset of Fig. 1) indicates that the alloy exhibits a distinct glass transition followed by a supercooled liquid region before crystallization. The glass transition temperature (Tg) and the onset temperature of crystallization (Tx) were determined to be 410 C and 448 C, respectively. This yields a supercooled liquid region (DTx = Tx Tg) of 38 K for this alloy. In addition, a unique multi-crystallization process can be observed in the DSC curve. To clarify the phase transformation underlying various crystallization stages, samples were subjected to heat-treatment to various annealing stages, followed by XRD examinations. Figure 2 shows the XRD patterns corresponding to the phase transformations occurred during annealing treatments. It can be seen that icosahedral quasicrystals are
exo Heat Flow (a.u.)
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Tg Tx Zr60Nb5Cu22.5Pd5Al7.5
200
300
400
500
600
Temperature (°C)
20
30
40
50
60
70
80
2 theta (degrees) Figure 1. X-ray diffraction patterns of as-cast Zr60Nb5Cu22.5Pd5Al7.5 BMG. The inset shows the DSC curve of the BMG at a heating rate of 20 C/min.
Zr2Cu QC
+ Unkonwn Intensity (a.u.)
606
(b)
+
+
+
(a)
30
40
50
60
70
80
2 theta (degrees) Figure 2. X-ray diffraction patterns of Zr60Nb5Cu22.5Pd5Al7.5 BMG after being annealed up to different stages. Curve (a) corresponds to the XRD pattern of the as-cast BMG annealed up to the end of the first DSC peak. Curve (b) corresponds to the BMG annealed to the end of the final DSC peak.
preferentially formed, although the final phase is still Zr2Cu, which is usually the stable crystalline phase for most Zr-based BMGs after the completion of crystallization. Figure 3 shows the stress–strain (r–e) curve of the Zr60Nb5Cu22.5Pd5Al7.5 BMG obtained under compressive loading at a strain rate of 10 4 s 1. The BMG exhibits very good mechanical properties with yield strength (ry) of 1554 MPa, fracture strength (rf) of 1720 MPa and a fracture strain of 5.3%. Interestingly, a relatively large plastic strain (around 3.5%) was obtained although the BMG is basically of a single amorphous structure. In addition, an elastic strain of 1.8% with a relatively low elastic modulus of about 82 GPa could be obtained in the test, which are closer to those
C. L. Qiu et al. / Scripta Materialia 55 (2006) 605–608 2000
Stress / MPa
1500
1000
500
100 μm 0 0
1
2
3
4
5
6
Strain / % Figure 3. Compressive stress–strain curve of the Zr60Nb5Cu22.5Pd5Al7.5 BMG at a strain rate of 10 4 s 1. The SEM image for lateral surfaces of the BMG rod after fracture is shown in the inset.
of human bone than 316L stainless steel and Ti–6Al–4V alloy are. The inset in Figure 3 is a scanning electron microscopy (SEM) image of the side surface of the sample after compression to fracture. It can be seen that a huge number of jagged and bifurcated shear bands are distributed on the whole sample, which are believed to contribute to the extended plasticity of the BMG. Figure 4 shows the polarization curve of the BMG in PBS open to air at 37 C. The results for 316L stainless steel and Ti–6Al–4V tested under the same condition are also presented for comparison. It can be seen that the BMG exhibits a spontaneous passivation with a wide stable passive region before pitting occurred. In comparison with 316L stainless steel and Ti–6Al–4V alloy, the BMG shows a lower passive current density, suggesting that the passive film formed on the BMG is more protective than on other two alloys. Moreover, the pitting potential of the BMG is much higher than that of 316L stainless steel and similar to that of Ti–6Al–4V alloy.
