Abstract
The Ti alloy with 7 wt% Mo revealed a microstructure that contained only the orthorhombic phase of a fine acicular martensitic structure. The corrosion resistance of the Ti-Mo alloys increased as the Mo content increased. Based on the results obtained from the polarization curve and electrochemical impedance, the Ti-Mo alloys were shown to be corrosion resistant because of the passive films formed on their surfaces. No ion release was detected in SBF (simulated body fluid) solution, while Ti ions were released in 0.1% lactic acid ranging from 0.05 to 0.12 μg/mL for the Ti-Mo alloys. In vitro tests showed that MC3T3-E1 cell proliferation on Ti-7 wt% Mo alloy was rather active compared to other Ti-Mo alloys and commercial-grade pure Ti.
1. Introduction
Both commercial-grade pure titanium (CP Ti) and the Ti-6Al-4V alloy have been widely used for orthopedic and dental implant materials because of their excellent combination of biocompatibility, corrosion resistance, and beneficial mechanical properties [1, 2]. However, an alternative to the Ti-6Al-4V alloy that is currently being utilized should be developed because the release of Al and V ions into the human body causes long-term health problems [3–5]. Another disadvantage of this material is the mismatch of Young’s modulus between the Ti implant (100–120 GPa) and bone (10–30 GPa), which is unfavorable for bone healing and remodeling [6, 7]. To address these problems and further improve the biological and mechanical properties of Ti alloys, many Ti alloys have been developed for biomedical applications [6–8]. These efforts have focused on the development of novel titanium alloys using nontoxic elements. For this purpose, β-type titanium alloys composed of nontoxic elements, such as niobium (Nb), tantalum (Ta), zirconium (Zr), molybdenum (Mo), and tin (Sn), with lower moduli of elasticity and greater strengths should be developed [7–10]. In recent studies, binary Ti alloys employed as biomaterials have been investigated, with emphasis on their microstructure and mechanical properties [10–13].
In the present study, the elastic modulus, corrosion behavior, electrochemical properties, and microstructures of homogenization-treated binary Ti-Mo alloys with Mo contents ranging from 3 to 15 wt% were investigated.
2. Experimental
The elastic modulus, corrosion resistance, and biocompatibility of binary Ti-Mo alloy specimens with Mo contents of 3, 7, 10, and 15 wt% were investigated. Ingots of the studied alloys, consisting of commercially pure sponge titanium (99.5%, grade 4) and pellet Mo (99.95%), were melted under an argon atmosphere using a nonconsumable tungsten electrode. These alloys were melted 10 times with inversion of the liquid metal to obtain homogeneous structures. This process was followed by homogenization treatments that were performed at 1000°C under an Ar atmosphere for 24 hrs and then quenched in water. The chemical compositions of the studied Ti-Mo alloys, as determined through wet chemical and gas analysis, are summarized in Table 1; this table shows that the actual chemical composition of each designed alloy was similar to its nominal composition.
Young’s modulus were measured using a dynamic ultramicrohardness tester (DUHW 201S, Shimadzu Co., Japan). The apparent elastic modulus of each sample, , was calculated using the load-displacement curve and the standard Hertzian contact theory [14]: where is the depth of elastic penetration of the indenter, is the load, and is the indenter radius. The true modulus of the sample, , can be calculated from the apparent modulus by accounting for the elastic properties of the indenter.
The microstructural morphologies of the homogenization-treated Ti-Mo alloys were observed using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan). Crystal phases were identified using an X-ray diffractometer (X’Pert MPD PW3040, Philips, Netherlands).
Anodic polarization tests and an ion-release test in a simulated body fluid (SBF) were conducted. The electrodes for the anodic polarization tests were prepared via cold mounting with epoxy resin followed by mechanical polishing with emery paper of up to 2000 grit. The SBF solution, which was composed of NaCl (8 g), KCl (0.22 g), Ka2HPO4·12H2O (0.15 g), NaSO4 (0.08 g), NaHCO3 (0.35 g), and CaCl2 (0.27 g), was used as an electrolyte solution at a pH of 7.4 and a temperature of 37°C. Potentiodynamic polarization scans were performed at a scan rate of 1 mV/s in the range from −1500 to 1500 mV/SCE. Immersion tests to assess the metal-ion release were conducted in the SBF solution (pH 7.4) and in 0.1% lactic acid (pH 3.4) at 37°C. Each solution was collected every 5 days for 30 days. The concentrations (average values of three times) of metal ions that were released into the solutions were analyzed up to 10 ppb (0.01 μg/mL) via inductively coupled plasma-mass spectrometry (ICP-MS, JY 70Plus, Longjumeau, France).
