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Improvements in the plasticity and biocompatibility of Zr–Cu–Ni–Al bulk metallic glass by the microalloying of Nb
Materials Science and Engineering A 449–451 (2007) 193–197
Improvements in the plasticity and biocompatibility of Zr–Cu–Ni–Al bulk metallic glass by the microalloying of Nb L. Liu a,∗ , C.L. Qiu a , M. Sun a , Q. Chen a , K.C. Chan b , Geoffrey K.H. Pang c a
The State Key Lab of Die & Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, China b Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, China c Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China Received 21 August 2005; received in revised form 20 December 2005; accepted 23 February 2006
Abstract Bulk metallic glasses (BMGs) of Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2, and 5 at.%) were prepared by copper mold casting. Room-temperature uniaxial compression tests revealed that the addition of a small amount of Nb could considerably enhance the compression plasticity of Zr-based BMGs. Electrochemical polarization measurements and ion release tests of the BMGs in an artificial body fluid indicated that Nb addition could highly improve the corrosion resistance and reduce the ion release of all kinds of the base alloy. The results demonstrated that Zr-based BMGs microalloyed with Nb hold promise for biomedical use. © 2006 Elsevier B.V. All rights reserved. Keywords: Zr-based bulk metallic glass; Room-temperature plasticity; Biocompatibility
1. Introduction Bulk metallic glasses (BMGs) have attracted increasing attention in the past decade due to their unique properties including high elastic strain, relatively low Young’s modulus and, excellent corrosion resistance [1–4]. These properties together with easy forming ability in a supercooled liquid state make them extremely promising for biomedical applications. This potential has recently motivated studies on the biocompatibility of a few Zr-based BMGs [5–8]. Hiromoto et al. [5–7] primarily investigated the effects of chloride-ion concentration, pH value, surface finishing, and dissolved oxygen pressure on the polarization behavior of Zr65 Cu17.5 Ni10 Al7.5 (numbers indicate at.%) bulk metallic glass in a phosphate buffered solution. Horton et al. [8] subsequently performed a series of tests on the corrosion behavior and biocompatibility of BAM-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni). The results demonstrated that Zr-based BMGs are promising for the biomedical application. The drawback of monolithic BMGs results from the limit plasticity prior to catastrophic failure [9]. The introduction of second phases into the amorphous matrix to form bulk metallic
∗
Corresponding author. Tel.: +86 27 87556894; fax: +86 27 87554405. E-mail address: [email protected] (L. Liu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.255
glass composites (BMGCs) is an effective approach to improve the plasticity of amorphous materials. However, the BMGCs usually show inferior corrosion resistance due to the existence of corrosion-susceptible boundaries between crystallites and the amorphous matrix [10]. On the other hand, recent research has demonstrated that the plasticity of Zr-based bulk metallic glasses can also be improved by the microalloying of refractory metals, such as Ta and Nb, even though no visible crystalline phases were formed [11–13]. Xing et al. [11] pointed out that the extended plasticity in Zr-based BMGs containing minor refractory metals is due to the formation of strong short-range ordering in the amorphous structure, which promotes the formation of multishear bands, and thus increases the global plasticity [11–13]. In this paper, based on the Zr65 Cu17.5 Ni10 Al7.5 amorphous system, we try to develop a new BMG by substituting part of the Zr with Nb with the aim of improving the plasticity and the biocompatibility of the base alloy. 2. Experimental Three Zr-based bulk metallic glasses with nominal compositions of Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2, and 5 at.%) and a diameter of 3 mm were prepared by copper mold casting. The structures of the BMGs prepared were examined by X-ray diffraction (XRD, χ Pert PRO) with Cu K␣ radiation and high-
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Table 1 Composition (10−3 mol/l) of the artificial body fluid used in the study Composition
