Microstructure, wear resistance, and corrosion performance of Ti35Zr28Nb alloy fabricated by powder metallurgy for orthopedic applications

1. Introduction

Titanium (Ti) and its alloys, e.g. commercially pure Ti (CP-Ti), Ti6Al4V (wt.% hereafter), Ti5Al2.5Fe, and Ti6Al7Nb, are currently preferred as orthopedic implant materials because of their excellent biocompatibility, high corrosion resistance and strength-to-weight ratio [[1], [2], [3], [4], [5]]. However, these alloys have a higher elastic modulus (∼110 GPa) than human bone (3-30 GPa for cortical bone and 0.02-3 GPa for trabecular bone). The mismatch of elastic modulus between implants and bone can cause severe stress shielding, which may lead to premature implant failure [6]. Additionally, toxic ions, e.g. aluminum (Al) and vanadium (V), also can dissolve out from these alloys after long-term implantation, which can cause some diseases such as Alzheimer’s disease and mental disorder [6]. Therefore, the development of β-type Ti alloys with lower elastic modulus and better biocompatibility has become the focus of current research.

Recently, TiNbZr alloys containing different levels of niobium (Nb) and zirconium (Zr) have received extensive attention due to the Nb and Zr are considered to display excellent biological responses [7]. Additionally, the addition of Nb and Zr not only improves the mechanical properties by solid solution, but also enhances the corrosion and wear resistance through forming a protective TiO₂-Nb₂O₅-ZrO₂ film on the surface of the alloy [[8], [9], [10], [11], [12], [13], [14]]. Based on these excellent performances of TiNbZr alloys, Wen et al. [15] has designed the Ti35Zr28Nb alloy through some alloy design theories, e.g. molybdenum equivalence (Mo_eq), d-electron, and electron to atom ratio (e/a) methods. The results proved that the Ti35Zr28Nb alloy exhibited satisfactory mechanical properties and biocompatibility [15,16]. However, currently the alloy was fabricated by cold-crucible levitation melting that has a high manufacturing cost. In order to lower the cost, the Ti35Zr28Nb alloy was fabricated by powder metallurgy (PM) by our group, and the microstructure, mechanical properties, and cytocompatibility of the manufactured Ti35Zr28Nb alloy were investigated [17]. The results demonstrated that the PM-fabricated Ti35Zr28Nb alloy not only exhibited high compressive yield strength and low elastic modulus, but also displayed no significantly negative effects on cell proliferation [17].

It is well known that the human body is a complex organism and therefore two practical considerations must be taken into account when using metal materials as load bearings in orthopedic applications. Firstly, the alloy should be highly wear-resistant to minimize the formation of wear debris during continuous dynamic movement. Otherwise, the accumulated wear debris can lead to adverse allergic reactions in the tissue [18]. Secondly, the implant alloy needs to be highly corrosion resistant, as its surfaces come into direct contact with different types of corrosive body fluids that encompassing lots of Na⁺, Cl-, and so on. These ions can attract the implants locally, and hence leads to pitting corrosion. So, the evaluation of corrosion and wear behavior of new materials is crucial. The corrosion behaviors of Ti35Zr28Nb alloy fabricated by the ingot metallurgy method [19] and the porous specimens of the alloy fabricated by selective laser melting (SLM) under crevice-corrosion conditions were assessed in Hanks’ solution at 37 °C and 3.5 wt% NaCl solution at 95 °C [20]. The results showed that both bulk and porous Ti35Zr28Nb alloy had low corrosion rates, and there was no crevice corrosion occurred. However, the wear resistance and the corrosion behavior of the alloy fabricated by PM is still rare in the available literature.

In this study, the microstructure, hardness, corrosion and wear behaviors of the PM-fabricated Ti35Zr28Nb alloy were systematically investigated using the methods of open-circuit potential (OCP), potentiodynamic (PD) polarization, electrochemical impedance spectroscopy (EIS), and reciprocating wear test in a simulated body fluid (SBF) solution. In addition to establishing necessary understanding of the corrosion and wear performance of the PM-fabricated Ti35Zr28Nb alloy, it is hoped that this work will also provide insights into the corrosion and wear mechanisms of this PM-fabricated Ti35Zr28Nb alloy.