In order to evaluate cytocompatibility of the alloys, MC3T3-E1 cells (RCB 1126, an osteoblast-like cell line from C57BL/6 mouse calvaria) were cultured on Ti-Mo alloy plates in DMEM-HG and in 5% CO2 at 37°C with a cell seeding density 1.5 × 104 cells/well. The cell viability was measured with a 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl tetrazolium bromide (MTT) assay and was carried out after 1 and 3 days of culturing using an ELISA plate reader (precision microplate reader, Molecular Devices, Sunnyvale, CA, USA).
3. Results and Discussion
3.1. Microstructures and Elastic Modulus of Ti-Mo Alloys
Figure 1 shows the microstructures of the Ti alloys with 3, 7, 10, and 15 wt% Mo after heat treatment for homogenization. The specimens of the Ti-Mo alloys with 3 and 7 wt% Mo exhibited a fine acicular, martensitic structure of the phase in their surface microstructures [11]. When the Mo content increased, the fine acicular patterns in the Ti-Mo alloys disappeared. These results confirmed that the Mo content influences the formation of the martensite phase and effectively suppresses grain-growth behavior [13, 14].
Figure 2 presents the elastic modulus of the homogenization-treated Ti-Mo alloys with respect to the varying Mo contents. The elastic modulus of the Ti-Mo alloys decreased with the addition of Mo, although the actual value of the modulus fluctuated with different Mo contents. The elastic modulus decreased as the Mo content increased up to 7 wt% but then increased as the Mo content was further increased to 15 wt%. It is interesting to note that the elastic modulus of Ti-7Mo was very low (i.e., approximately 55 GPa); this result is in good agreement with that of Ho et al. [15].
Figure 3 shows the X-ray diffraction patterns of the Ti-Mo alloys. When a Mo content of 3 wt% was added, the homogenized alloy exhibited peaks corresponding to the hexagonal and orthorhombic phases. It was found that, in certain alloys (i.e., Ti-Mo, Ti-Nb, Ti-Ta, etc.), the hexagonal phase distorts and transforms into an orthorhombic phase [16]. The Ti-7 wt% Mo showed just the martensitic phase. When the Mo content increased to 10 wt% or higher, only the phase was observed in the XRD patterns. The retention of the phase at higher Mo contents is consistent with the results of Ho et al. [15].
3.2. Corrosion Resistance of Ti-Mo Alloys
Figure 4 presents the potentiodynamic polarization curves for the Ti-Mo alloys compared with CP Ti as tested in the SBF solution at 37°C. Some shift in the corrosion potential () and a remarkable reduction in the corrosion current densities () are evident, which indicate enhanced corrosion resistance. The differences in corrosion potential among the alloys may be attributable to tiny differences in surface conditions [17], despite the fact that the specimens were prepared using the same procedure. The corrosion potentials (), corrosion current densities (), and passive current densities () for the alloys, as obtained from their polarization curves, are summarized in Table 2. The corrosion densities () were obtained from the polarization curves using the Tafel extrapolation method and ranged from 0.358 to 0.524 μA/cm2 for the Ti-Mo alloys. The passive current densities () were in the range of 1.78–2.22 μA/cm2 for the Ti-Mo alloys, compared with a value of 3.77 μA/cm2 for the CP Ti. These results suggest that a protective passive film was formed for the Ti-Mo alloys. The potentiodynamic polarization curve of the Ti-7 wt% Mo exhibited stable passivation behavior throughout the entire range of potentials. These results indicate that the addition of Mo to pure Ti does not appear to modify the protection characteristics of its spontaneous oxides [18].
3.3. Electrochemical Impedance Spectroscopy (EIS) of Ti-Mo Alloys
Electrochemical impedance analysis between 10 mHz and 105 Hz with an amplitude of 10 mV at an applied potential was performed for the surfaces of the Ti-Mo alloy during immersion to investigate the electrode/electrolyte interfaces and the processes that occur on these surfaces.
Figure 5 presents the Bode magnitude and phase plots for the Ti-Mo alloys. The Bode magnitude plots are similar for all alloys; these plots are characterized by two distinct regions. In the high-frequency range (103–105 Hz), there is a flat portion (slope ≈ 0) that is associated with the response of the electrolyte resistance. In the broad low- and moderate-frequency ranges, the impedance spectra exhibit linear slopes of approximately −1, which is the characteristic response associated with the capacitive behavior of a passive film [19–21].