Concentration (10−3 mol/l)
Na+ K+ Ca2+ Mg2+ HCO3 − Cl− HPO4 2− SO4 2−
142.0 5.0 2.5 1.5 4.2 147.8 1.0 0.5
resolution transmission electron microscopy (HRTEM, Jeol2010). The thermal response of the BMGs was investigated using differential scanning calorimetry (DSC, Perkin-Elmer DSC-7) at a heating rate of 20 K/min. Room-temperature uniaxial compression tests were carried out using an MTS 458.20 testing machine at a strain rate of 10−4 s−1 . Samples for mechanical test were 5 mm in length and 3 mm in diameter. Electrochemical polarization was conducted in a threeelectrode cell using a platinum counter electrode and a saturated calomel reference electrode (SCE) at 37 ◦ C. The electrolyte used in the present study is artificial body fluid (ABF, pH 7.4), the composition of ABF is listed in Table 1. The potentiodynamic polarization curves of the specimens were recorded at a potential sweep rate of 1 mV/s. Ion release measurements of samples with various Nb contents were carried out by using an inductively coupled plasma mass spectrometry (ICP-MS, ThermoElemental X7) after being immersed in ABF at 37 ◦ C for 20 days. The analytical detection resolutions for all metals under these conditions were below 0.5 ng/ml. 3. Results The amorphous feature of as-cast alloys with various Nb contents has been verified by X-ray diffraction [14]. The DSC curves of the BMGs are shown in Fig. 1(a). It can be seen that all samples exhibit a distinct glass transition and wide supercooled liquid region before crystallization. It can be seen that,
Table 2 Mechanical properties of yield stress (σ y ), fracture stress (σ f ), elastic strain (εe ), plastic strain (εp ), and fracture strain (εf ) of the Zr65−x Nbx Cu17.5 Al7.5 Ni10 (x = 0, 2, and 5 at.%) BMGs under uniaxial comparison x (at.% Nb)
σ y (MPa)
σ f (MPa)
εe (%)
εp (%)
εf (%)
0 2 5
1620 1619 1625
1647 1713 1741
2.98 2.54 2.41
– 3.00 5.01
2.98 5.54 7.41
with the increase in Nb content, the glass transition temperature (Tg ) increases while the onset temperature of crystallization (Tx ) decreases, leading to a significant decrease in Tx from 103 ◦ C for the Nb-free BMG to 68 ◦ C for the one containing 5 at.% Nb. On the other hand, the addition of Nb also had a significant effect on the crystallization behavior of the base alloy, as indicated by the change from a single crystallization event for the Nbfree BMG to a multi-crystallization process for the Nb-bearing BMGs. XRD demonstrated that the first exothermic peak for the Nb-bearing BMGs corresponds to the formation of icosahedral quasicrystals (see Fig. 1(b)), which were finally transformed to the stable intermetallic Zr2 Cu after the completion of crystallization. The formation of quasicrystals was further confirmed by TEM electron diffraction, which indicated clearly a five-fold symmetry of the crystals formed after annealing to the end of first DSC peak [15]. However, the exothermic event for Nb-free BMG is related to the transformation of the amorphous phase to Zr2 Ni and Zr2 Cu compounds (see Fig. 1(b)), which are the common phases formed primarily during annealing for other Zr-based BMGs. Fig. 2 shows the stress–strain (σ–ε) curves of the BMGs with various Nb contents under compressive loading. The yield stress (σ y ), ultimate compression stress (σ f ), elastic strain (εe ), plastic strain (εp ), and ultimate compression strain (εf ) are summarized in Table 2. It can be seen that both σ f and εp increased remarkably with the increase in Nb. In particular, the plastic strain (εp ) and global strain (εf ) increased from 0 and 2.98% for the Nb-free alloy to 5.01 and 7.40% for the alloy with 5 at.% Nb, respectively. The Young’s modulus (Ey ) of the BMGs was calculated to be 60–70 GPa on the basis of the linear part in σ–ε curves, which is
Fig. 1. (a) DSC curves of the as-cast Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2 and 5 at.%) BMGs at a heating rate of 20 K/min. (b) XRD patterns of the as-cast Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2 and 5 at.%) BMGs after DSC scanning to the end of the first exothermic peak.
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195
Zr65−x Nbx
Fig. 4. Potentiodynamic polarization curves of Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2, 5 at.%) BMGs in artificial body fluids open to air at 37 ◦ C.