2. Experiments and methods

2.1. Material preparation and characterization

Atomized Ti28Nb35.4 Zr powders (purity ≥ 99.9%, 75 ≤ particle size ≤ 150 μm) was supplied by Wen group (RMIT University, Australia) who fabricated the powders using continuous inert gas atomization method. Due to it is difficulty to press into blocks, this powder was milled by ball milling machine for 30 min. The frequency of vibration of ball milling machine and the ball-to-powder weight ratio were 1400 r/min and 3:1, respectively. A cylindrical compact was obtained by pressing the milled powder under 450 MPa at room temperature. The compacted cylindrical was sintered in an argon (Ar) protection environment at 1550 °C for 2 h. The detailed fabrication process was described in our previous study [17,[21], [22], [23], [24]]. The Archimedes method was used to measure the density of the alloys based on the ASTM B962-14 [25], and the relative density was calculated by the follow formula:

Relativedensity = the density the theoretical density (1)

where the theoretical density of Ti35Zr28Nb is 6.36 g/cm³. A micro-hardness tester (Buehler Micromet 2100) with a 0.5 N load was used to measure the micro-hardness of the alloy in accordance with the ASTM E384-11 [26]. Phase constituents were performed a by X-ray diffractometer (Rigaku) using Cu radiation. The microstructure of the alloy was observed by a scanning electron microscope (SEM, JSM-6480LV, Japan). Before SEM observation, each specimen was ground with by abrasive paper and polished. Then the polished specimen was etched using Kroll’s etchant. The composition of the Kroll’s solution was as follows: hydrofluoric acid (5%), nitric acid (10%), and distilled water (85%). For density and hardness tests, five samples were measured and the average values were obtained.

2.2. Corrosion test

The corrosion resistance of the PM-fabricated Ti35Zr28Nb alloy was tested by America Princeton VersaSTAT MC electrochemical workstation in SBF at 37 ± 0.5 °C according to ASTM G59-97 [27]. The composition of the SBF solution was as follows: NaCl 8.035 g L^-1, NaHCO₃ 0.355 g L^-1, KCl 0.225 g L^-1, K₂HPO₄·3H₂O 0.231 g L^-1, MgCl₂·6H₂O 0.311 g L^-1, 1.0 M HCl solution 39 mL L^-1, CaCl₂ 0.292 g L^-1, NaSO₄ 0.072 g L^-1, Tris 6.118 g L^-1, 1.0 M HCl solution 0-5 mL L^-1, pH = 7.4. For comparison, as-cast Ti6Al4V and pure Ti were studied simultaneously, which is commonly used in clinical as orthopedic and dental implants and provided by Beijing Institute of Aerial Materials. The specimens were prepared as follows: 1. A copper (Cu) wire was laser connected to each specimen; 2. Each welded specimen was set in an epoxy resin with an exposed working surface of 1 cm²; 3. All specimens were ground with abrasive paper to 2000 grit; 4. All specimens were ultrasonically cleaned with ethanol and distilled water, and then dried in vacuum for further electrochemical testing.

The corrosion resistance of each specimens was studied by three-electrode system. The working electrode, reference electrode, and counter electrode were each specimen, saturated calomel electrode (SCE), and platinum foil, respectively. To stabilize the potential, each specimen was immersed into the solution for 2 h, and the corrosion potential vs. time curve was recorded. For the EIS measurements, the electrochemical impedance (Z) was measured in a frequency range from 10⁵ Hz to 0.01 Hz. The obtained data was simulated by the simulation software (Zsimpwin). Finally, the PD polarization curves were tested. The scan rate and scan range are 0.5 mV/s and 0.3-2.0 V vs. OCP, respectively. The corrosion parameters, including corrosion potential (E_corr), corrosion current density (I_corr), passive current density, determined at 0.5 V (I_p), film breakdown potential (E_b), passive potential (E_p) and passivation range (E_p-E_b), were calculated from PD polarization curves. All experiments were repeated five times to verify the reproducibility.

