A. Inoue

A. L. Greer

A. Ochsenbein

A. Yamamoto

C. L. Qiu

C. Zhang

E. A. Vogler

G. F. Nordberg

I. Catelas

K. F. Jin

L. Liu

L. Ponsonnet

Lu Huang

M. Niinomi

M. Z. Ma

N. B. Hua

Q. K. Jiang

S. Buzzi

S. J. Pang

ShuJie Pang

T. Wada

T. Zhang

Tao Zhang

W. H. Wang

W. L. Johnson

XuYang Lu

Y. B. Wang

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    © The Author(s) 2012. This article is published with open access at Springerlink.com csb.scichina.com   www.springer.com/scp                       *Corresponding author (email: pangshujie@buaa.edu.cn) Article Materials Science May 2012  Vol.57  No.14: 17231727  doi: 10.1007/s11434-012-5027-0  Formation and biocorrosion behavior of Zr-Al-Co-Nb bulk  metallic glasses LU XuYang, HUANG Lu, PANG ShuJie* & ZHANG Tao Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang  University, Beijing 100191, China Received November 14, 2011; accepted December 13, 2011; published online March 8, 2012  Ni- and Cu-free Zr-Al-Co-Nb glassy alloys with different Nb and Co contents were synthesized by melt spinning and copper mold casting. The effects of Nb addition to partially replace Co in the Zr55Al20Co25 glassy alloy on the glass-forming ability, thermal properties, in-vitro biocorrosion behavior and surface wettability of the metallic glasses were investigated. Although addition of Nb up to 5 at.% slightly decreased the supercooled liquid region and the glass-forming ability (GFA), the alloys could be casted in a bulk glassy rod form with diameters up to 3 mm. The Zr-Al-Co-Nb glassy alloys were spontaneously passivated with low pas-sive current densities in phosphate buffered saline and Hanks’ solution. Substitution of “toxic” Co by Nb is effective in improved the corrosion resistance of the Zr-Al-Co glassy alloy. Water contact angle measurements showed that Nb addition increased the hydrophilicity of the glassy alloys, which may enhance cell adhesion of the alloys in biomedical applications.  Zr-based alloy, Nb, metallic glass, corrosion, biocompatibility  Citation:  Lu X Y, Huang L, Pang S J, et al. Formation and biocorrosion behavior of Zr-Al-Co-Nb bulk metallic glasses. Chin Sci Bull, 2012, 57: 17231727, doi: 10.1007/s11434-012-5027-0     Because of the lack of long-range atomic order, Zr-based bulk metallic glasses (BMGs) exhibit superior strength, high elastic strain limits and fatigue endurance limits, relatively low Young’s modulus, and excellent wear and corrosion resistance [1–5], which attracted attention in biomedical applications. However, toxic elements such as nickel and copper are often included in Zr-based metallic glasses to achieve high glass-forming ability (GFA), which are thought to impair cellular metabolism [6]. In recent years, many efforts have been made to remove these elements from Zr-based BMGs to further improve their biocompatibility. Major efforts have been made in removing Ni from the al-loying elements, because Ni is a known allergen. Ni-free glass forming systems such as Zr-Al-Cu [7], Zr-Al-Co [8–11], Zr-Al-Co-Cu [12], Zr-Al-Fe-Cu [13], Zr-Cu-Pd- Al-Nb [14], Zr-(Cu, Ag)-Al [15], Zr-Al-Cu-Fe-(Ti/Nb) [16], and Zr-Al-Co-Ag [17,18] have been developed. Among the Ni-free Zr-based BMGs, Zr-Al-Co BMGs are attractive because they are free from copper, which may result in high cytotoxicity if excessive release occurs during corrosion and is also an element of concern in biomaterials [19]. Based on the ternary Zr55Al20Co25 alloy with high GFA, quaternary Zr55Al20xCo25Nbx (x = 2.5 at.% and 5 at.%) metallic glasses have been developed, and addition of Nb were found to enhance their resistance to pitting corrosion in 3 mass% NaCl solution [20]. From the viewpoint of biomedical en-gineering, Nb is recognized as a highly biocompatible ele-ment. In the quaternary Zr-Al-Co-Nb alloys, Co is more cytotoxic than Al, and excessive amounts of Co or pro-longed exposure to Co may result in Co poisoning [21–23]. Therefore, rather than substitution of Al with Nb as in the previous study [20], substitution of Co with Nb could further improve the biocompatibility of the alloys. In consideration of potential medical applications, the GFA and in vitro bio-corrosion behavior of Zr55Al20Co25xNbx glassy alloys were investigated in the present work. Surface wettability of the 1724 Lu X Y, et al.   Chin Sci Bull   May (2012) Vol.57 No.14 Zr55Al20Co25xNbx glassy alloys was also studied, which can be related to cell attachment/adhesion with relatively higher hydrophilicity enhancing cell adhesion [24,25]. The effects of substitution of Co with Nb on the formation and corro-sion resistance of the Zr-Al-Co glassy alloy are discussed. 