607
To better understand the origin of the excellent corrosion resistance of the BMG in PBS, XPS was employed to characterize the composition of the passive film and the chemical states of the elements therein, which was formed after potentiodynamic polarization in its passive region. Analysis on the XPS spectra (not shown here) reveals that the passive film was mainly composed of Zr-, Al- and Nb-oxides. The concentration of each oxide could be determined from the integrated intensity of each XPS spectrum, which yielded around 51.3% of ZrO2, 29.2% of Al2O3, and 5.6% of Nb2O5, respectively (see Table 1). However, no spectra corresponding to any forms of copper could be detected in the passive film. This is considered to be most significant for the compatibility of the BMG. Figure 5 shows the results of MTT assays for the Nifree BMG and Ti-6Al-4V alloy after cell culture for seven days. The BMG shows a considerably higher absorbance as compared with Ti–6Al–4V alloy, indicating that the BMG has higher cell viability and proliferation activity. SEM observation demonstrated that NIH/3T3 cells can closely adhere to and extend across the surfaces of the Ni-free BMG (see the inset of Fig. 5). This indicates that the BMG developed in this work has excellent biocompatibility. The experimental results demonstrate that the Ni-free Zr60Nb5Cu22.5Pd5Al7.5 BMG developed in this work exhibits not only superior strength but also reasonably good plasticity, even though the BMG is apparently of a single amorphous structure. The extended plasticity is reflected by the high density of shear bands on the side surface of the sample after compression (see Fig. 3). Table 1. Composition (at.%) of passive film and the chemical states of elements thereina
a
Alloy
Zr ox
Al ox
Nb ox
Phosphate
Zr60Nb5Cu22.5Pd5Al7.5
51.3
29.2
5.6
13.8
ox represents oxide-state.
0 -2
stainless steel 316L
log(i)/(Acm-2)
-4
Ti-6Al-4V
-6
Zr60Nb5Cu22.5Pd5Al7.5
-8 -10 -12
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
E(vs.SCE)/V Figure 4. Potentiodynamic polarization curves of Zr60Nb5Cu22.5Pd5Al7.5 BMG in phosphate buffered solution open to air at 37 C.
Figure 5. Cytotoxicity tests of Zr60Nb5Cu22.5Pd5Al7.5 BMG and Ti– 6Al–4V alloy after cell culture for seven days in comparison with the control. The product of viability and proliferation activity was evaluated by mitochondrial integrity assay (MTT assay). Values are the mean of at least three independent samples. The morphology of cells grown on the surface of the BMG is shown in the inset.
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C. L. Qiu et al. / Scripta Materialia 55 (2006) 605–608
Similar enhancement of plasticity has been reported in a few other BMG systems, including Zr59Ta5Cu18Ni8Al10 [14], (Zr70Cu20Ni10)86Ta4Al10 [15], and Ni59Zr16Ti13Si3Sn2Nb7 [16] BMGs. Xing [14] and Lee [16] argued that the enhanced plasticity of monolithic BMGs upon deformation is closely related to the structural inhomogeneity or existence of short/middle-range order clusters in the amorphous phase, which affect the free volume distribution and therefore shear band formation and propagation. In fact, the preferential precipitation of quasicrystals during annealing in the present study (see Fig. 2) implies that a strong icosahedral short/middlerange ordering structure may have already existed in the as-cast BMG, which caused the inhomogeneity of the amorphous phase and led to the extension of the plasticity. This is consistent with the previous work by Inoue, who argued