much less than that of stainless steel (316L, Ey = 200 GPa) and Ti–6Al–4V alloy (Ey = 102 GPa), but close to the Young’s modulus of human bones (Ey = 20–30 GPa). The results demonstrate that the present Zr-based BMGs are mechanically compatible for medical use. Fig. 3 shows the SEM morphologies of the side surface of Zr65 Cu17.5 Al7.5 Ni10 and Zr60 Cu17.5 Al7.5 Ni10 Nb5 BMGs after fracture. It was found that much more shear bands existed in the Nb-bearing BMG than in the Nb-free BMG, and most of shear bands were deflected and bifurcated. Since the plastic deformation achieved by bulk metallic glasses is confined almost entirely to the narrow regions near shear bands, the high density and extended distribution of the shear bands should account for the enhanced plasticity in the Nb-bearing BMG. Fig. 4 shows the polarization curves of the BMGs with different Nb contents in ABF open to air at 37 ◦ C. The results for 316L stainless steel and Ti–6Al–4V alloy tested under the same conditions are also presented for comparison. It can be seen that all BMGs showed similar polarization behaviors, i.e., they were all
spontaneously passivated with similar passive current densities and distinct passive regions before pitting occurred. However, with the increase in Nb content, the corrosion potential of the BMGs shifted moderately to a more positive regime and the pitting potential increased significantly, indicating that the addition of Nb enhances the corrosion resistance of the BMGs in ABF. Compared with 316L stainless steel and Ti–6Al–4V alloy, the Nb-bearing BMGs exhibited a lower passive current density. This means that the passive films formed in the BMGs containing Nb are more protective. In order to evaluate the biocompatibility of the BMGs, the metal ion release of the BMGs with 5 at.% Nb and without Nb was measured by ICP-MS after immersion in ABF for 20 days. The results are summarized in Table 3. It is clear that the addition of Nb greatly reduced the ion release of all kinds of metals. In particular, Ni and Cu ions, that are usually considered to be harmful to body tissue, showed a remarkable reduction. The total amount of the ion release of the two BMGs was lower than that of 316L stainless steel, which is consistent with the
Fig. 2. Compressive stress–stain curves of the as-cast Cu17.5 Al7.5 Ni10 (x = 0, 2, 5 at.%) BMGs at room temperature.
Fig. 3. SEM micrographs of shear bands on the surfaces of samples of Zr65 Cu17.5 Al7.5 Ni10 BMG (a) and Zr60 Nb5 Cu17.5 Al7.5 Ni10 BMG (b) after compression test.
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Table 3 The concentrations (ng/ml) of various metal ions released from Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0 and 5 at.%) and 316L stainless steels after static immersion in ABF for 20 days Alloys
Ni
Cu
Zr
Nb
Fe
Cr
Zr65 Cu17.5 Ni10 Al7.5 Zr60 Nb5 Cu17.5 Ni10 Al7.5 316L stainless steel
33.4 11.0 12.4
48.7 17.5 –
259.6 34.6 –
– 35.3 –
– – 1210.3
– – 9.74
tals during annealing in the Nb-bearing BMG (see Fig. 1(b)) in the present study indicates that the addition of Nb enhances in reality the formation of icosahedral short-range ordering structures in the amorphous phase. The enhancement in the formation of the short/middle-range order structure was also reported in the Zr65 Cu17.5 Ni10 Al7.5 system by the addition of Ag, Pd, Au or Pt [16]. However, the detailed mechanism for the reaction between the short/middle range order structure and the shear band still needs to be further investigated. The addition of Nb also greatly enhanced the corrosion resistance of the Zr-based BMG in artificial body fluid, as indicated by a much higher pitting potential and lower passive current density in the Nb-bearing BMG than in the Nb-free BMG. XPS measurements revealed that the passive film formed on the Nb-bearing BMGs after anodic polarization was enriched in aluminum oxide, but depleted in phosphate ions [14]. Due to the fact that aluminum oxide is a highly corrosion-resistant, and that the corrosion attacks in a solution containing chloride ions mainly occurred at the sites with phosphate ions. Passive film with a high content of aluminum oxide and less phosphate ions should be more protective. The beneficial effect of Nb on the biocompatibility of Zr-based BMGs is also reflected in the release of metal ions. ICP-MS measurements revealed that the addition of Nb could significantly reduce the ion release of all kinds of metals in the base alloy (see Table 3). This can be attributed to the formation of protective passive film in ABF, and the element of Nb is believed to promote this process [17]. 5. Conclusions
Fig. 5. High-resolution TEM image and selected area electron diffraction pattern of as-cast Zr60 Nb5 Cu17.5 Ni10 Al7.5 BMG.