2.3. Wear test

Wear resistance of the Ti35Zr28Nb alloy was investigated by a reciprocating wear test using a UMT-Ⅱ multi-functional friction and wear tester in SBF solution. The as-cast Ti6Al4V and pure Ti alloys were also studied for comparison. Specimens were cut by electrical discharge machining in a size of 20 mm (length) × 6 mm (width) × 3 mm (thickness). Like the specimens for corrosion testing, all specimens were embedded in epoxy resin, and then ground, polished, ultrasonically cleaned, and dried in vacuum. The slip speed was 0.03 m/s (slip frequency was 1 Hz), and the stroke length was 15 mm. A silicon nitride sphere with a diameter of 5 mm was used as the grinding material. All tests were carried out at room temperature with a load of 3 N for 30 min. Coefficient of friction (COF) of each specimen was continuously recorded by tester system during the wear tests. The wear volume (V) was determined gravimetrically using:

V=$\frac{M_{loss}}{ρ}$ (2)

Where M_loss is the weight loss of the specimens after wear tests and ρ is the theoretical density of the alloy. Weight loss was measured at room temperature using an electronic balance with an accuracy of 0.0001 g. The wear rate was evaluated by the following formula [28]:

WR=$\frac{V}{F×v×t}$ (3)

where wR is the wear rate, mm / m · N, V is the wear loss volume during sliding, mm³; t is the sliding time, s; v is the sliding speed, m/s. Wear morphology was observed by an SEM. All experiments were repeated five times to verify the reproducibility.

2.4. Surface examination after PD polarization test

After the potentiodynamic polarization tests, each specimen was ultrasonically cleaned with ethanol, distilled water, and then dried in vacuum. The surface compositions of each specimens were examined by America Thermo ESCAL ultrasonically AB 250Xi X-ray Photoelectron Spectroscopy (XPS) analyzer. The output of Al-Kα source and X-ray spot size were 150 W and 500 μm, respectively.

3. Results and discussion

3.1. Microstructure and hardness of PM-fabricated Ti35Zr28Nb alloy

XRD patterns (a) and corresponding microstructures (b-d) of the PM-fabricated Ti35Zr28Nb, as-cast pure Ti, and Ti6Al4V are shown in Fig. 1. The chemical compositions of Ti35Zr28Nb alloy are presented in Table 1. The diffraction peaks of the Ti35Zr28Nb alloy proved that the alloy consisted of a single β phase. Additionally, as the Fig. 1(a) shown, the pure Ti consisted of a single α phase while the Ti6Al4V consisted of α phase with a small amount of β phase.

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Fig. 1.   (a) XRD spectra, and SEM images of: (b) PM-fabricated Ti35Zr28Nb, (c) as-cast Ti6Al4V, and (d) as-cast pure Ti.

Fig. 1(b-d) depicted the microstructures of three different Ti alloys. It can be seen that all three Ti specimens were almost completely dense with no visible pores and they all exhibited uniform microstructures. The microstructure of the Ti35Zr28Nb alloy was characterized by a single β phase with an average grain size of 100 ± 10 μm, while the microstructures of the pure Ti and Ti6Al4V exhibited coarse equiaxial α phase structures and Widmanstätten α + β phase structures, respectively. These results are in agreement with the XRD analysis.

The density test result showed that the density of Ti35Zr28Nb, pure Ti, and Ti6Al4V was 6.239 g/cm³, 4.469 g/cm³, and 4.45 g/cm³, and the relative density of Ti35Zr28Nb, pure Ti, and Ti6Al4V was 98.1%, 99.1%, and 98.9%, respectively. As shown in Fig. 2, the hardness value of the Ti35Zr28Nb was 310 ± 1.8 HV, which is nearly 2.8 times of that of pure Ti (106 ± 1.3 HV) but lower than that of the Ti6Al4V (382 ± 2.1 HV), which is similar to the previous results [15].

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Fig. 2.   Micro-Vickers of PM-fabricated Ti35Zr28Nb, as-cast Ti6Al4V, and as-cast pure Ti.

3.2. Corrosion properties of PM-fabricated Ti35Zr28Nb alloy

Initial OCP curves of the PM-fabricated Ti35Zr28Nb, as-cast pure Ti and Ti6Al4V are shown in Fig. 3. It can be seen that all curves exhibited similar tendency. After the specimens were immersed into the solution, the E_ocp rose quickly during the first few minutes. Thereafter the OCP stabilized to -0.27 V for pure Ti, -0.31 V for Ti6Al4V, and -0.22 V for Ti35Zr28Nb, respectively. After the OCP was stabilized, the Ti35Zr28Nb alloy showed higher OCP values than both the Ti6Al4V and pure Ti, which means that the PM-fabricated Ti35Zr28Nb alloy potentially has higher corrosion resistance and self-passivating ability.

