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Cu ratios for biomedical application
Intermetallics 72 (2016) 36e43
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Formation and properties of Ti-based TieZreCueFeeSneSi bulk metallic glasses with different (Ti þ Zr)/Cu ratios for biomedical application Ying Liu, Shujie Pang*, Haifei Li, Qiao Hu, Bin Chen, Tao Zhang** Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2015 Received in revised form 21 January 2016 Accepted 22 January 2016 Available online xxx
Ti-based TieZreCueFeeSneSi bulk metallic glasses (BMGs) free from highly toxic elements Ni and Be were developed as promising biomaterials. The influence of (Ti þ Zr)/Cu ratio on glass-formation, thermal stability, mechanical properties, bio-corrosion resistance, surface wettability and biocompatibility were investigated. In the present Ti-based BMG system, the Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy exhibited the highest glass forming ability (GFA) corresponding to the largest supercooled liquid region, and a glassy rod with a critical diameter of 3 mm was prepared by copper-mold casting. The Ti-based BMGs possess high compressive strength of 2014e2185 MPa and microhardness of 606e613 Hv. Young's modulus of the Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy was about 100 GPa, which is slightly lower than that of Tie6Ale4V alloy. The Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy with high GFA exhibited high biocorrosion resistance, and good surface hydrophilia and cytocompatibility. The mechanisms for glass formation as well as the effect of (Ti þ Zr)/Cu ratio on bio-corrosion behavior and biocompatibility are discussed. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bulk metallic glasses Ti-based alloys Mechanical properties Bio-corrosion resistance Biocompatibility
1. Introduction Ti-based bulk metallic glasses with excellent mechanical and chemical properties, and biocompatibility as well as good netshaping and thermoplastic forming ability [1e3] attribute to the unique amorphous feature, are attractive to be potential biomaterials. Particularly, the combination of high strength, low elastic modulus, and high wear and corrosion resistance of the Ti-based BMGs is favorable for reducing the failures of biomedical implants, which is usually induced by the mismatch of mechanical properties between the implants and bones and leading to particle disease, stress shielding as well as fatigue during long term clinical application [4e6]. As well known, most Ti-based BMGs have been developed based on the Tie(Zr)eCueNi glassy alloy systems [7e11], for which the critical diameters for glass formation were in the range from 1 mm to 6 mm by copper-mold casting, while Ni is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Pang), [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.intermet.2016.01.007 0966-9795/© 2016 Elsevier Ltd. All rights reserved.
highly toxic. Besides, element Be, which is blamed for the antiproliferative effects on human bodies, is also commonly alloyed in Ti-based BMGs to attain good glass formers [12e14]. Therefore, exploring Ni- and Be-free Ti-based bulk glassy alloys with high GFA, and good mechano- and bio-compatibility is one of the challenges to promote the potential application of BMGs as biomaterials. On another hand, for the application of metallic materials used as orthopedic or dental implants, it is crucial to evaluate their biocorrosion behavior, cytocompatibility, and surface wettability which regulates the cell response and further affects the cytocompatibility of implanted biomaterials [15e17], however, studies on these properties of Ti-based BMGs are limited [3,18e20]. It is generally recognized that the main constituent elements of the multicomponent alloys with high GFA have significant difference in atomic size and negative heats of mixing, which can be defined as dissimilar elements [21,22]. It has also been found that the alloying with an element similar to the components of a glassy alloy is effective on improving the GFA [22]. Therefore, based on the TieCu alloy system, considering the coexistence criterion of dissimilar and similar elements and the positive effect of Sn and Si microalloying Ti-based BMGs on GFA [7,10], Ni- and Be-free TieZreCueFeeSneSi bulk glassy alloys bearing similar element pair of CueFe are
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designed and synthesized in the present study. The Ti-based TieZreCueFeeSneSi glassy alloy system with critical diameter up to 3 mm is fabricated by copper-mold casting. In addition, the formation, thermal properties, mechanical properties, biocorrosion behaviors, surface wettability and cytocompatibility of the Ti-based BMGs are investigated, and the related mechanisms are also discussed. 2. Materials and methods TieZreCueFeeSneSi alloy ingots with nominal compositions (at.%) of Ti45Zr5.5Cu44Fe2.5Sn2Si1 ((Ti þ Zr)/Cu ¼ 1.15), Ti45Zr7.5Cu42Fe2.5Sn2Si1 ((Ti þ Zr)/Cu ¼ 1.25), Ti47Zr7.5Cu40Fe2.5Sn2Si1 ((Ti þ Zr)/Cu ¼ 1.36) and Ti48Zr7.5Cu39Fe2.5Sn2Si1 ((Ti þ Zr)/ Cu ¼ 1.42), denoted as Ti45Zr5.5Cu44M, Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M in the following text, respectively, were prepared by arc-melting the mixture of pure elements under a high-purity argon atmosphere. The alloy ingots were melted four times to ensure the compositional homogeneity. From the master alloys, under an argon atmosphere, ribbons and rod samples with different diameters were prepared by melt spinning and copper mold casting, respectively. Microstructure of the samples was characterized by X-ray diffraction (XRD, Bruker AXS D8). Thermal behaviors of the ribbons and the rod samples with critical diameters were investigated by differential scanning calorimetry (DSC, NETZSCH 404C) at a heating rate of 0.33 K/s. Compressive mechanical properties of the Ti-based BMGs were evaluated by a materials testing machine using glassy rod specimens with a dimension of f 1 2 mm for the Ti45Zr5.5Cu44M BMG and f 2 4 mm for other Ti-based BMGs in present study, respectively, at a constant strain rate of 2.1 104 s1. Fracture and lateral surface of the deformed samples were examined by scanning electron microscope (SEM, Cam Scan 3400). Vickers hardness of the glassy samples was measured under a load of 200 gf for 15 s using a Vickers microhardness tester (Future-tech FM800). Bio-corrosion behaviors of the Ti-based glassy alloys were investigated by electrochemical measurements in phosphate buffered saline (PBS, pH ¼ 7.4) solution at about 310 K aerated with a 4 vol.% O2/N2 gas mixture at a flowing rate of 50 ml/min. The Tie6Ale4V alloy, widely used in clinical application as orthopedic implants, was employed as the reference material. Prior to corrosion test, the samples were mechanically polished up to 2000-grit silicon carbide paper in cyclohexane, and then ultrasonic cleaning in acetone, alcohol and de-ionized water, successively, followed by exposing to air for about 24 h for good reproducibility. Electrochemical measurements were conducted using an electrochemical workstation (EG&G Princeton Applied Research VersaSTAT II) with a three-electrode system consisting of a sample as the working electrode, a platinum counter electrode and a saturated calomel reference electrode (SCE, SCE ¼ 0.242 V). After immersion for about 1800 s when the open-circuit potentials (OCP) became almost steady, potentiodynamic polarizations were conducted at a potential sweep rate of 0.833 mV/s. The surface film formed on the Ti47Zr7.5Cu40M BMG with a dimension of f 2.5 3.5 mm after immersion in PBS at 37 C for 30 days were characterized by X-ray photoelectron spectroscopy (XPS) using an ESCALab250 photoelectron spectroscopy with monochromatized Al Ka excitation. XPS spectra were analyzed by XPSPEAK analytical software to obtain further information about chemical states. The concentrations of metal ions released into solution were investigated by inductively coupled plasma-mass spectrometry (ICP-MS, Yoko gawa Analytical Systems HP4500 spectrometer). Surface wettability of the Ti-based glassy alloys was measured by sessile drop method using 5-mm-wide ribbon samples at 298 K. The sample surfaces were polished and cleaned in the same