polarization result whereby the BMGs showed a lower passive current density than 316L stainless steel. 4. Discussion The experimental results demonstrate that the addition of Nb can remarkably improve the plasticity of the base alloy, although all of the alloys studied are apparently of an amorphous structure from X-ray diffraction. Similar enhancement of plasticity has been reported in a few of the other BMG systems [11–13]. On the other hand, the enhanced plasticity has also been observed in some BMGs containing a small volume of nanocrystals, which may be too fine in size and too small in amount to be detected by ordinary XRD. We have considered this possibility by carefully examining the structure of the sample containing 5 at.% Nb using HRTEM. Fig. 5 shows the HR micrograph and electron diffraction pattern of the sample. No trace of crystalline phases could be observed in the sample within the solution of our TEM apparatus (∼1 nm). Xing et al. [11] and Lee et al. [13] 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 promotes the formation and propagation of shear bands. In fact, the preferential precipitation of quasicrys-
Zr65−x Nbx Cu17.5 Ni10 Al7.5 bulk metallic glasses with Nb = 0, 2 and 5 at.% were prepared by copper mold casting. The results demonstrated that the addition of Nb remarkably enhanced the plasticity and improved the biocompatibility of the base BMG. The superior corrosion resistance of Nb-bearing BMGs is attributed to the enrichment in aluminum oxides and depletion in phosphate ions in the passive film formed. The present results imply that Zr-based BMGs microalloyed with Nb are promising for biomedical applications. Acknowledgments This work was financially supported by the Natural Science Foundation of China under grant No. 50571039 and the Excellent Young Teachers Program of the Ministry of Education, the People’s Republic of China. References [1] H.A. Bruck, T. Christman, A.J. Rosakis, W.L. Johnson, Scripta Mater. 30 (1994) 429. [2] C.J. Gilbert, R.O. Ritchie, W.L. Johnson, Appl. Phys. Lett. 71 (1997) 476. [3] S. Pang, T. Zhang, H. Kimura, K. Asami, A. Inoue, Mater. Trans. JIM 41 (2000) 1490. [4] J.G. Wang, B.W. Choi, T.G. Nieh, C.T. Liu, J. Mater. Res. 15 (2000) 913. [5] S. Hiromoto, A.P. Tsai, M. Sumita, T. Hanawa, Corros. Sci. 42 (2000) 1651. [6] S. Hiromoto, A.P. Tsai, M. Sumita, T. Hanawa, Corros. Sci. 42 (2000) 2193. [7] S. Hiromoto, A.P. Tsai, M. Sumita, T. Hanawa, Corros. Sci. 42 (2000) 2167.
L. Liu et al. / Materials Science and Engineering A 449–451 (2007) 193–197 [8] J.A. Horton, D.E. Parsell, Mater. Res. Soc. Symp. Proc. 754 (2003), CC1.5.1. [9] H.A. Bruck, T. Christman, A.J. Rosakis, W.L. Johnson, Scripta Metall. Mater. 30 (1994) 429. [10] L. Liu, C.L. Qiu, K.C. Chan, J. Alloys Compd. 399 (2005) 144. [11] L.Q. Xing, Y. Li, K.T. Ramesh, Phys. Rev. B 64 (2001) 180211. [12] T.C. Hufnagel, C. Fan, R.T. Ott, Intermetallics 10 (2002) 1163.
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[13] M.H. Lee, J.Y. Lee, D.H. Bae, Intermetallics 12 (2003) 1133. [14] C.L. Qiu, L. Liu, M. Sun, S.M. Zhang, J. Biomed. Mater. Res., Part A 75 (2005) 950. [15] M. Sun, Master Thesis, Huazhong University of Science and Technology, 2005. [16] A. Inoue, T. Zhang, J. Saida, Mater. Trans. JIM 40 (1999) 1181. [17] T. Hanawa, Mater. Sci. Eng. C 24 (2004) 745.