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Fig. 3.   Open-circuit potential vs. time for as-cast pure Ti, as-cast Ti6Al4V and PM-fabricated Ti35Zr28Nb specimens in naturally aerated SBF solution at 37 ± 0.5 °C.

The PD polarization curves of Ti35Zr28Nb, Ti6Al4V and pure Ti alloys are shown in Fig. 4. As regards the cathodic branches, no significant difference was observed, indicating that similar cathodic reaction was happened on the surface of Ti35Zr28Nb, Ti6Al4V and pure Ti alloys. As for the anodic curves for each specimen, there were three characteristic potential domains. Taking the Ti35Zr28Nb alloy as an example, in the first potential domain (-0.22 V (E_corr) - 0.05 V (E_p)), the current density increased gradually with the potential raise. This is mainly because the spontaneously formed oxide film is replaced by a less protective oxide layer, or the TiO or Ti₂O₃ is oxidize to TiO₂ [[29], [30], [31], [32]]. When the potential reached to 0.05 V (E_p), it entered the second potential domain. In this potential domain, the current density unchanged with the potential increased, indicating that the alloy entered into the stable passivation region. When the potential continually increased to 1.34 V (E_p), it entered the third potential domain. In this potential domain, the current density increased with the potential increase again, which means that the films started to breakdown due to high overpotential. As for the anodic branches of Ti6Al4V and pure Ti alloys, no significant difference was observed compared with that of Ti35Zr28Nb alloy. It also divided into three potential domains, although the values of E_corr, E_p and E_b varied. This result is similar with the findings given by Cai et al. [33], who systematically investigated the electrochemical characterization of cast titanium alloys including pure Ti, Ti6Al4V, Ti6Al7Nb, and Ti13Nb13Zr.

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Fig. 4.   Potentiodynamic polarization curves for as-cast pure Ti, as-cast Ti6Al4V, and PM-fabricated Ti35Zr28Nb specimens in naturally aerated SBF solution at 37 ± 0.5 °C.

The corrosion parameters calculated from PD polarization curves are listed in Table 2. Based on the results given by McCafferty [34], there is a dissolution reaction before passivation reaction in the anodic polarization curves, which will lead to a loosely-defined anodic Tafel region. So, the I_corr was determined from cathodic polarization curve by Tafel extrapolation method, as shown in Fig. 4. It can be seen from Table 2 that all three specimens showed similar I_p values in an extended potential region within the statistical error ranges. This result means that all of three specimens have formed stability film in the surface in naturally aerated SBF solution. However, the Ti35Zr28Nb exhibited higher E_corr (-0.22 ± 0.01 V) than pure Ti (-0.27 ± 0.03 V) and Ti6Al4V (-0.31 ± 0.02 V,), while shown smaller I_corr (57.45 ± 1.88 nA cm^-2) than pure Ti (63.31 ± 1.21 nA cm^-2) and Ti6Al4V (95.31 ± 1.56 nA cm^-2). In addition, the Ti35Zr28Nb alloy presented higher E_b and E_p-E_b than those of the pure Ti and Ti6Al4V, while showing a lower E_p. The I_corr, E_b, E_p-E_b, and E_p are elements that reflect the anti-corrosion properties of materials. Generally, the smaller that E_p and I_corr and the larger that E_b and E_p-E_b are, the higher the corrosion properties of the material. These results indicate that the Ti35Zr28Nb alloy is more corrosion resistant than that of the as-cast pure Ti and Ti6Al4V.

Fig. 5 shows the Nyquist (a) and Bode (b) plots for pure Ti, Ti6Al4V and Ti35Zr28Nb specimens at the E_ocp. It is obvious from Fig. 5(a) that all the Nyquist plots exhibited the complete semicircle, which means that a near capacitive response [35,36]. Additionally, the Ti35Zr28Nb alloy exhibited bigger dimeter of the capacitive loop than those of the pure Ti and Ti6Al4V, indicating that Ti35Zr28Nb alloy is more corrosion resistant. The Bode magnitude and phase plots of all three specimens are shown in Fig. 5(b). For the Bode magnitude plots, two salient regions were observed. In the range of 10³-10⁵ Hz, the slope is close to 0, which is related with the response of R_s. When the frequency less than 10³ Hz, the Bode impedance spectra exhibited a slope of approximately -1, which is a capacitive behavior’s respone of a passive film [37]. For Bode phase plots, in moderate-frequency and low-frequency, all three specimens exhibited high negative phase angle (70-80°). Such phenomenon indicates that the formation of a passive film on the surface [38,39]. In addition, it also can seen from Fig. 5(b) that negative phase angle drops to near 0 ° in the range of 10³-10⁵ Hz, which is also because of the response of R_s.