that the icosahedral-type ordered atomic configuration does exist in Zr-based BMGs, and that the refractory metals, such as Nb, Ta and V, can promote the formation of the structure [17]. The above results also demonstrate that Zr60Nb5Cu22.5Pd5Al7.5 BMG has excellent corrosion resistance in PBS, as indicated by its high pitting potential and significantly low passive current density. The passive current density is even smaller than that of Ti–6Al–4V, indicating that the passive film formed on the BMG is more protective than that on Ti alloy. The excellent corrosion resistance can be explained well by the formation of a series of highly corrosion-resistant oxides, such as Nb-, Zr- and Al-oxides in the passive film, as verified by XPS. The elements of Nb, Zr and Al are known to be typical valve elements, whose oxides are dense in structure and stable in chemistry, and thus are highly immune to the attack of the chloride ions in a nearly neutral medium. On the other hand, the absence of Ni in the alloy and depletion of Cu in the passive film can further improve the biocompatibility of the BMG, which is actually confirmed by MTT assays. In conclusion, we have developed a novel Ni-free BMG of composition Zr60Nb5Cu22.5Pd5Al7.5. This BMG exhibits superior strength and reasonably good plasticity under compression. The extended plasticity is probably due to the existence of a strong icosahedral short/middle-range ordering structure in the amorphous phase. The Ni-free BMG also shows excellent corrosion
resistance in phosphate buffered solution, as indicated by a high pitting potential and low passive current density in electrochemical polarization. The excellent corrosion resistance of Zr60Nb5Cu22.5Pd5Al7.5 BMG is attributed to the formation of a highly corrosion-resistant passive film that was enriched in Zr-, Nb- and Al-oxides, but depleted in Cu-oxide. Cytotoxicity test suggests that the BMG has very good biocompatibility. The present results demonstrate that the Zr60Nb5Cu22.5Pd5Al7.5 BMG developed in this work is promising for biomedical application. This work was financially supported by the Natural Science Foundation of China under grant No. 50571039 and the Excellent Young Teachers Program of MOE, People’s Republic of China. [1] H.A. Bruck, T. Christman, A.J. Rosakis, W.L. Johnson, Scripta Mater. 30 (1994) 429. [2] A. Inoue, Mater. Trans. JIM 36 (1995) 866. [3] C.J. Gilbert, R.O. Ritchie, W.L. Johnson, Appl. Phys. Lett. 71 (1997) 476. [4] W.L. Johnson, MRS Bull. 24 (1999) 42. [5] S. Pang, T. Zhang, H. Kimura, K. Asami, A. Inoue, Mater. Trans. JIM 41 (2000) 1490. [6] J.G. Wang, B.W. Choi, T.G. Nieh, C.T. Liu, J. Mater. Res. 15 (2000) 913. [7] C.L. Qiu, L. Liu, M. Sun, S.M. Zhang, J. Biomed. Mater Res. Part A 75 (2005) 950. [8] L. Liu, C.L. Qiu, Q. Chen, S.M. Zhang, J. Alloys Compd, in press. [9] L. Liu, C.L. Qiu, M. Sun, Q. Chen, K.C. Chan, G.K.H. Pang, Mater. Sci. Eng. A, in press. [10] J.C. Wataha, P.E. Lockwood, A. Schedle, J. Biomed. Mater Res. 52 (2000) 360. [11] K. Jin, J.F. Lo¨ffler, Appl. Phys. Lett. 86 (2005) 241909. [12] S. Buzzi, K.F. Jin, P.J. Uggowitzer, S. Tosatti, I. Gerber, J.F. Lo¨ffler, Intermetallics 14 (2006) 729. [13] F.X. Qin, H.F. Zhang, Y.F. Deng, B.Z. Ding, Z.Q. Hu, J. Alloys Compd. 375 (2004) 318. [14] L.Q. Xing, Y. Li, K.T. Ramesh, Phys. Rev. B 64 (2001) 180211. [15] T.C. Hufnagel, C. Fan, R.T. Ott, Intermetallics 10 (2002) 1163. [16] M.H. Lee, J.Y. Lee, D.H. Bae, Intermetallics 12 (2003) 1133. [17] A. Inoue, Proc. Japan Acad. 81 (2005) 156.