Improvements in the plasticity and biocompatibility of Zr–Cu–Ni–Al bulk metallic glass by the microalloying of Nb L. Liu a,∗ , C.L. Qiu a , M. Sun a , Q. Chen a , K.C. Chan b , Geoffrey K.H. Pang c a
The State Key Lab of Die & Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, China b Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, China c Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China Received 21 August 2005; received in revised form 20 December 2005; accepted 23 February 2006
Abstract Bulk metallic glasses (BMGs) of Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2, and 5 at.%) were prepared by copper mold casting. Room-temperature uniaxial compression tests revealed that the addition of a small amount of Nb could considerably enhance the compression plasticity of Zr-based BMGs. Electrochemical polarization measurements and ion release tests of the BMGs in an artificial body fluid indicated that Nb addition could highly improve the corrosion resistance and reduce the ion release of all kinds of the base alloy. The results demonstrated that Zr-based BMGs microalloyed with Nb hold promise for biomedical use. © 2006 Elsevier B.V. All rights reserved. Keywords: Zr-based bulk metallic glass; Room-temperature plasticity; Biocompatibility
1. Introduction Bulk metallic glasses (BMGs) have attracted increasing attention in the past decade due to their unique properties including high elastic strain, relatively low Young’s modulus and, excellent corrosion resistance [1–4]. These properties together with easy forming ability in a supercooled liquid state make them extremely promising for biomedical applications. This potential has recently motivated studies on the biocompatibility of a few Zr-based BMGs [5–8]. Hiromoto et al. [5–7] primarily investigated the effects of chloride-ion concentration, pH value, surface finishing, and dissolved oxygen pressure on the polarization behavior of Zr65 Cu17.5 Ni10 Al7.5 (numbers indicate at.%) bulk metallic glass in a phosphate buffered solution. Horton et al. [8] subsequently performed a series of tests on the corrosion behavior and biocompatibility of BAM-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni). The results demonstrated that Zr-based BMGs are promising for the biomedical application. The drawback of monolithic BMGs results from the limit plasticity prior to catastrophic failure [9]. The introduction of second phases into the amorphous matrix to form bulk metallic
∗
Corresponding author. Tel.: +86 27 87556894; fax: +86 27 87554405. E-mail address: [email protected] (L. Liu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.255
glass composites (BMGCs) is an effective approach to improve the plasticity of amorphous materials. However, the BMGCs usually show inferior corrosion resistance due to the existence of corrosion-susceptible boundaries between crystallites and the amorphous matrix [10]. On the other hand, recent research has demonstrated that the plasticity of Zr-based bulk metallic glasses can also be improved by the microalloying of refractory metals, such as Ta and Nb, even though no visible crystalline phases were formed [11–13]. Xing et al. [11] pointed out that the extended plasticity in Zr-based BMGs containing minor refractory metals is due to the formation of strong short-range ordering in the amorphous structure, which promotes the formation of multishear bands, and thus increases the global plasticity [11–13]. In this paper, based on the Zr65 Cu17.5 Ni10 Al7.5 amorphous system, we try to develop a new BMG by substituting part of the Zr with Nb with the aim of improving the plasticity and the biocompatibility of the base alloy. 2. Experimental Three Zr-based bulk metallic glasses with nominal compositions of Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2, and 5 at.%) and a diameter of 3 mm were prepared by copper mold casting. The structures of the BMGs prepared were examined by X-ray diffraction (XRD, χ Pert PRO) with Cu K␣ radiation and high-
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Table 1 Composition (10−3 mol/l) of the artificial body fluid used in the study Composition
Concentration (10−3 mol/l)
Na+ K+ Ca2+ Mg2+ HCO3 − Cl− HPO4 2− SO4 2−
142.0 5.0 2.5 1.5 4.2 147.8 1.0 0.5
resolution transmission electron microscopy (HRTEM, Jeol2010). The thermal response of the BMGs was investigated using differential scanning calorimetry (DSC, Perkin-Elmer DSC-7) at a heating rate of 20 K/min. Room-temperature uniaxial compression tests were carried out using an MTS 458.20 testing machine at a strain rate of 10−4 s−1 . Samples for mechanical test were 5 mm in length and 3 mm in diameter. Electrochemical polarization was conducted in a threeelectrode cell using a platinum counter electrode and a saturated calomel reference electrode (SCE) at 37 ◦ C. The electrolyte used in the present study is artificial body fluid (ABF, pH 7.4), the composition of ABF is listed in Table 1. The potentiodynamic polarization curves of the specimens were recorded at a potential sweep rate of 1 mV/s. Ion release measurements of samples with various Nb contents were carried out by using an inductively coupled plasma mass spectrometry (ICP-MS, ThermoElemental X7) after being immersed in ABF at 37 ◦ C for 20 days. The analytical detection resolutions for all metals under these conditions were below 0.5 ng/ml. 3. Results The amorphous feature of as-cast alloys with various Nb contents has been verified by X-ray diffraction [14]. The DSC curves of the BMGs are shown in Fig. 1(a). It can be seen that all samples exhibit a distinct glass transition and wide supercooled liquid region before crystallization. It can be seen that,