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Fig. 5.   (a) Nyquist, and (b) Bode plots for as-cast pure Ti, as-cast Ti6Al4V and PM-fabricated Ti35Zr28Nb specimens in naturally aerated SBF solution at 37 ± 0.5 °C compared with the simulation results.

The change of the impedance with frequency can be explained by equivalent circuit. According to the EIS data as shown in Fig. 5, there is a large semicircular capacitive loop (Fig. 5(a)) and one relaxation time constant (Fig. 5(b)). Hence, a simplified Randles’ circuit R_s(R_bQ) model was used (Fig. 6), which has been used in many Ti alloys [[40], [41], [42], [43]]. It assumes an oxide film that acts as a barrier-type compact layer to hinder the corrosion of a passive metal. The simulated curves were also plotted in Fig. 5, and it can be seen that the simulate values were consistent with the experimental values. The simulated impedance parameters are listed in Table 3. It can be seen that the chi-squared (χ²) values all on the order of 10^-4, indicating a good agreement. In addition, the n_b values given by an equivalent circuit for each Ti specimen approached 1.0 (≥ 0.88). This means that near capacitive behavior occurred on each specimen [41]. The polarization resistance (R_p) value, which is a parameter that associates with the rate of dissolution reaction and the ability of the charge transfer, is 10.24 ± 0.18 MΩ cm² for the Ti35Zr28Nb alloy, and it is about 17.7 times that of the Ti6Al4V and 4 times that of the pure Ti. Additionally, the sequence of Q_b values follows: Ti6Al4V > pure Ti > Ti35Zr28Nb. All these results demonstrated that the Ti35Zr28Nb alloy is more highly corrosion resistant than either the pure Ti or Ti6Al4V, which is consisted with the results of PD polarization.

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Fig. 6.   Randle equivalent circuit for simulation results of impedance spectra of as-cast pure Ti, as-cast Ti6Al4V and PM-fabricated Ti35Zr28Nb specimens in naturally aerated SBF solution at 37 ± 0.5 °C.

XPS was used to identify the composition of the passive films formed on the surface of the Ti35Zr28Nb, pure Ti, and Ti6Al4V. The high resolution spectra and the corresponding binding energy are shown in Fig. 7 and Table 4, respectively. It can be seen that the primary elements for the Ti35Zr28Nb alloy were titanium (Ti), niobium (Nb), zirconium (Zr), and oxygen (O). Further comparison indicated that the surface oxidation films formed on the Ti35Zr28Nb were mainly composed of TiO₂, Nb₂O₅, and ZrO₂. Because the binding energies of Ti at Ti_2p, Nb at Nb_3d, and Zr at Zr_3d are in agreement with standard data for TiO₂, Nb₂O₅, and ZrO₂ [44]. In the case of the pure Ti and Ti6Al4V, the main elements on the surface were Ti, O, and Ti, O, Al, respectively. The binding energies for pure Ti and Ti6Al4V of Ti at Ti_2p and Al at Al_2p were in agreement with the standard data for TiO₂ and Al₂O₃ [[45], [46], [47]], indicating that the oxides formed on the surfaces of pure Ti and Ti6Al4V were TiO₂ and TiO₂-Al₂O₃, respectively.

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Fig. 7.   High resolution XPS spectra of CP Ti, Ti-6Al-4V and Ti35Zr28Nb in SBF after potentiodynamic polarization tests (a) Ti 2p, (b) O 1s, (c) Al 2p, (d) Nb 3d, (e) Zr 3d.

The corrosion resistance of metal material is related with the following three relevant facts: (1) structural of the passive film; (2) component of the passive film; (3) microstructure of the alloys. In the present study, the high corrosion resistance of the Ti35Zr28Nb alloy can be attributed to the high Nb and Zr content. The high Nb and Zr content can form high concentrations of Nb₂O₅ and ZrO₂ on the surface of the alloy. The present of Nb₂O₅ and ZrO₂ on the passive TiO₂ layer can improve the structural integrity of the oxide film [48], and hence enhances the corrosion resistance. Additionally, Nb and Zr are much nobler than Ti from the electrochemical point of view. So, when there are Nb and Zr-enriched layer formed on the surface, the corrosion performance of materials certainly would be improved [48]. Furthermore, the microstructure of the alloys also had a significant effect on the corrosion resistance. It is well known that the β-type Ti alloys are usually more corrosion resistance than α-type Ti alloys due to the oxide film formed on β phase is more stable than that on α phase [49].