37
procedure as that for electrochemical measurements. Surface roughness of the samples was confirmed to be similar by atomic force microscope (AFM, Veeco/Bruker ICON), and the roughness values were lower than 100 nm. Deionized water was used as the wetting liquid with a drop size of 3 ml. Sessile drop contact angles of the airewateresubstrate interface were measured using a drop shape analysis system equipped with an automated stage and droplet dispenser, a digital camera, and image analysis software. Five different positions of each sample were assessed to ensure authenticity of the results. Mouse MC3T3-E1 pre-osteoblast cell line was adopted in this study for preliminarily evaluating the cytocompatibility of the Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs with relatively high GFA as well as Tie6Ale4V alloy. Samples with a dimension of f 2 1.5 mm were prepared in the same procedure as reported in our previous study [23]. The MC3T3-E1 cells were cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 1% 100U ml1 penicillin and 100 mg ml1 streptomycin in a 5% CO2 balanced incubator at 37 C. Cells suspension at a density of 2.6 104 cells/cm2 were seeded onto the surface of each substrate directly to observed the morphologies and quantities of the cells adhered on the samples after cultured for 7 days. At the harvest time the samples were fetched out, followed by washing, fixation and dehydration in sequence, and the details were reported in our previous report [23]. Finally the morphologies of the cells adhered on the samples were observed by SEM. Quadruplicate samples of each alloy were adopted for good reproducibility. According to the instructions in ISO 10993-5, in vitro cytotoxicity test of the Ti-based BMGs and Tie6Ale4V alloy was performed using extraction media of the samples. The sterilized samples were immersed in DMEM medium with a surface area/extraction medium ratio 1 cm2/ml in a humidified atmosphere with 5% CO2 at 37 C for 24 h and the extraction medium was stored at 4 C before the cytotoxicity test. The DMEM medium and DMEM medium containing 10% dimethyl sulfoxide (DMSO) were used as a negative control and a positive control, respectively. 100 ml of cells suspension at a density of 2 104 cells/cm2 were seeded in 96-well plate and cultured for 24 h to allow attachment. The media was then replaced with 100 ml of the extraction media. After incubating for 3 days, each well was added with 20 ml of the MTT solution following a further incubation for 4 h. Afterwards, 100 ml of formazan solubilization solution (DMSO) were added. The samples were then removed for spectrophotometric absorbance measurements using a microplate reader at 492 nm. Five replicates (both the samples and the controls) were used for all tests. 3. Results and discussion 3.1. Formation and thermal properties XRD patterns of the Ti-based cast rods with their critical diameters (Dc) are shown in Fig. 1. All the patterns exhibit a main broad halo without any sharp Bragg peaks, indicating the amorphous structure. The critical diameter of the Ti-based BMGs increases from 1 mm to 3 mm with increasing (Ti þ Zr) content from 50.5 at% to 54.5 at%, and then decreases to 2 mm with further increasing (Ti þ Zr) content to 55.5 at%. As depicted in Fig. 2(a), DSC traces of the Ti-based glassy rods with their critical diameters are characterized by an endothermic peak of glass transition, a subsequent supercooled liquid region prior to crystallization. The values of Dc, glass transition temperature (Tg), onset temperature of crystallization (Tx) and supercooled liquid region (DTx ¼ Tx Tg) as a function of (Ti þ Zr)/Cu ratios are revealed in Fig. 2(b). With increasing (Ti þ Zr)/Cu ratio from 1.15 to 1.36, both Tg and Tx
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Y. Liu et al. / Intermetallics 72 (2016) 36e43
Fig. 1. XRD patterns of TieZreCueFeeSneSi glassy alloy rods with their critical diameters prepared by injection copper-mold casting.
decrease to the minimum values of 646 K and 702 K, respectively, and then increase to 659 K and 703 K, respectively, with further increased (Ti þ Zr)/Cu ratio up to 1.42. The DTx, known as a GFA parameter, exhibit opposite variation with increasing (Ti þ Zr)/Cu ratio, and the largest DTx is 56 K for Ti47Zr7.5Cu40M glassy alloy which possesses the highest GFA among the TieCueZreFeeSneSi system BMGs. There was no distinct difference in the values of Tg, Tx and heat release of crystallization between the bulk and ribbon samples of the same compositions, further confirming the glassy structure of the alloy rod. It suggested that properly altering the Ti, Zr and Cu contents is effective for designing the Ti-based BMGs with high GFA. It is well known that the addition of Be or coexistence of Cu and Ni are commonly involved for designing Ti-based alloy systems with high GFA, therefore, synthesis of Ni- and Be-free Ti-based BMGs with high GFA is challenging. The present TieCueZreFeeSneSi BMGs and the noble element-free Ti-based BMGs with low Ni content less than 10 at% [7e11] are summarized in Table 1, and their (Ti þ Zr)/(Cuþ(Ni)) ratio and corresponding Dc are shown for comparison. Among the Ti-based BMGs, the present Ni-free TieCueZreFeeSneSi ones achieved the integration of the lowest Cu(þNi) contents in the range of 39e42 at%, the highest (Ti þ Zr)/(Cuþ(Ni)) ratio in the range of 1.25e1.42 and relatively large critical diameters up to 3 mm, which is of great significance for designing Ti-based BMGs with potential for biomedical application. Moreover, the high (Ti þ Zr)/Cu ratios of the TieCueZreFeeSneSi BMGs would be favorable for improving the bio-corrosion resistance and biocompatibility. Regarding the mechanism of high GFA for the TieCueZreFeeSneSi BMGs, it is considered that the alloy system consists of multicomponent with large mismatch in atomic size, including relatively large Zr element, medium size Ti, Cu, Sn and Fe, and small Si [24], which is advantageous to increase the atomic packing density, leading to the enhanced resistance of the supercool liquid against crystallization
Fig. 2. (a) DSC curves of TieZreCueFeeSneSi BMGs with their critical diameters and (b) changes in Dc, Tg, Tx and DTx with (Ti þ Zr)/Cu ratios of the Ti-based BMGs.
Y. Liu et al. / Intermetallics 72 (2016) 36e43
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Table 1 The (Cuþ(Ni)) contents, (Ti þ Zr)/(Cuþ(Ni)) ratios and critical diameter (Dc) for the noble element-free Ti-based BMGs with relatively low Ni content. Alloy (at.%)
Cu (þNi) content (at.%)
(Ti þ Zr)/(Cuþ(Ni))
Dc (mm)
Ref.