Table 2 Mechanical properties of yield stress (σ y ), fracture stress (σ f ), elastic strain (εe ), plastic strain (εp ), and fracture strain (εf ) of the Zr65−x Nbx Cu17.5 Al7.5 Ni10 (x = 0, 2, and 5 at.%) BMGs under uniaxial comparison x (at.% Nb)
σ y (MPa)
σ f (MPa)
εe (%)
εp (%)
εf (%)
0 2 5
1620 1619 1625
1647 1713 1741
2.98 2.54 2.41
– 3.00 5.01
2.98 5.54 7.41
with the increase in Nb content, the glass transition temperature (Tg ) increases while the onset temperature of crystallization (Tx ) decreases, leading to a significant decrease in Tx from 103 ◦ C for the Nb-free BMG to 68 ◦ C for the one containing 5 at.% Nb. On the other hand, the addition of Nb also had a significant effect on the crystallization behavior of the base alloy, as indicated by the change from a single crystallization event for the Nbfree BMG to a multi-crystallization process for the Nb-bearing BMGs. XRD demonstrated that the first exothermic peak for the Nb-bearing BMGs corresponds to the formation of icosahedral quasicrystals (see Fig. 1(b)), which were finally transformed to the stable intermetallic Zr2 Cu after the completion of crystallization. The formation of quasicrystals was further confirmed by TEM electron diffraction, which indicated clearly a five-fold symmetry of the crystals formed after annealing to the end of first DSC peak [15]. However, the exothermic event for Nb-free BMG is related to the transformation of the amorphous phase to Zr2 Ni and Zr2 Cu compounds (see Fig. 1(b)), which are the common phases formed primarily during annealing for other Zr-based BMGs. Fig. 2 shows the stress–strain (σ–ε) curves of the BMGs with various Nb contents under compressive loading. The yield stress (σ y ), ultimate compression stress (σ f ), elastic strain (εe ), plastic strain (εp ), and ultimate compression strain (εf ) are summarized in Table 2. It can be seen that both σ f and εp increased remarkably with the increase in Nb. In particular, the plastic strain (εp ) and global strain (εf ) increased from 0 and 2.98% for the Nb-free alloy to 5.01 and 7.40% for the alloy with 5 at.% Nb, respectively. The Young’s modulus (Ey ) of the BMGs was calculated to be 60–70 GPa on the basis of the linear part in σ–ε curves, which is
Fig. 1. (a) DSC curves of the as-cast Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2 and 5 at.%) BMGs at a heating rate of 20 K/min. (b) XRD patterns of the as-cast Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2 and 5 at.%) BMGs after DSC scanning to the end of the first exothermic peak.
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195
Zr65−x Nbx
Fig. 4. Potentiodynamic polarization curves of Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0, 2, 5 at.%) BMGs in artificial body fluids open to air at 37 ◦ C.
much less than that of stainless steel (316L, Ey = 200 GPa) and Ti–6Al–4V alloy (Ey = 102 GPa), but close to the Young’s modulus of human bones (Ey = 20–30 GPa). The results demonstrate that the present Zr-based BMGs are mechanically compatible for medical use. Fig. 3 shows the SEM morphologies of the side surface of Zr65 Cu17.5 Al7.5 Ni10 and Zr60 Cu17.5 Al7.5 Ni10 Nb5 BMGs after fracture. It was found that much more shear bands existed in the Nb-bearing BMG than in the Nb-free BMG, and most of shear bands were deflected and bifurcated. Since the plastic deformation achieved by bulk metallic glasses is confined almost entirely to the narrow regions near shear bands, the high density and extended distribution of the shear bands should account for the enhanced plasticity in the Nb-bearing BMG. Fig. 4 shows the polarization curves of the BMGs with different Nb contents in ABF open to air at 37 ◦ C. The results for 316L stainless steel and Ti–6Al–4V alloy tested under the same conditions are also presented for comparison. It can be seen that all BMGs showed similar polarization behaviors, i.e., they were all
spontaneously passivated with similar passive current densities and distinct passive regions before pitting occurred. However, with the increase in Nb content, the corrosion potential of the BMGs shifted moderately to a more positive regime and the pitting potential increased significantly, indicating that the addition of Nb enhances the corrosion resistance of the BMGs in ABF. Compared with 316L stainless steel and Ti–6Al–4V alloy, the Nb-bearing BMGs exhibited a lower passive current density. This means that the passive films formed in the BMGs containing Nb are more protective. In order to evaluate the biocompatibility of the BMGs, the metal ion release of the BMGs with 5 at.% Nb and without Nb was measured by ICP-MS after immersion in ABF for 20 days. The results are summarized in Table 3. It is clear that the addition of Nb greatly reduced the ion release of all kinds of metals. In particular, Ni and Cu ions, that are usually considered to be harmful to body tissue, showed a remarkable reduction. The total amount of the ion release of the two BMGs was lower than that of 316L stainless steel, which is consistent with the
Fig. 2. Compressive stress–stain curves of the as-cast Cu17.5 Al7.5 Ni10 (x = 0, 2, 5 at.%) BMGs at room temperature.