1  Experimental  Master alloys with nominal compositions of Zr55Al20Co25x-   Nbx (x = 0, 2.5 at.% and 5 at.%) were prepared by arc melting mixtures of the pure metals in a Ti-gettered high-purity argon atmosphere. Ribbon samples (thickness 0.02 mm, width 1 or 4 mm) and cylindrical alloy rods with different diameters were prepared from crushed alloy ingots in an argon atmosphere by melt spinning and injection copper mold casting, respec-tively. The microstructures of the specimens were examined by X-ray diffraction (XRD) with Cu-K radiation. The thermal stability of the alloys was investigated by differential scan-ning calorimetry (DSC) at a heating rate of 0.33 K/s.  The electrochemical behaviors of the Zr-Al-Co-Nb glassy alloys were characterized using ribbon samples of 1 mm wide in phosphate-buffered saline (PBS) and Hanks’ bal-anced salt solution. Before the tests, the solutions were heated to 310 K in a water bath and aerated with 4 vol.% O2/N2 gas mixture at a flow rate of 50 mL/min. Aeration was continued throughout the tests. The surface of each specimen was mechanically polished in cyclohexane with silicon carbide paper up to # 2000 (average particle diame-ter 10 m), degreased in acetone, cleaned in ethanol and distilled water, dried in air, and then exposed to air for about 24 h for good reproducibility. A three-electrode cell system consisting of a corrosion sample as the working electrode, a saturated calomel reference electrode (SCE, ESCE= 0.242 V), and a platinum counter electrode was adopted. Prior to potentiodynamic polarizations, the test samples were immersed in the electrolytes for 4000 s to allow open circuit potentials (OCPs) to reach steady state. Anodic potentiody-namic polarization tests were started at about 0.05 V below the OCPs at a potential sweep rate of 50 mV/min.  For the characterization of the surface wettability of the glassy alloys, water contact angles were measured using the sessile drop technique at 298 K with 4-mm-wide ribbon samples. The surface of each ribbon sample was polished with silicon carbide paper up to # 2000, cleaned in an ultra-sonic cleaner with acetone, ethanol and distilled water, and then dried in air. Deionized water droplets (about 0.5 L) were dropped carefully onto the surface. Measurements were performed at five different positions along each sample and the average contact angle was calculated.  2  Results and discussion XRD patterns of the Zr55Al20Co25-xNbx (at.%) alloy samples with different geometries are shown in Figure 1. The XRD patterns of the Zr55Al20Co25 alloys with diameters up to 5 mm show a broad peak at around 2 = 37°, which is characteris-tic of the amorphous structure. Nb addition negatively im-pact glass formation of the Zr-Al-Co alloy. Substitution of Co by Nb at 2.5 at.% decreases the critical diameter for glass formation to 3 mm. Crystallization peaks for Zr6Al2Co and ZrCo can be observed in the diffraction pattern of the  5 mm sample. When the Nb content is increased to 5 at.%, the critical diameter of the Zr55Al20Co20Nb5 BMG decreases to 2 mm. The XRD patterns of partially crystallized Zr-Al- Co-Nb alloys indicate that Nb addition influences the GFA of the Zr-Al-Co BMGs by enhancing formation of Zr6Al2Co   Figure 1  XRD patterns of Zr55Al20Co25xNbx (x = 0, 2.5 at.% and 5 at.%) ribbon and rod samples.  Lu X Y, et al.   Chin Sci Bull   May (2012) Vol.57 No.14 1725 and ZrCo phases. DSC curves of the Zr-Al-Co-Nb alloy ribbons and rods with their critical diameters are shown in Figure 2, where Tg and Tx are glass transition temperature and onset tempera-ture of crystallization, respectively. Upon heating, each of the samples exhibits a distinct glass transition followed by a wide supercooled liquid region (Tx = Tx–Tg) before crystal-lization. The values of Tg, Tx, and heat release of the crystal-lization for the rod samples are almost identical to those of the ribbon samples with the same compositions, which fur-ther confirms the glassy structure of the bulk alloys. With the increase in Nb content, Tg decreases from 746 to 728 K, and Tx decreases from 821 to 790 K, which causes a de-crease in Tx from 75 to 62 K. The decrease in Tx can be correlated with the decrease in the GFA of the alloys.  To simulate body fluid, PBS and Hank’s solution were used in corrosion tests. The former is a simple solution with a similar Cl concentration and pH value to body fluid, and the later is more complex, which additionally contains Mg2+, Ca2+, and glucose. Changes in the open circuit potential with immersion time for the Zr55Al20Co25xNbx (x = 0, 2.5 at.% and 5 at.