Ti45Zr5.5Cu44Fe2.5Sn2Si1 Ti45Zr7.5Cu42Fe2.5Sn2Si1 Ti47Zr7.5Cu40Fe2.5Sn2Si1 Ti48Zr7.5Cu39Fe2.5Sn2Si1 Ti45.2Zr7.2Cu40.5Ni5.1Sn2 Ti45.8Zr6.2Cu39.9Ni5.1Sn2Si1 Ti45.4Zr6.1Cu39.5Ni5Sn2Si2 Ti50Cu40Zr5Ni5 Ti45Cu45Zr5Ni5 Ti42.5Zr2.5Hf5Cu42.5Ni7.5 Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 Ti43.15Zr9.59Cu36.24Ni9.06Sn1.96
44 42 40 39 45.6 45 44.5 45 50 50 50 45 45.3
1.15 1.25 1.36 1.42 1.15 1.16 1.16 1.22 1.00 0.90 0.99 0.98 1.16
1 2 3 2 3 4 2 2 3 2e2.5 ~5 6 3
This This This This [7] [7] [7] [8] [8] [9] [9] [10] [11]
[25]. The large negative heat of mixing of TieFe, TieSn, ZreSn, SieTi, SieCu and SieZr atomic pairs [24], could promote the formation of chemical short range orders or atomic clusters which may hinder the diffusion of constituent elements and stabilize the supercooled liquid [25,26]. The existence of similar element pair of CueFe and the coexistence of Zr element may promote the formation of icosahedral short-range order in the TieZreCueFeeSneSi alloy system, which may cause the decreasing potential energy and the increasing GFA [27]. The minor addition of Si and Sn element in the present Ti-based BMG system is also effective in enhancing the GFA, which has also been confirmed in other Ti-based BMG systems [7,10]. Besides, the Ti47Zr7.5Cu40M BMG possesses large supercooled liquid region (DTx) of 56 K [7e12], implying the high thermal stability of the supercooled liquid and the easy glass formation. The synthesis of TieZreCueFeeSneSi BMGs free from Ni and Be elements is beneficial to promote the development of Ti-based BMGs with potential for the application in biomedical fields. Additionally, the GFA of the present Ti-based BMGs may be further enhanced by minor addition of suitable elements, which is effective for many BMG systems [28,29]. 3.2. Mechanical properties Typical compressive stressestrain curves for the Ti-based BMGs with different (Ti þ Zr)/Cu ratio are shown in Fig. 3. All the samples exhibit similar elastic strain around 2%, followed by yielding at a high stress in the range of 1869e1981 MPa, and then deformed in a plastic manner prior to fracture at a stress above 2000 MPa, which
work work work work
is higher than that of Tie6Ale4V alloy [30]. With increasing (Ti þ Zr)/Cu ratio, the fracture strength of the Ti-based BMGs gradually decrease from 2185 MPa to 2014 MPa, and the largest plastic strain up to 1.5% is observed for Ti47Zr7.5Cu40M BMG. The typical SEM images of fractured Ti47Zr7.5Cu40M BMG sample are shown in Fig. 4. As shown in Fig. 4(a), the fracture took place along the maximum shear plane, inclining about 42 to the direction of the applied compressive load, which suggests dominant effort of shear stress in the failure process. Multiple shear bands including primary shear bands paralleling to the fracture plane and irregularly shaped secondary shear bands on the lateral surface (Fig. 4(a)) and well-developed vein pattern in the fracture surface (Fig. 4(b)) can be observed, which is in response to the relatively large plasticity of the Ti47Zr7.5Cu40M BMG. Liquid-like droplets (indicated by black arrows) are also detected on the fracture surface, implying local softening and/or melting during compression. By ultrasound velocity measurement, the Young's modulus of the Ti47Zr7.5Cu40M glassy alloy with the highest GFA in the present system was measured to be ~100 GPa, which is lower than those of typical clinical metallic biomaterials, such as Tie6Ale4V alloy (~110 GPa), 316L stainless steels (~210 GPa) and CoeCreMo alloys (~240 GPa) [6]. For a healthy person, as depicted by Wolff's law, bone will remodel and adapt itself to the loads under which it is placed. Therefore, if the loading on a bone decreases, the bone will become less dense and weaker, causing osteopenia which is a precursor to osteoporosis and eventual loosening and failure of the implants [31]. Thus the relatively low Young's modulus of the Tibased BMG is beneficial to minimizing stress shielding and improving implant longevity. The microhardness of the Ti44Zr5.5Cu44M, Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs were measured to be 613, 611, 609 and 606 Hv, respectively, implying good anti-wear performance of the Ti-based BMGs which is favorable in reducing the occurrence of particle diseases induced by wear debris. The Ti47Zr7.5Cu40M BMG with good mechanical properties, manifesting as low Young's modulus, high strength and microhardness, is a promising potential biomaterial. Moreover, tuning (Ti þ Zr)/Cu ratios may also be effective for designing Tibased BMGs satisfying the mechanical demands of biomedical application.
3.3. Bio-corrosion behaviors
Fig. 3. Compressive stressestrain curves of TieZreCueFeeSneSi BMGs.
Superior bio-corrosion resistance is an essential property for biomedical metals used for long-term implants in order to endure the aggressive human body environment and suppress the release of corrosion-induced metallic-ions which may yield adverse effects on the host. Bio-corrosion behaviors of the Ti-based glassy alloys as well as Tie6Ale4V alloy are investigated by electrochemical
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Y. Liu et al. / Intermetallics 72 (2016) 36e43
Fig. 4. SEM images of (a) lateral surface and (b) fracture surface of the Ti47Zr7.5Cu40Fe2.5Sn2Si1 BMG after compressive test.
Table 2 The cationic fractions in the oxide film on the Ti47Cu40Zr7.5Fe2.5Sn2Si1 BMG after 30 days immersion in PBS at 37 C. Elements
Ti
Zr
Cu
Fe
Sn
Si
Ag
Cationic fractions (at.%)
67.4
17.1
0
6.6
1.2
7.7
0
depicted in Fig. 5(b), the potentiodynamic polarization curves for the Ti alloys exhibit spontaneous passivation behavior with a wide passive region. The paralleled passive current densities (ipass) of the Ti-based glassy alloys in the range of 103 and 102 A/m2 are notably lower than that of Tie6Ale4V alloy, though pitting corrosion occurs at high anodic polarization potential for the glassy alloys. Additionally, as shown in Fig. 5, with increasing (Ti þ Zr)/Cu ratio the OPCs and pitting potentials (Epits) of the Ti-based BGMs tend to reach higher values, particularly the Epit greatly increases from 0.45 V to 1.22 V. The chemical compositions of the surface film formed on the Ti47Cu40Zr7.5M BMG after immersion in PBS at 37 C for 30 days were analyzed by XPS. The Ti 2p, Zr 3d, Cu 2p, Fe 2p, Sn 3d, Si 2p, O ls and C 1s core level peaks are easily detected on the XPS spectra of the Ti-based BMG. The C1s spectrum is related to a contaminant hydrocarbon layer formed on the exposed surface of the samples. Based on the integrated intensities of the peaks for individual species, the cationic ions friction of constitute elements in the passive film were determined quantitatively, as listed in Table 2.
Fig. 5. (a) Changes in open circuit potential with immersion time and (b) potentiodynamic polarization curves of TieZreCueFeeSneSi glassy alloys and Tie6Ale4V alloy in PBS at 310 K.
measurements in PBS solution at 37 C with 4 vol.% O2/N2 gas mixture. Fig. 5(a) shows the changes in the open circuit potentials (OCPs) with immersion time of the Ti alloys. The OCPs increase abruptly in the initial 100 s upon immersion and then slow down to a stable value after immersion for 1800 s. The Ti-based glassy alloys possess higher OCPs in the range of 0.25e0.05 V than that (~0.42 V) of Tie6Ale4V alloy, suggesting the formation of more stable oxide films on surface of the Ti-based glassy alloys. As
Fig. 6. Contact angles of water droplets on TieZreCueFeeSneSi glassy alloys and Tie6Ale4V alloy.
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Fig. 7. Morphologies of MC3T3-E1 cells cultured on (a)e(b) Ti45Zr7.5Cu42Fe2.5Sn2Si1 BMG, (c)e(d) Ti47Zr7.5Cu40Fe2.5Sn2Si1 BMG, (e)e(f) Ti48Zr7.5Cu39Fe2.5Sn2Si1 BMG and (g)e(h) Tie6Ale4V alloy for 7 days.