Fig. 3. SEM micrographs of shear bands on the surfaces of samples of Zr65 Cu17.5 Al7.5 Ni10 BMG (a) and Zr60 Nb5 Cu17.5 Al7.5 Ni10 BMG (b) after compression test.
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Table 3 The concentrations (ng/ml) of various metal ions released from Zr65−x Nbx Cu17.5 Ni10 Al7.5 (x = 0 and 5 at.%) and 316L stainless steels after static immersion in ABF for 20 days Alloys
Ni
Cu
Zr
Nb
Fe
Cr
Zr65 Cu17.5 Ni10 Al7.5 Zr60 Nb5 Cu17.5 Ni10 Al7.5 316L stainless steel
33.4 11.0 12.4
48.7 17.5 –
259.6 34.6 –
– 35.3 –
– – 1210.3
– – 9.74
tals during annealing in the Nb-bearing BMG (see Fig. 1(b)) in the present study indicates that the addition of Nb enhances in reality the formation of icosahedral short-range ordering structures in the amorphous phase. The enhancement in the formation of the short/middle-range order structure was also reported in the Zr65 Cu17.5 Ni10 Al7.5 system by the addition of Ag, Pd, Au or Pt [16]. However, the detailed mechanism for the reaction between the short/middle range order structure and the shear band still needs to be further investigated. The addition of Nb also greatly enhanced the corrosion resistance of the Zr-based BMG in artificial body fluid, as indicated by a much higher pitting potential and lower passive current density in the Nb-bearing BMG than in the Nb-free BMG. XPS measurements revealed that the passive film formed on the Nb-bearing BMGs after anodic polarization was enriched in aluminum oxide, but depleted in phosphate ions [14]. Due to the fact that aluminum oxide is a highly corrosion-resistant, and that the corrosion attacks in a solution containing chloride ions mainly occurred at the sites with phosphate ions. Passive film with a high content of aluminum oxide and less phosphate ions should be more protective. The beneficial effect of Nb on the biocompatibility of Zr-based BMGs is also reflected in the release of metal ions. ICP-MS measurements revealed that the addition of Nb could significantly reduce the ion release of all kinds of metals in the base alloy (see Table 3). This can be attributed to the formation of protective passive film in ABF, and the element of Nb is believed to promote this process [17]. 5. Conclusions
Fig. 5. High-resolution TEM image and selected area electron diffraction pattern of as-cast Zr60 Nb5 Cu17.5 Ni10 Al7.5 BMG.
polarization result whereby the BMGs showed a lower passive current density than 316L stainless steel. 4. Discussion The experimental results demonstrate that the addition of Nb can remarkably improve the plasticity of the base alloy, although all of the alloys studied are apparently of an amorphous structure from X-ray diffraction. Similar enhancement of plasticity has been reported in a few of the other BMG systems [11–13]. On the other hand, the enhanced plasticity has also been observed in some BMGs containing a small volume of nanocrystals, which may be too fine in size and too small in amount to be detected by ordinary XRD. We have considered this possibility by carefully examining the structure of the sample containing 5 at.% Nb using HRTEM. Fig. 5 shows the HR micrograph and electron diffraction pattern of the sample. No trace of crystalline phases could be observed in the sample within the solution of our TEM apparatus (∼1 nm). Xing et al. [11] and Lee et al. [13] 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 promotes the formation and propagation of shear bands. In fact, the preferential precipitation of quasicrys-
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