A novel Ni-free Zr-based bulk metallic glass with enhanced plasticity and good biocompatibility C.L. Qiu,a Q. Chen,a L. Liu,a,* K.C. Chan,b J.X. Zhou,c P.P. Chenc and S.M. Zhangc a
The State Key Lab of Die and Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, China b Department of Industrial and System Engineering, The Hong Kong Polytechnic University, Hong Kong, China c Advanced Biomaterials and Tissue Engineering Center, Huazhong University of Science and Technology, 430074 Wuhan, PR China Received 24 April 2006; revised 15 June 2006; accepted 16 June 2006 Available online 21 July 2006
A novel Ni-free bulk metallic glass (BMG) of Zr60Cu22.5Pd5Al7.5Nb5 was successfully prepared by copper mold casting. The BMG exhibited excellent mechanical properties, and superior corrosion resistance in an artificial body fluid and reasonably good biocompatibility, thus demonstrating its promise for biomedical application. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ni-free bulk metallic glass; Room-temperature plasticity; Corrosion resistance; Cell culture; Rapid solidification
Owing to their superior strength (2 Gpa), high elastic strain limit (2%), relatively low Young’s modulus (50–100 GPa), improved wear resistance and excellent corrosion resistance in artificial body fluids, Zr-based bulk metallic glasses (BMGs) are considered promising biomedical materials [1–9]. However, most Zr-based bulk metallic glasses developed up to date contain the element Ni [1–7], which is usually blamed for the occurrence of an allergy and has antiproliferative effects on cell cultures [10]. Recently, Jin et al. have developed a series of Ni-free Zr-based BMGs with the composition of (ZrxCu100 x)80(Fe40Al60)20 (where x = 62–81) [11], which show excellent glass forming ability and quite satisfactory biocompatibility [12]. However, little work has been done on the mechanical properties and corrosion resistance in vivo of this kind of BMG, which is also essential for the evaluation of a material for biomedical use. Qin et al. [13] did suggest that Ni in Zr55Cu30Al10Ni5 could be substituted by Pd because Ni and Pd belong to the same group in the periodic table. However, this substitution seems to deteriorate the glass forming ability of the BMG due to the formation of nanocrystals, which, in turn, causes the decrease in corrosion resistance in NaCl solution. In our previous works, we have prepared a few Zr-based BMGs with the structure * Corresponding author. Tel.: +86 27 87556894; fax: +86 27 87554405; e-mail: [email protected]
being modified by a small amount of Nb (e.g. Zr60Ni10Cu17.5Al7.5Nb5, Zr60Ni5Cu17.5Al7.5Pd5Nb5), and found that the addition of Nb can considerably enhance the plasticity and corrosion resistance of the base alloy in different artificial body fluids (ABFs) [7–9]. However, the BMGs also contained 10 at.% Ni, which as explained above is unfavorable for biomedical use. In this paper, we have developed a novel Ni-free, Zr-based BMG with the composition Zr60Nb5Cu22.5Pd5Al7.5. It will be shown that this BMG exhibits not only good biocompatibility, but also excellent mechanical properties and corrosion resistance in ABFs. An alloy ingot with the nominal composition Zr60Nb5Cu22.5Pd5Al7.5 was prepared by arc-melting a mixture of pure Zr, Nb, Cu, Pd and Al metals (purity>99.5%) under an argon atmosphere. From the master alloys, sample rods with a diameter of 3 mm and a length of 50 mm were produced by copper mould casting. The structure of the sample obtained was examined by X-ray diffraction (XRD, v’Pert PRO) with CuKa radiation and high resolution transmission electron microscopy (HRTEM, Jeol-2010). The thermal response of the BMG was examined using differential scanning calorimetry (DSC, Perkin–Elmer DSC-7) at a heating rate of 20 C/min under a constant flow of argon. A uniaxial quasi-static compression test was carried out at room temperature by using a MTS machine at a strain rate of 1 · 10 4s 1. Samples of length 6 mm and diameter 3 mm were cut from the rods for the test.
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.06.018
Intensity (a.u.)