Compared with the nominal composition of the Ti47Cu40Zr7.5M BMG, the oxide film formed on the sample surface after immersion is rich in Ti and Zr with the presence as TiO2 and ZrO2. It is well known that the Ti- and Zr-oxides are chemically stable and structurally dense, which mainly contribute to the superior biocorrosion resistance of the Ti-based glassy alloys. Meanwhile, the composition homogeneity and single-phase nature without grain boundaries and second-phase particles of the Ti-based glassy
alloys, may further promote the formation of a uniform passive film on the alloy surface, which could also be responsible for the superior bio-corrosion resistance [24,32]. As abovementioned, the improvement in bio-corrosion resistance of the Ti-based glassy alloys by increasing (Ti þ Zr)/Cu ratio could be due to the increase in Ti and Zr concentrations in the passive film, which present as TiO2 and ZrO2 with high pitting resistance, respectively, in response to the change in the nominal compositions [32]. The concentrations of
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ions released into the PBS solution from the Ti47Cu40Zr7.5M BMG were also measured to further study the corrosion resistance as well as biocompatibility. For the Ti-based BMG, only Cu, Fe and Si ions were detectable with concentrations of 3.1, 2.1 and 65 ppb (defined as mg/L), respectively, after 30 days immersion at 37 C. It is clear that the amount of each metal ion is far below their safe limited values allowed in human body [33]. Moreover, the low concentrations of the metallic ions released from the Ti47Cu40Zr7.5M BMG implying superior corrosion resistance of the Ti-based BMGs, which is favorable for biomedical application. 3.4. In vitro biocompatibility The contact angles of water on the Ti-based BMGs and Tie6Ale4V alloy were measured to evaluate the surface wettability, which could influence the protein absorption and further regulate cell response [15e17]. As presented in Fig. 6, all the Ti alloys exhibit hydrophilicity as the values of contact angles is in the range of 72 e78 . In comparison with Tie6Ale4V alloy, the Ti-based BMGs show relatively small water contact angles around 74 , indicating more hydrophilic surface of the Ti-based BMGs, which is conducive to cell attachment and spreading. Refer to previous reports, when fibronectin adhered on hydrophilic surfaces, they would expose more arginyl-glycyl-aspartic acid (RGD) which would mediate cellular attachment [34]. Mammalian cells can also efficiently attach to hydrophilic surfaces in comparison with hydrophobic surfaces the cells inefficiently attach to [35]. The cytocompatibility of Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs with relatively high GFA and good biocorrosion resistance among this system alloys were further studied by adopted mouse MC3T3-E1 pre-osteoblast, and Tie6Ale4V alloy utilized as a crystalline counterpart. It should be noted that the effect of roughness on cell behaviors can be eliminated, because the samples was mechanically polished in the same procedure and similar surface roughness was confirmed by AFM. Fig. 7 exhibits the morphologies of MC3T3-E1 cells cultured on the three substrates for 7 days. A large number of interacted cells adhered on the surfaces of the Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs and Tie6Ale4V alloy, as shown in Fig. 7(a), (c), (e) and (g), respectively. It can be seen that more adhesive cells on the surface of the BMGs merged into a confluent layer, implying a better cytocompatibility of the TieCueZreFeeSneSi BMGs. Fig. 7(b), (d), (f) and (h) show the enlarged cell morphologies for the four substrates. The cells present similar irregular and polygonal shapes on the different substrates with long and thin filopodia (indicated by black arrows in the images), which are essential for cellular migration and supporting the exchange of substance and message between interacted cells. It can also be observed that the Ti-based BMGs with different (Ti þ Zr)/Cu ratios of 1.25e1.42 shows similar cellular behaviors. Fig. 8 illustrates the cytotoxicity of the Ti-based BMGs, Tie6Ale4V alloy and positive control expressed as a percentage of result for the negative control after the 3-day incubation. It can be seen that the MC3T3-E1 cells cultured in the four Ti alloys extracts show much higher cell viability, compared with the positive control. The cell viabilities in the Ti-based BMGs extracts are slightly higher than that in Tie6Ale4V alloy extract. In addition, all the Tibased BMGs with different (Ti þ Zr)/Cu ratio in the range of 1.25e1.42 show no significant difference in cell viability. The abovementioned results imply that the Cu-containing Tibased BMGs with proper (Ti þ Zr)/Cu ratio exhibit no significant negative effects on the cytocompatibility. The elimination of highly toxic Ni and Be elements, and high concentrations of biocompatible Ti and Zr elements in the alloy as well as the formation of the protective passive surface film are beneficial to the cell adhesion
Fig. 8. Cytotoxicity of MC3T3-E1 cells cultured in Ti45Zr7.5Cu42Fe2.5Sn2Si1, Ti47Zr7.5Cu40Fe2.5Sn2Si1, Ti48Zr7.5Cu39Fe2.5Sn2Si1 glassy alloy and Tie6Ale4V alloy extraction media.
and proliferation [6,36]. The high bio-corrosion resistance of the Tibased BMGs demonstrated by the electrochemical measurement results will suppress the release of metal ions to the human body environment and reduce the occurrences of toxic reactions during the biomedical service, which is also an advantage for the biomedical applications. 4. Conclusions Ni-free TieZreCueFeeSneSi BMGs with high (Ti þ Zr)/Cu ratios in the range of 1.15e1.42 have been synthesized, and Ti47Zr7.5Cu40Fe2.5Sn2Si1 BMG with a diameter of 3 mm can be fabricated by copper-mold casting. The Ti-based BMGs possess high compressive strength and microhardness in the range of 2014e2185 MPa and 606e613 Hv, respectively. The Young's modulus of the Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy was measured to be about 100 GPa. The Ti-based BMGs exhibited superior bio-corrosion resistance characterized by high corrosion potential and low passive current density, and particularly the pitting corrosion resistance is greatly enhanced with increasing (Ti þ Zr)/Cu ratio due to the formation of surface film mainly composed of TiO2 and ZrO2. Good surface hydrophilicity of the Ti-based glassy alloys was revealed by the sessile drop contact angle method. Cell culture indicated good cytocompatibility of the Ti-based BMGs with (Ti þ Zr)/Cu ratio over 1.2. The Ti47Cu40Zr7.5Fe2.5Sn2Si1 BMG with high GFA, good mechanical properties, high bio-corrosion resistance and good biocompatibility are promising as potential biomaterials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51271008 and 51161130526). References [1] C. Suryanarayana, A. Inoue, Bulk Metallic Glasses, CRC Press, Boca Raton, 2011. [2] J. Schroers, Processing of bulk metallic glass, Adv. Mater. 22 (2010) 1566e1597. [3] G.Q. Xie, F.X. Qin, S.L. Zhu, Recent progress in Ti-based metallic glasses for application as biomaterials, Mater. Trans. 54 (2013) 1314e1323. [4] J. Schroers, G. Kumar, T.M. Hodges, S. Chan, T.R. Kyriakides, Bulk metallic glasses for biomedical applications, JOM 61 (2009) 21e28. [5] M.D. Demetriou, A. Wiest, D.C. Hofmann, W.L. Johnson, B. Han, Amorphous metals for hard-tissue prosthesis, JOM 62 (2010) 83e91. [6] M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Ti based biomaterials, the