%) glassy alloys in PBS and Hanks’ solution are illus-trated in Figure 3. During immersion, the potential of each alloy initially rises steeply and then slows down until reaches constant values, which are determined as OCPs. The OCP values of the alloys in both solutions increase with the increase in Nb content, indicating that Nb addition im-proved the stability of the surface films of the alloys.  Figure 4 shows the anodic polarization curves of the glassy alloys in PBS and Hanks’ solution at 310 K with aeration of O2/N2 gas mixture at 50 mL/min. All alloys are spontaneously passivated with wide passive regions before pitting in both electrolytes, and their passive current densi-ties are around 0.08 A/m2. Figure 5 shows that the pitting potential of the Zr55Al20Co25xNbx BMGs increases consid-erably with increasing Nb content. Therefore, partial sub-stitution of Co by Nb effectively improves the corrosion resistance of the Zr55Al20Co25 BMG in the simulated body   Figure 2  DSC curves of Zr55Al20Co25xNbx (x = 0, 2.5 at.% and 5 at.%) ribbon and rod samples.  Figure 3  Changes in the open-circuit potential with immersion time for Zr55Al20Co25xNbx (x = 0, 2.5 at.% and 5 at.%) glassy alloys in PBS and Hanks’ solution.  Figure 4  Anodic polarization curves of Zr55Al20Co25xNbx (x = 0, 2.5 at.% and 5 at.%) glassy alloys in PBS and Hanks’ solution. 1726 Lu X Y, et al.   Chin Sci Bull   May (2012) Vol.57 No.14  Figure 5  Changes in the pitting potentials of Zr55Al20Co25xNbx (x = 0, 2.5 at.% and 5 at.%) glassy alloys with Nb content in PBS and Hanks’ solution. fluids. The positive effect on the corrosion resistance of the Zr-Al-Co glassy alloy by Nb addition can be attributed to the change in the composition of the passive film. It has been reported that Nb addition contributes to the formation of a protective surface film with higher chemical stability [20]. Water contact angle results for the Zr-Al-Co-Nb glassy alloys are presented in Figure 6. The same abrasion proce-dure diminishes the effect of surface roughness on the sur-face wettability. The water contact angle decreases consid-erably with the addition of Nb, which indicates higher hy-drophilicity of the Nb-containing alloys. When the Nb con-tent reaches 5 at.%, the water contact angles become small-er than the Berg limit ( = 65°), which is beneficial for bio-medical applications [26]. The present experimental results indicate that Nb addition simultaneously improves both the hydrophilicity and the corrosion resistance of the Zr-Al-Co- Nb glassy alloys. This is different from an earlier finding that related high corrosion resistance with low hydrophilic-ity for ZrTiCuNiBe metallic glasses [27]. The improvement in the corrosion resistance of the Zr-Al-Co glassy alloy with Nb addition is attributed to the change in the composition of the passive film, because Nb addition to Zr-based glassy alloys induces the formation of a more protective passive film enriched in Nb [20]. Further research will be carried out to clarify whether the composition or wettability is dominant in determining the corrosion resistance of various glassy alloy systems.  3  Conclusions Ni- and Cu-free Zr55Al20Co25xNbx (x = 2.5 at.% and 5 at.%)   Figure 6  Contact angles of water droplets measured on Zr55Al20Co25xNbx (x = 0, 2.5 at.% and 5 at.%) glassy alloys. BMGs with critical diameters up to 3 mm were developed. The glassy alloys exhibited high thermal stability, and good biocorrosion resistance and surface wettability. Nb addition was slightly detrimental to the GFA. The corrosion re-sistance of the Zr-Al-Co BMG in PBS and Hanks’ solution was improved considerably with the increase in Nb content, which was indicated by the widening of the passive region, and the increases in pitting potential and OCP. Water con-tact angle measurements showed that Nb addition improved the hydrophilicity of the alloy, which is beneficial for bio-medical applications. This work was supported by the National Natural Science Foundation of China (51161130526 and 50771005) and the Innovation Foundation of Bei-hang University (BUAA) for PhD Graduates (special program for DSAE).  Lu X Y, et al.   Chin Sci Bull   May (2012) Vol.57 No.14 1727 1 Johnson W L. Bulk glass-forming metallic alloys: Science and tech-nology. MRS Bull, 1999, 24: 42–56 2 Wang W H, Dong C, Shek C H. Bulk metallic glasses. Mater Sci Eng: R: Reports, 2004, 44: 45–89 3 Greer A L, Ma E. Bulk metallic glasses: At the cutting edge of metals research. 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Formation and biocorrosion behavior of Zr-Al-Co-Nb bulk metallic glasses

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Formation and biocorrosion behavior of Zr-Al-Co-Nb bulk metallic glasses

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