Electrochemical polarization was conducted in a three-electrode cell using a platinum counter electrode and a saturated calomel reference electrode (SCE). The whole cell was kept at 37 C throughout the test. The electrolyte used in the present study is phosphate buffered solution (PBS, pH = 7.4) which is composed of 8 g/L NaCl, 0.2 g/L KCl, 0.14 g/L NaH2PO4, and 0.2 g/L KH2PO4. Prior to a corrosion test, the specimens were mechanically polished to mirror finish, then degreased in acetone, washed in distilled water and dried in air. The potentiodynamic polarization curves of the specimens were recorded at a potential sweep rate of 1 mV/s when the open-circuit potential became almost steady after immersion in artificial body fluid for at least 20 min. In order to characterize the composition and chemical states of the passive film formed after electrochemical treatment, the specimen was potentiodynamically polarized roughly to the pitting potential and then taken out immediately for X-ray photoelectron spectroscopy (XPS) measurements (Kratos XSAM 800, with Mg Ka excitation). XPS spectra were analyzed by a least-squares fit using XPSPEAK analytical software to obtain further information about chemical states. The potential cytotoxicity of the BMG was evaluated via a cell culture for one week followed by a 3-(4,5dimethylthiazol-2-yl-)-2,5-diphenyltetrazolium bromide (MTT) assay. In this process, NIH/3T3 cells (embryonic mouse fibroblast cell line) were first seeded at a density of 1.44 · 105cells/ml on the polished and sterilized alloy slices with a size of 5 mm · 3 mm · 1.5 mm in a 96-well plate, and then cultured in Dulbecco’s modified Eagle’s medium (DMEM; Gibco), supplemented with 10% fetal bovine serum (FBS) in a 37 C, humidified, 5%CO2/95% air environment. The culture medium was changed every three days. After seven days, the BMG specimens were taken out for MTT assay. Three specimens were tested simultaneously to get a statistically sound result. The cytotoxicity test of Ti–6Al–4V alloy was also performed under the same condition for comparison and a blank test (i.e., cells were cultured in wells without alloy samples) was carried out as a control. Figure 1 shows the XRD pattern of the Zr60Nb5Cu22.5Pd5Al7.5 rod prepared. A broad diffraction hump without any trace of crystalline peaks in the XRD pattern indicates that the as-cast sample is basically amorphous. The single phase amorphous structure of the sample was further verified by TEM. The DSC scan (as shown in the inset of Fig. 1) indicates that the alloy exhibits a distinct glass transition followed by a supercooled liquid region before crystallization. The glass transition temperature (Tg) and the onset temperature of crystallization (Tx) were determined to be 410 C and 448 C, respectively. This yields a supercooled liquid region (DTx = Tx Tg) of 38 K for this alloy. In addition, a unique multi-crystallization process can be observed in the DSC curve. To clarify the phase transformation underlying various crystallization stages, samples were subjected to heat-treatment to various annealing stages, followed by XRD examinations. Figure 2 shows the XRD patterns corresponding to the phase transformations occurred during annealing treatments. It can be seen that icosahedral quasicrystals are
exo Heat Flow (a.u.)
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Tg Tx Zr60Nb5Cu22.5Pd5Al7.5
200
300
400
500
600
Temperature (°C)
20
30
40
50
60
70
80
2 theta (degrees) Figure 1. X-ray diffraction patterns of as-cast Zr60Nb5Cu22.5Pd5Al7.5 BMG. The inset shows the DSC curve of the BMG at a heating rate of 20 C/min.
Zr2Cu QC
+ Unkonwn Intensity (a.u.)
606
(b)
+
+
+
(a)
30
40
50
60
70
80
2 theta (degrees) Figure 2. X-ray diffraction patterns of Zr60Nb5Cu22.5Pd5Al7.5 BMG after being annealed up to different stages. Curve (a) corresponds to the XRD pattern of the as-cast BMG annealed up to the end of the first DSC peak. Curve (b) corresponds to the BMG annealed to the end of the final DSC peak.
preferentially formed, although the final phase is still Zr2Cu, which is usually the stable crystalline phase for most Zr-based BMGs after the completion of crystallization. Figure 3 shows the stress–strain (r–e) curve of the Zr60Nb5Cu22.5Pd5Al7.5 BMG obtained under compressive loading at a strain rate of 10 4 s 1. The BMG exhibits very good mechanical properties with yield strength (ry) of 1554 MPa, fracture strength (rf) of 1720 MPa and a fracture strain of 5.3%. Interestingly, a relatively large plastic strain (around 3.5%) was obtained although the BMG is basically of a single amorphous structure. In addition, an elastic strain of 1.8% with a relatively low elastic modulus of about 82 GPa could be obtained in the test, which are closer to those
C. L. Qiu et al. / Scripta Materialia 55 (2006) 605–608 2000
Stress / MPa
1500
1000
500
100 μm 0 0
1
2
3
4
5
6
Strain / % Figure 3. Compressive stress–strain curve of the Zr60Nb5Cu22.5Pd5Al7.5 BMG at a strain rate of 10 4 s 1. The SEM image for lateral surfaces of the BMG rod after fracture is shown in the inset.