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Formation and properties of Ti-based TieZreCueFeeSneSi bulk metallic glasses with different (Ti þ Zr)/Cu ratios for biomedical application Ying Liu, Shujie Pang*, Haifei Li, Qiao Hu, Bin Chen, Tao Zhang** Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2015 Received in revised form 21 January 2016 Accepted 22 January 2016 Available online xxx
Ti-based TieZreCueFeeSneSi bulk metallic glasses (BMGs) free from highly toxic elements Ni and Be were developed as promising biomaterials. The influence of (Ti þ Zr)/Cu ratio on glass-formation, thermal stability, mechanical properties, bio-corrosion resistance, surface wettability and biocompatibility were investigated. In the present Ti-based BMG system, the Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy exhibited the highest glass forming ability (GFA) corresponding to the largest supercooled liquid region, and a glassy rod with a critical diameter of 3 mm was prepared by copper-mold casting. The Ti-based BMGs possess high compressive strength of 2014e2185 MPa and microhardness of 606e613 Hv. Young's modulus of the Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy was about 100 GPa, which is slightly lower than that of Tie6Ale4V alloy. The Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy with high GFA exhibited high biocorrosion resistance, and good surface hydrophilia and cytocompatibility. The mechanisms for glass formation as well as the effect of (Ti þ Zr)/Cu ratio on bio-corrosion behavior and biocompatibility are discussed. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Bulk metallic glasses Ti-based alloys Mechanical properties Bio-corrosion resistance Biocompatibility
1. Introduction Ti-based bulk metallic glasses with excellent mechanical and chemical properties, and biocompatibility as well as good netshaping and thermoplastic forming ability [1e3] attribute to the unique amorphous feature, are attractive to be potential biomaterials. Particularly, the combination of high strength, low elastic modulus, and high wear and corrosion resistance of the Ti-based BMGs is favorable for reducing the failures of biomedical implants, which is usually induced by the mismatch of mechanical properties between the implants and bones and leading to particle disease, stress shielding as well as fatigue during long term clinical application [4e6]. As well known, most Ti-based BMGs have been developed based on the Tie(Zr)eCueNi glassy alloy systems [7e11], for which the critical diameters for glass formation were in the range from 1 mm to 6 mm by copper-mold casting, while Ni is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Pang), [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.intermet.2016.01.007 0966-9795/© 2016 Elsevier Ltd. All rights reserved.
highly toxic. Besides, element Be, which is blamed for the antiproliferative effects on human bodies, is also commonly alloyed in Ti-based BMGs to attain good glass formers [12e14]. Therefore, exploring Ni- and Be-free Ti-based bulk glassy alloys with high GFA, and good mechano- and bio-compatibility is one of the challenges to promote the potential application of BMGs as biomaterials. On another hand, for the application of metallic materials used as orthopedic or dental implants, it is crucial to evaluate their biocorrosion behavior, cytocompatibility, and surface wettability which regulates the cell response and further affects the cytocompatibility of implanted biomaterials [15e17], however, studies on these properties of Ti-based BMGs are limited [3,18e20]. It is generally recognized that the main constituent elements of the multicomponent alloys with high GFA have significant difference in atomic size and negative heats of mixing, which can be defined as dissimilar elements [21,22]. It has also been found that the alloying with an element similar to the components of a glassy alloy is effective on improving the GFA [22]. Therefore, based on the TieCu alloy system, considering the coexistence criterion of dissimilar and similar elements and the positive effect of Sn and Si microalloying Ti-based BMGs on GFA [7,10], Ni- and Be-free TieZreCueFeeSneSi bulk glassy alloys bearing similar element pair of CueFe are
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designed and synthesized in the present study. The Ti-based TieZreCueFeeSneSi glassy alloy system with critical diameter up to 3 mm is fabricated by copper-mold casting. In addition, the formation, thermal properties, mechanical properties, biocorrosion behaviors, surface wettability and cytocompatibility of the Ti-based BMGs are investigated, and the related mechanisms are also discussed. 2. Materials and methods TieZreCueFeeSneSi alloy ingots with nominal compositions (at.%) of Ti45Zr5.5Cu44Fe2.5Sn2Si1 ((Ti þ Zr)/Cu ¼ 1.15), Ti45Zr7.5Cu42Fe2.5Sn2Si1 ((Ti þ Zr)/Cu ¼ 1.25), Ti47Zr7.5Cu40Fe2.5Sn2Si1 ((Ti þ Zr)/Cu ¼ 1.36) and Ti48Zr7.5Cu39Fe2.5Sn2Si1 ((Ti þ Zr)/ Cu ¼ 1.42), denoted as Ti45Zr5.5Cu44M, Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M in the following text, respectively, were prepared by arc-melting the mixture of pure elements under a high-purity argon atmosphere. The alloy ingots were melted four times to ensure the compositional homogeneity. From the master alloys, under an argon atmosphere, ribbons and rod samples with different diameters were prepared by melt spinning and copper mold casting, respectively. Microstructure of the samples was characterized by X-ray diffraction (XRD, Bruker AXS D8). Thermal behaviors of the ribbons and the rod samples with critical diameters were investigated by differential scanning calorimetry (DSC, NETZSCH 404C) at a heating rate of 0.33 K/s. Compressive mechanical properties of the Ti-based BMGs were evaluated by a materials testing machine using glassy rod specimens with a dimension of f 1 2 mm for the Ti45Zr5.5Cu44M BMG and f 2 4 mm for other Ti-based BMGs in present study, respectively, at a constant strain rate of 2.1 104 s1. Fracture and lateral surface of the deformed samples were examined by scanning electron microscope (SEM, Cam Scan 3400). Vickers hardness of the glassy samples was measured under a load of 200 gf for 15 s using a Vickers microhardness tester (Future-tech FM800). Bio-corrosion behaviors of the Ti-based glassy alloys were investigated by electrochemical measurements in phosphate buffered saline (PBS, pH ¼ 7.4) solution at about 310 K aerated with a 4 vol.% O2/N2 gas mixture at a flowing rate of 50 ml/min. The Tie6Ale4V alloy, widely used in clinical application as orthopedic implants, was employed as the reference material. Prior to corrosion test, the samples were mechanically polished up to 2000-grit silicon carbide paper in cyclohexane, and then ultrasonic cleaning in acetone, alcohol and de-ionized water, successively, followed by exposing to air for about 24 h for good reproducibility. Electrochemical measurements were conducted using an electrochemical workstation (EG&G Princeton Applied Research VersaSTAT II) with a three-electrode system consisting of a sample as the working electrode, a platinum counter electrode and a saturated calomel reference electrode (SCE, SCE ¼ 0.242 V). After immersion for about 1800 s when the open-circuit potentials (OCP) became almost steady, potentiodynamic polarizations were conducted at a potential sweep rate of 0.833 mV/s. The surface film formed on the Ti47Zr7.5Cu40M BMG with a dimension of f 2.5 3.5 mm after immersion in PBS at 37 C for 30 days were characterized by X-ray photoelectron spectroscopy (XPS) using an ESCALab250 photoelectron spectroscopy with monochromatized Al Ka excitation. XPS spectra were analyzed by XPSPEAK analytical software to obtain further information about chemical states. The concentrations of metal ions released into solution were investigated by inductively coupled plasma-mass spectrometry (ICP-MS, Yoko gawa Analytical Systems HP4500 spectrometer). Surface wettability of the Ti-based glassy alloys was measured by sessile drop method using 5-mm-wide ribbon samples at 298 K. The sample surfaces were polished and cleaned in the same