(a) Bode plot
(b) Bode phase plot
For all alloys, two distinct regions characterize the Bode phase plots. In the high-frequency range (over 1 kHz), the plots exhibit a constant impedance modulus that corresponds to a phase angle near zero degrees. The fact that the phase angle drops to near zero degrees in this high-frequency range indicates that the impedance is dominated by the solution resistance and by the surface-film resistance for the high- and low-frequency cases, respectively. In the high-frequency region, it seems that the phase angle drops to near 0° along with the response of the electrolyte resistance. In the moderate-frequency region, the phase angle remains near 80° over a wide frequency range, indicating the presence of a typical passive film on the surface and a near-capacitive response of the passive film [20]. In the low-frequency region, the phase angle decreases to a lower value because of the contribution of the passive-film resistance. In the low- and moderate-frequency ranges below 1 kHz, the plots of the impedance modulus versus the frequency exhibit linear slopes. This is the characteristic response associated with the capacitive behavior of a passive film [22, 23].
To obtain the electrochemical parameters by fitting the experimental EIS results, the equivalent circuit shown in Figure 6 was used for the alloys. Here, denotes the solution resistance, represents the polarization resistance, and CPE is the constant phase element. The physical model for this equivalent circuit corresponds to the presence of a single layer with both resistance and capacitance on a metal surface. By analyzing the data presented in Table 3, the following conclusions were derived. The values of were very close to one () for all alloys, indicating a near-capacitive behavior of the passive films formed on the Ti-Mo alloys. The corrosion resistance of the Ti-Mo alloys increased as the Mo content increased.
3.4. Metal-Ion Release of Ti-Mo Alloys
Table 4 presents the concentrations of metal ions released into the SBF solution and the 0.1% lactic acid over a period of 30 days. In the SBF solution, the elution of Ti and Mo ions was below the detection limit of 0.01 μg/mL, regardless of the alloy composition. By contrast, in the 0.1 lactic acid, Ti-ion release was detected in the range from 0.04 to 0.11 μg/mL, depending on the Mo content of the alloy. Moreover, the Ti-ion release from the Ti-7 wt% Mo alloy was slightly lower than that of conventional Ti-Mo alloys. This result indicates that the corrosion resistance of the Ti alloy with 7 wt% Mo was slightly better than the corrosion resistance of conventional Ti-Mo alloys, consistent with the results of the electrochemical tests.
3.5. Biocompatibility of Ti-Mo Alloys
The MTT assay was used to determine mitochondrial activity of the cultured osteoblasts on the surface of CP Ti and Ti-Mo alloys. Figure 7(a) shows the proliferation behavior in MC3T3-E1 cell adhesion after 1 day and 3 days during cell culturing. A sharp difference in the viability of MC3T3-E1 cells was scarcely found in all samples. It seems that the initial cell attachment was not affected by the alloy chemistry. After 3 days, proliferation in cell adhesion slightly increased compared to that of 1 day. Figure 7(b) shows the absorbance obtained from cells adhered to the surface at 3 days. The proliferation and absorbance of the cell showed rather high values for Ti-7 wt% Mo alloy.
(a)
(b)
4. Conclusion
The elastic modulus, electrochemical properties, and microstructures of homogenization-treated binary Ti-Mo alloys with Mo contents ranging from 3 to 15 wt% were investigated. The elastic modulus decreased when the Mo content increased up to 7% and then increased as the Mo content was further increased to 15%. Microstructure morphology of the Ti-7 wt% Mo alloy showed only a fine acicular martensitic structure of the orthorhombic () phase. An EIS data analysis confirmed that the polarization resistance of the Ti-Mo alloys increased as the Mo content was increased. The corrosion resistance of the Ti-Mo alloys increased with increasing Mo content. The Ti-7Mo alloy exhibited a relatively low elastic modulus and low metal-ion release. Near-capacitive behavior of the passive films was observed in the electrochemical impedance analysis, which reflected the corrosion resistance of the Ti-Mo alloys. MC3T3-E1 cell proliferation on Ti-7 wt% Mo was rather active than other Ti-Mo alloys and CP Ti. The low elastic modulus and good biocompatibility suggest that Ti-7Mo alloy is promising metallic biomaterials.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This study was supported by research fund from Chosun University, 2013.