of human bone than 316L stainless steel and Ti–6Al–4V alloy are. The inset in Figure 3 is a scanning electron microscopy (SEM) image of the side surface of the sample after compression to fracture. It can be seen that a huge number of jagged and bifurcated shear bands are distributed on the whole sample, which are believed to contribute to the extended plasticity of the BMG. Figure 4 shows the polarization curve of the BMG in PBS open to air at 37 C. The results for 316L stainless steel and Ti–6Al–4V tested under the same condition are also presented for comparison. It can be seen that the BMG exhibits a spontaneous passivation with a wide stable passive region before pitting occurred. In comparison with 316L stainless steel and Ti–6Al–4V alloy, the BMG shows a lower passive current density, suggesting that the passive film formed on the BMG is more protective than on other two alloys. Moreover, the pitting potential of the BMG is much higher than that of 316L stainless steel and similar to that of Ti–6Al–4V alloy.
607
To better understand the origin of the excellent corrosion resistance of the BMG in PBS, XPS was employed to characterize the composition of the passive film and the chemical states of the elements therein, which was formed after potentiodynamic polarization in its passive region. Analysis on the XPS spectra (not shown here) reveals that the passive film was mainly composed of Zr-, Al- and Nb-oxides. The concentration of each oxide could be determined from the integrated intensity of each XPS spectrum, which yielded around 51.3% of ZrO2, 29.2% of Al2O3, and 5.6% of Nb2O5, respectively (see Table 1). However, no spectra corresponding to any forms of copper could be detected in the passive film. This is considered to be most significant for the compatibility of the BMG. Figure 5 shows the results of MTT assays for the Nifree BMG and Ti-6Al-4V alloy after cell culture for seven days. The BMG shows a considerably higher absorbance as compared with Ti–6Al–4V alloy, indicating that the BMG has higher cell viability and proliferation activity. SEM observation demonstrated that NIH/3T3 cells can closely adhere to and extend across the surfaces of the Ni-free BMG (see the inset of Fig. 5). This indicates that the BMG developed in this work has excellent biocompatibility. The experimental results demonstrate that the Ni-free Zr60Nb5Cu22.5Pd5Al7.5 BMG developed in this work exhibits not only superior strength but also reasonably good plasticity, even though the BMG is apparently of a single amorphous structure. The extended plasticity is reflected by the high density of shear bands on the side surface of the sample after compression (see Fig. 3). Table 1. Composition (at.%) of passive film and the chemical states of elements thereina
a
Alloy
Zr ox
Al ox
Nb ox
Phosphate
Zr60Nb5Cu22.5Pd5Al7.5
51.3
29.2
5.6
13.8
ox represents oxide-state.
0 -2
stainless steel 316L
log(i)/(Acm-2)
-4
Ti-6Al-4V
-6
Zr60Nb5Cu22.5Pd5Al7.5
-8 -10 -12
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
E(vs.SCE)/V Figure 4. Potentiodynamic polarization curves of Zr60Nb5Cu22.5Pd5Al7.5 BMG in phosphate buffered solution open to air at 37 C.
Figure 5. Cytotoxicity tests of Zr60Nb5Cu22.5Pd5Al7.5 BMG and Ti– 6Al–4V alloy after cell culture for seven days in comparison with the control. The product of viability and proliferation activity was evaluated by mitochondrial integrity assay (MTT assay). Values are the mean of at least three independent samples. The morphology of cells grown on the surface of the BMG is shown in the inset.