37
procedure as that for electrochemical measurements. Surface roughness of the samples was confirmed to be similar by atomic force microscope (AFM, Veeco/Bruker ICON), and the roughness values were lower than 100 nm. Deionized water was used as the wetting liquid with a drop size of 3 ml. Sessile drop contact angles of the airewateresubstrate interface were measured using a drop shape analysis system equipped with an automated stage and droplet dispenser, a digital camera, and image analysis software. Five different positions of each sample were assessed to ensure authenticity of the results. Mouse MC3T3-E1 pre-osteoblast cell line was adopted in this study for preliminarily evaluating the cytocompatibility of the Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs with relatively high GFA as well as Tie6Ale4V alloy. Samples with a dimension of f 2 1.5 mm were prepared in the same procedure as reported in our previous study [23]. The MC3T3-E1 cells were cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 1% 100U ml1 penicillin and 100 mg ml1 streptomycin in a 5% CO2 balanced incubator at 37 C. Cells suspension at a density of 2.6 104 cells/cm2 were seeded onto the surface of each substrate directly to observed the morphologies and quantities of the cells adhered on the samples after cultured for 7 days. At the harvest time the samples were fetched out, followed by washing, fixation and dehydration in sequence, and the details were reported in our previous report [23]. Finally the morphologies of the cells adhered on the samples were observed by SEM. Quadruplicate samples of each alloy were adopted for good reproducibility. According to the instructions in ISO 10993-5, in vitro cytotoxicity test of the Ti-based BMGs and Tie6Ale4V alloy was performed using extraction media of the samples. The sterilized samples were immersed in DMEM medium with a surface area/extraction medium ratio 1 cm2/ml in a humidified atmosphere with 5% CO2 at 37 C for 24 h and the extraction medium was stored at 4 C before the cytotoxicity test. The DMEM medium and DMEM medium containing 10% dimethyl sulfoxide (DMSO) were used as a negative control and a positive control, respectively. 100 ml of cells suspension at a density of 2 104 cells/cm2 were seeded in 96-well plate and cultured for 24 h to allow attachment. The media was then replaced with 100 ml of the extraction media. After incubating for 3 days, each well was added with 20 ml of the MTT solution following a further incubation for 4 h. Afterwards, 100 ml of formazan solubilization solution (DMSO) were added. The samples were then removed for spectrophotometric absorbance measurements using a microplate reader at 492 nm. Five replicates (both the samples and the controls) were used for all tests. 3. Results and discussion 3.1. Formation and thermal properties XRD patterns of the Ti-based cast rods with their critical diameters (Dc) are shown in Fig. 1. All the patterns exhibit a main broad halo without any sharp Bragg peaks, indicating the amorphous structure. The critical diameter of the Ti-based BMGs increases from 1 mm to 3 mm with increasing (Ti þ Zr) content from 50.5 at% to 54.5 at%, and then decreases to 2 mm with further increasing (Ti þ Zr) content to 55.5 at%. As depicted in Fig. 2(a), DSC traces of the Ti-based glassy rods with their critical diameters are characterized by an endothermic peak of glass transition, a subsequent supercooled liquid region prior to crystallization. The values of Dc, glass transition temperature (Tg), onset temperature of crystallization (Tx) and supercooled liquid region (DTx ¼ Tx Tg) as a function of (Ti þ Zr)/Cu ratios are revealed in Fig. 2(b). With increasing (Ti þ Zr)/Cu ratio from 1.15 to 1.36, both Tg and Tx
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Y. Liu et al. / Intermetallics 72 (2016) 36e43
Fig. 1. XRD patterns of TieZreCueFeeSneSi glassy alloy rods with their critical diameters prepared by injection copper-mold casting.
decrease to the minimum values of 646 K and 702 K, respectively, and then increase to 659 K and 703 K, respectively, with further increased (Ti þ Zr)/Cu ratio up to 1.42. The DTx, known as a GFA parameter, exhibit opposite variation with increasing (Ti þ Zr)/Cu ratio, and the largest DTx is 56 K for Ti47Zr7.5Cu40M glassy alloy which possesses the highest GFA among the TieCueZreFeeSneSi system BMGs. There was no distinct difference in the values of Tg, Tx and heat release of crystallization between the bulk and ribbon samples of the same compositions, further confirming the glassy structure of the alloy rod. It suggested that properly altering the Ti, Zr and Cu contents is effective for designing the Ti-based BMGs with high GFA. It is well known that the addition of Be or coexistence of Cu and Ni are commonly involved for designing Ti-based alloy systems with high GFA, therefore, synthesis of Ni- and Be-free Ti-based BMGs with high GFA is challenging. The present TieCueZreFeeSneSi BMGs and the noble element-free Ti-based BMGs with low Ni content less than 10 at% [7e11] are summarized in Table 1, and their (Ti þ Zr)/(Cuþ(Ni)) ratio and corresponding Dc are shown for comparison. Among the Ti-based BMGs, the present Ni-free TieCueZreFeeSneSi ones achieved the integration of the lowest Cu(þNi) contents in the range of 39e42 at%, the highest (Ti þ Zr)/(Cuþ(Ni)) ratio in the range of 1.25e1.42 and relatively large critical diameters up to 3 mm, which is of great significance for designing Ti-based BMGs with potential for biomedical application. Moreover, the high (Ti þ Zr)/Cu ratios of the TieCueZreFeeSneSi BMGs would be favorable for improving the bio-corrosion resistance and biocompatibility. Regarding the mechanism of high GFA for the TieCueZreFeeSneSi BMGs, it is considered that the alloy system consists of multicomponent with large mismatch in atomic size, including relatively large Zr element, medium size Ti, Cu, Sn and Fe, and small Si [24], which is advantageous to increase the atomic packing density, leading to the enhanced resistance of the supercool liquid against crystallization
Fig. 2. (a) DSC curves of TieZreCueFeeSneSi BMGs with their critical diameters and (b) changes in Dc, Tg, Tx and DTx with (Ti þ Zr)/Cu ratios of the Ti-based BMGs.
Y. Liu et al. / Intermetallics 72 (2016) 36e43
39
Table 1 The (Cuþ(Ni)) contents, (Ti þ Zr)/(Cuþ(Ni)) ratios and critical diameter (Dc) for the noble element-free Ti-based BMGs with relatively low Ni content. Alloy (at.%)
Cu (þNi) content (at.%)
(Ti þ Zr)/(Cuþ(Ni))
Dc (mm)
Ref.
Ti45Zr5.5Cu44Fe2.5Sn2Si1 Ti45Zr7.5Cu42Fe2.5Sn2Si1 Ti47Zr7.5Cu40Fe2.5Sn2Si1 Ti48Zr7.5Cu39Fe2.5Sn2Si1 Ti45.2Zr7.2Cu40.5Ni5.1Sn2 Ti45.8Zr6.2Cu39.9Ni5.1Sn2Si1 Ti45.4Zr6.1Cu39.5Ni5Sn2Si2 Ti50Cu40Zr5Ni5 Ti45Cu45Zr5Ni5 Ti42.5Zr2.5Hf5Cu42.5Ni7.5 Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 Ti43.15Zr9.59Cu36.24Ni9.06Sn1.96
44 42 40 39 45.6 45 44.5 45 50 50 50 45 45.3
1.15 1.25 1.36 1.42 1.15 1.16 1.16 1.22 1.00 0.90 0.99 0.98 1.16
1 2 3 2 3 4 2 2 3 2e2.5 ~5 6 3
This This This This [7] [7] [7] [8] [8] [9] [9] [10] [11]
[25]. The large negative heat of mixing of TieFe, TieSn, ZreSn, SieTi, SieCu and SieZr atomic pairs [24], could promote the formation of chemical short range orders or atomic clusters which may hinder the diffusion of constituent elements and stabilize the supercooled liquid [25,26]. The existence of similar element pair of CueFe and the coexistence of Zr element may promote the formation of icosahedral short-range order in the TieZreCueFeeSneSi alloy system, which may cause the decreasing potential energy and the increasing GFA [27]. The minor addition of Si and Sn element in the present Ti-based BMG system is also effective in enhancing the GFA, which has also been confirmed in other Ti-based BMG systems [7,10]. Besides, the Ti47Zr7.5Cu40M BMG possesses large supercooled liquid region (DTx) of 56 K [7e12], implying the high thermal stability of the supercooled liquid and the easy glass formation. The synthesis of TieZreCueFeeSneSi BMGs free from Ni and Be elements is beneficial to promote the development of Ti-based BMGs with potential for the application in biomedical fields. Additionally, the GFA of the present Ti-based BMGs may be further enhanced by minor addition of suitable elements, which is effective for many BMG systems [28,29]. 3.2. Mechanical properties Typical compressive stressestrain curves for the Ti-based BMGs with different (Ti þ Zr)/Cu ratio are shown in Fig. 3. All the samples exhibit similar elastic strain around 2%, followed by yielding at a high stress in the range of 1869e1981 MPa, and then deformed in a plastic manner prior to fracture at a stress above 2000 MPa, which
work work work work
is higher than that of Tie6Ale4V alloy [30]. With increasing (Ti þ Zr)/Cu ratio, the fracture strength of the Ti-based BMGs gradually decrease from 2185 MPa to 2014 MPa, and the largest plastic strain up to 1.5% is observed for Ti47Zr7.5Cu40M BMG. The typical SEM images of fractured Ti47Zr7.5Cu40M BMG sample are shown in Fig. 4. As shown in Fig. 4(a), the fracture took place along the maximum shear plane, inclining about 42 to the direction of the applied compressive load, which suggests dominant effort of shear stress in the failure process. Multiple shear bands including primary shear bands paralleling to the fracture plane and irregularly shaped secondary shear bands on the lateral surface (Fig. 4(a)) and well-developed vein pattern in the fracture surface (Fig. 4(b)) can be observed, which is in response to the relatively large plasticity of the Ti47Zr7.5Cu40M BMG. Liquid-like droplets (indicated by black arrows) are also detected on the fracture surface, implying local softening and/or melting during compression. By ultrasound velocity measurement, the Young's modulus of the Ti47Zr7.5Cu40M glassy alloy with the highest GFA in the present system was measured to be ~100 GPa, which is lower than those of typical clinical metallic biomaterials, such as Tie6Ale4V alloy (~110 GPa), 316L stainless steels (~210 GPa) and CoeCreMo alloys (~240 GPa) [6]. For a healthy person, as depicted by Wolff's law, bone will remodel and adapt itself to the loads under which it is placed. Therefore, if the loading on a bone decreases, the bone will become less dense and weaker, causing osteopenia which is a precursor to osteoporosis and eventual loosening and failure of the implants [31]. Thus the relatively low Young's modulus of the Tibased BMG is beneficial to minimizing stress shielding and improving implant longevity. The microhardness of the Ti44Zr5.5Cu44M, Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs were measured to be 613, 611, 609 and 606 Hv, respectively, implying good anti-wear performance of the Ti-based BMGs which is favorable in reducing the occurrence of particle diseases induced by wear debris. The Ti47Zr7.5Cu40M BMG with good mechanical properties, manifesting as low Young's modulus, high strength and microhardness, is a promising potential biomaterial. Moreover, tuning (Ti þ Zr)/Cu ratios may also be effective for designing Tibased BMGs satisfying the mechanical demands of biomedical application.
3.3. Bio-corrosion behaviors
Fig. 3. Compressive stressestrain curves of TieZreCueFeeSneSi BMGs.
Superior bio-corrosion resistance is an essential property for biomedical metals used for long-term implants in order to endure the aggressive human body environment and suppress the release of corrosion-induced metallic-ions which may yield adverse effects on the host. Bio-corrosion behaviors of the Ti-based glassy alloys as well as Tie6Ale4V alloy are investigated by electrochemical
40
Y. Liu et al. / Intermetallics 72 (2016) 36e43
Fig. 4. SEM images of (a) lateral surface and (b) fracture surface of the Ti47Zr7.5Cu40Fe2.5Sn2Si1 BMG after compressive test.
Table 2 The cationic fractions in the oxide film on the Ti47Cu40Zr7.5Fe2.5Sn2Si1 BMG after 30 days immersion in PBS at 37 C. Elements
Ti
Zr
Cu
Fe
Sn
Si
Ag
Cationic fractions (at.%)
67.4
17.1
0
6.6
1.2
7.7
0
depicted in Fig. 5(b), the potentiodynamic polarization curves for the Ti alloys exhibit spontaneous passivation behavior with a wide passive region. The paralleled passive current densities (ipass) of the Ti-based glassy alloys in the range of 103 and 102 A/m2 are notably lower than that of Tie6Ale4V alloy, though pitting corrosion occurs at high anodic polarization potential for the glassy alloys. Additionally, as shown in Fig. 5, with increasing (Ti þ Zr)/Cu ratio the OPCs and pitting potentials (Epits) of the Ti-based BGMs tend to reach higher values, particularly the Epit greatly increases from 0.45 V to 1.22 V. The chemical compositions of the surface film formed on the Ti47Cu40Zr7.5M BMG after immersion in PBS at 37 C for 30 days were analyzed by XPS. The Ti 2p, Zr 3d, Cu 2p, Fe 2p, Sn 3d, Si 2p, O ls and C 1s core level peaks are easily detected on the XPS spectra of the Ti-based BMG. The C1s spectrum is related to a contaminant hydrocarbon layer formed on the exposed surface of the samples. Based on the integrated intensities of the peaks for individual species, the cationic ions friction of constitute elements in the passive film were determined quantitatively, as listed in Table 2.
Fig. 5. (a) Changes in open circuit potential with immersion time and (b) potentiodynamic polarization curves of TieZreCueFeeSneSi glassy alloys and Tie6Ale4V alloy in PBS at 310 K.
measurements in PBS solution at 37 C with 4 vol.% O2/N2 gas mixture. Fig. 5(a) shows the changes in the open circuit potentials (OCPs) with immersion time of the Ti alloys. The OCPs increase abruptly in the initial 100 s upon immersion and then slow down to a stable value after immersion for 1800 s. The Ti-based glassy alloys possess higher OCPs in the range of 0.25e0.05 V than that (~0.42 V) of Tie6Ale4V alloy, suggesting the formation of more stable oxide films on surface of the Ti-based glassy alloys. As
Fig. 6. Contact angles of water droplets on TieZreCueFeeSneSi glassy alloys and Tie6Ale4V alloy.
Y. Liu et al. / Intermetallics 72 (2016) 36e43
41
Fig. 7. Morphologies of MC3T3-E1 cells cultured on (a)e(b) Ti45Zr7.5Cu42Fe2.5Sn2Si1 BMG, (c)e(d) Ti47Zr7.5Cu40Fe2.5Sn2Si1 BMG, (e)e(f) Ti48Zr7.5Cu39Fe2.5Sn2Si1 BMG and (g)e(h) Tie6Ale4V alloy for 7 days.