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Similar enhancement of plasticity has been reported in a few other BMG systems, including Zr59Ta5Cu18Ni8Al10 [14], (Zr70Cu20Ni10)86Ta4Al10 [15], and Ni59Zr16Ti13Si3Sn2Nb7 [16] BMGs. Xing [14] and Lee [16] argued that the enhanced plasticity of monolithic BMGs upon deformation is closely related to the structural inhomogeneity or existence of short/middle-range order clusters in the amorphous phase, which affect the free volume distribution and therefore shear band formation and propagation. In fact, the preferential precipitation of quasicrystals during annealing in the present study (see Fig. 2) implies that a strong icosahedral short/middlerange ordering structure may have already existed in the as-cast BMG, which caused the inhomogeneity of the amorphous phase and led to the extension of the plasticity. This is consistent with the previous work by Inoue, who argued that the icosahedral-type ordered atomic configuration does exist in Zr-based BMGs, and that the refractory metals, such as Nb, Ta and V, can promote the formation of the structure [17]. The above results also demonstrate that Zr60Nb5Cu22.5Pd5Al7.5 BMG has excellent corrosion resistance in PBS, as indicated by its high pitting potential and significantly low passive current density. The passive current density is even smaller than that of Ti–6Al–4V, indicating that the passive film formed on the BMG is more protective than that on Ti alloy. The excellent corrosion resistance can be explained well by the formation of a series of highly corrosion-resistant oxides, such as Nb-, Zr- and Al-oxides in the passive film, as verified by XPS. The elements of Nb, Zr and Al are known to be typical valve elements, whose oxides are dense in structure and stable in chemistry, and thus are highly immune to the attack of the chloride ions in a nearly neutral medium. On the other hand, the absence of Ni in the alloy and depletion of Cu in the passive film can further improve the biocompatibility of the BMG, which is actually confirmed by MTT assays. In conclusion, we have developed a novel Ni-free BMG of composition Zr60Nb5Cu22.5Pd5Al7.5. This BMG exhibits superior strength and reasonably good plasticity under compression. The extended plasticity is probably due to the existence of a strong icosahedral short/middle-range ordering structure in the amorphous phase. The Ni-free BMG also shows excellent corrosion
resistance in phosphate buffered solution, as indicated by a high pitting potential and low passive current density in electrochemical polarization. The excellent corrosion resistance of Zr60Nb5Cu22.5Pd5Al7.5 BMG is attributed to the formation of a highly corrosion-resistant passive film that was enriched in Zr-, Nb- and Al-oxides, but depleted in Cu-oxide. Cytotoxicity test suggests that the BMG has very good biocompatibility. The present results demonstrate that the Zr60Nb5Cu22.5Pd5Al7.5 BMG developed in this work is promising for biomedical application. This work was financially supported by the Natural Science Foundation of China under grant No. 50571039 and the Excellent Young Teachers Program of MOE, People’s Republic of China. [1] H.A. Bruck, T. Christman, A.J. Rosakis, W.L. Johnson, Scripta Mater. 30 (1994) 429. [2] A. Inoue, Mater. Trans. JIM 36 (1995) 866. [3] C.J. Gilbert, R.O. Ritchie, W.L. Johnson, Appl. Phys. Lett. 71 (1997) 476. [4] W.L. Johnson, MRS Bull. 24 (1999) 42. [5] S. Pang, T. Zhang, H. Kimura, K. Asami, A. Inoue, Mater. Trans. JIM 41 (2000) 1490. [6] J.G. Wang, B.W. Choi, T.G. Nieh, C.T. Liu, J. Mater. Res. 15 (2000) 913. [7] C.L. Qiu, L. Liu, M. Sun, S.M. Zhang, J. Biomed. Mater Res. Part A 75 (2005) 950. [8] L. Liu, C.L. Qiu, Q. Chen, S.M. Zhang, J. Alloys Compd, in press. [9] L. Liu, C.L. Qiu, M. Sun, Q. Chen, K.C. Chan, G.K.H. Pang, Mater. Sci. Eng. A, in press. [10] J.C. Wataha, P.E. Lockwood, A. Schedle, J. Biomed. Mater Res. 52 (2000) 360. [11] K. Jin, J.F. Lo¨ffler, Appl. Phys. Lett. 86 (2005) 241909. [12] S. Buzzi, K.F. Jin, P.J. Uggowitzer, S. Tosatti, I. Gerber, J.F. Lo¨ffler, Intermetallics 14 (2006) 729. [13] F.X. Qin, H.F. Zhang, Y.F. Deng, B.Z. Ding, Z.Q. Hu, J. Alloys Compd. 375 (2004) 318. [14] L.Q. Xing, Y. Li, K.T. Ramesh, Phys. Rev. B 64 (2001) 180211. [15] T.C. Hufnagel, C. Fan, R.T. Ott, Intermetallics 10 (2002) 1163. [16] M.H. Lee, J.Y. Lee, D.H. Bae, Intermetallics 12 (2003) 1133. [17] A. Inoue, Proc. Japan Acad. 81 (2005) 156.