Compared with the nominal composition of the Ti47Cu40Zr7.5M BMG, the oxide film formed on the sample surface after immersion is rich in Ti and Zr with the presence as TiO2 and ZrO2. It is well known that the Ti- and Zr-oxides are chemically stable and structurally dense, which mainly contribute to the superior biocorrosion resistance of the Ti-based glassy alloys. Meanwhile, the composition homogeneity and single-phase nature without grain boundaries and second-phase particles of the Ti-based glassy
alloys, may further promote the formation of a uniform passive film on the alloy surface, which could also be responsible for the superior bio-corrosion resistance [24,32]. As abovementioned, the improvement in bio-corrosion resistance of the Ti-based glassy alloys by increasing (Ti þ Zr)/Cu ratio could be due to the increase in Ti and Zr concentrations in the passive film, which present as TiO2 and ZrO2 with high pitting resistance, respectively, in response to the change in the nominal compositions [32]. The concentrations of
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ions released into the PBS solution from the Ti47Cu40Zr7.5M BMG were also measured to further study the corrosion resistance as well as biocompatibility. For the Ti-based BMG, only Cu, Fe and Si ions were detectable with concentrations of 3.1, 2.1 and 65 ppb (defined as mg/L), respectively, after 30 days immersion at 37 C. It is clear that the amount of each metal ion is far below their safe limited values allowed in human body [33]. Moreover, the low concentrations of the metallic ions released from the Ti47Cu40Zr7.5M BMG implying superior corrosion resistance of the Ti-based BMGs, which is favorable for biomedical application. 3.4. In vitro biocompatibility The contact angles of water on the Ti-based BMGs and Tie6Ale4V alloy were measured to evaluate the surface wettability, which could influence the protein absorption and further regulate cell response [15e17]. As presented in Fig. 6, all the Ti alloys exhibit hydrophilicity as the values of contact angles is in the range of 72 e78 . In comparison with Tie6Ale4V alloy, the Ti-based BMGs show relatively small water contact angles around 74 , indicating more hydrophilic surface of the Ti-based BMGs, which is conducive to cell attachment and spreading. Refer to previous reports, when fibronectin adhered on hydrophilic surfaces, they would expose more arginyl-glycyl-aspartic acid (RGD) which would mediate cellular attachment [34]. Mammalian cells can also efficiently attach to hydrophilic surfaces in comparison with hydrophobic surfaces the cells inefficiently attach to [35]. The cytocompatibility of Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs with relatively high GFA and good biocorrosion resistance among this system alloys were further studied by adopted mouse MC3T3-E1 pre-osteoblast, and Tie6Ale4V alloy utilized as a crystalline counterpart. It should be noted that the effect of roughness on cell behaviors can be eliminated, because the samples was mechanically polished in the same procedure and similar surface roughness was confirmed by AFM. Fig. 7 exhibits the morphologies of MC3T3-E1 cells cultured on the three substrates for 7 days. A large number of interacted cells adhered on the surfaces of the Ti45Zr7.5Cu42M, Ti47Zr7.5Cu40M and Ti48Zr7.5Cu39M BMGs and Tie6Ale4V alloy, as shown in Fig. 7(a), (c), (e) and (g), respectively. It can be seen that more adhesive cells on the surface of the BMGs merged into a confluent layer, implying a better cytocompatibility of the TieCueZreFeeSneSi BMGs. Fig. 7(b), (d), (f) and (h) show the enlarged cell morphologies for the four substrates. The cells present similar irregular and polygonal shapes on the different substrates with long and thin filopodia (indicated by black arrows in the images), which are essential for cellular migration and supporting the exchange of substance and message between interacted cells. It can also be observed that the Ti-based BMGs with different (Ti þ Zr)/Cu ratios of 1.25e1.42 shows similar cellular behaviors. Fig. 8 illustrates the cytotoxicity of the Ti-based BMGs, Tie6Ale4V alloy and positive control expressed as a percentage of result for the negative control after the 3-day incubation. It can be seen that the MC3T3-E1 cells cultured in the four Ti alloys extracts show much higher cell viability, compared with the positive control. The cell viabilities in the Ti-based BMGs extracts are slightly higher than that in Tie6Ale4V alloy extract. In addition, all the Tibased BMGs with different (Ti þ Zr)/Cu ratio in the range of 1.25e1.42 show no significant difference in cell viability. The abovementioned results imply that the Cu-containing Tibased BMGs with proper (Ti þ Zr)/Cu ratio exhibit no significant negative effects on the cytocompatibility. The elimination of highly toxic Ni and Be elements, and high concentrations of biocompatible Ti and Zr elements in the alloy as well as the formation of the protective passive surface film are beneficial to the cell adhesion
Fig. 8. Cytotoxicity of MC3T3-E1 cells cultured in Ti45Zr7.5Cu42Fe2.5Sn2Si1, Ti47Zr7.5Cu40Fe2.5Sn2Si1, Ti48Zr7.5Cu39Fe2.5Sn2Si1 glassy alloy and Tie6Ale4V alloy extraction media.
and proliferation [6,36]. The high bio-corrosion resistance of the Tibased BMGs demonstrated by the electrochemical measurement results will suppress the release of metal ions to the human body environment and reduce the occurrences of toxic reactions during the biomedical service, which is also an advantage for the biomedical applications. 4. Conclusions Ni-free TieZreCueFeeSneSi BMGs with high (Ti þ Zr)/Cu ratios in the range of 1.15e1.42 have been synthesized, and Ti47Zr7.5Cu40Fe2.5Sn2Si1 BMG with a diameter of 3 mm can be fabricated by copper-mold casting. The Ti-based BMGs possess high compressive strength and microhardness in the range of 2014e2185 MPa and 606e613 Hv, respectively. The Young's modulus of the Ti47Zr7.5Cu40Fe2.5Sn2Si1 glassy alloy was measured to be about 100 GPa. The Ti-based BMGs exhibited superior bio-corrosion resistance characterized by high corrosion potential and low passive current density, and particularly the pitting corrosion resistance is greatly enhanced with increasing (Ti þ Zr)/Cu ratio due to the formation of surface film mainly composed of TiO2 and ZrO2. Good surface hydrophilicity of the Ti-based glassy alloys was revealed by the sessile drop contact angle method. Cell culture indicated good cytocompatibility of the Ti-based BMGs with (Ti þ Zr)/Cu ratio over 1.2. The Ti47Cu40Zr7.5Fe2.5Sn2Si1 BMG with high GFA, good mechanical properties, high bio-corrosion resistance and good biocompatibility are promising as potential biomaterials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51271008 and 51161130526). References [1] C. Suryanarayana, A. Inoue, Bulk Metallic Glasses, CRC Press, Boca Raton, 2011. [2] J. Schroers, Processing of bulk metallic glass, Adv. Mater. 22 (2010) 1566e1597. [3] G.Q. Xie, F.X. Qin, S.L. Zhu, Recent progress in Ti-based metallic glasses for application as biomaterials, Mater. Trans. 54 (2013) 1314e1323. [4] J. Schroers, G. Kumar, T.M. Hodges, S. Chan, T.R. Kyriakides, Bulk metallic glasses for biomedical applications, JOM 61 (2009) 21e28. [5] M.D. Demetriou, A. Wiest, D.C. Hofmann, W.L. Johnson, B. Han, Amorphous metals for hard-tissue prosthesis, JOM 62 (2010) 83e91. [6] M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Ti based biomaterials, the
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