As regards the inferior corrosion resistance of the Ti6Al4V than Ti35Zr28Nb and pure Ti alloys, it mainly because of the V element. It has been proved that the presence of V in the oxide film will increase the number of anion vacancies, because of the atomic radius between V and Ti is very large [50,51]. The increase of anion vacancies reduces the stability of passive film, and hence lower the corrosion resistance of the materials. In addition, the compositional difference between the α and β phases can result in the formation of a galvanic potential, which will also decrease the corrosion resistance of Ti-6Al-4 V alloy [52]. As a result, the Ti35Zr28Nb alloy exhibited higher corrosion resistance than as-cast pure Ti and Ti-6Al-4 V alloys. This is in agreement with the findings of Robin et al. [53], who demonstrated that the TixNb13Zr alloys (x = 5, 13, and 20) were more corrosion resistant than as-cast pure Ti and Ti-6Al-4 V alloys.

3.3. Wear properties of PM-fabricated Ti35Zr28Nb

The wear volumetric loss wear rate, and friction coefficient curves of the pure Ti, Ti6Al4V and Ti35Zr28Nb specimens are shown in Fig. 8. As shown in Fig. 8(a), the average wear rate of the Ti35Zr28Nb is 0.0021 ± 0.0002 mm³/m·N, while the values for the pure Ti and Ti6Al4V are 0.0029 ± 0.0004 mm³/m·N and 0.0020 ± 0.0003 mm³/m·N, respectively. The COF of the three different Ti specimens is shown in Fig. 8(b). It can be seen that their COF values exhibited relatively steady-state behavior with few local fluctuations. The average COF values of the Ti35Zr28Nb, pure Ti and Ti6Al4V alloys are in the order: pure Ti (0.47) > Ti35Zr28Nb (0.42) > Ti6Al4V (0.35). Overall, the COF and wear rate results indicate that the wear resistance of the Ti35Zr28Nb is higher than that of the pure Ti and close to that of the Ti6Al4V.

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Fig. 8.   (a) wear rate and wear volumetric loss, and (b) friction coefficient curves of as-cast pure Ti, as-cast Ti6Al4V and PM-fabricated Ti35Zr28Nb specimens.

The differences in wear rate is associated with the hardness of the specimens and the oxide films formed on the surface. It has been demonstrated that an improvement in the hardness of alloys can enhance their wear resistance [54]. In this study, the Ti35Zr28Nb and Ti6Al4V alloys showed greater hardness than the pure Ti, and hence both of these displayed lower wear rates than the pure Ti. In addition to hardness, the oxide films formed on the surfaces also play a crucial role [55]. It has been reported that Nb₂O₅ is an oxide with very good lubricating [56,57], and hence the formation of Nb₂O₅ can improve the wear resistance. As discussed above, passive protective films including Nb₂O₅ oxide can form on the surface of the Ti35Zr28Nb. This protective film hence can enhance the wear resistance of the Ti35Zr28Nb. This is consistent with the results of Guo et al. [58] and Li et al. [59], who proved that both Ti-29Nb-13Ta-4.6 Zr and Ti-25Nb-2Mo-4Sn alloys shown better wear resistance than Ti6Al4V alloy due to the formation of Nb₂O₅ oxide films on the surface of the alloys. Hence, although the hardness of the Ti35Zr28Nb is lower than that of the Ti6Al4V alloy, they show similar wear resistance.

The morphologies of the worn surfaces generated and the wear debris on the pure Ti, Ti6Al4V, and Ti35Zr28Nb after wear tests are shown in Fig. 9. It can be seen that all three of the Ti specimens exhibit furrows with different depths and plastic deformation. Also, there is some wear debris distributed on the edges. The worn surface of the Ti35Zr28Nb is relatively flat, and there are continuous furrows, which indicates that abrasive wear occurred (Fig. 9(a)). This is caused by the squeezing and scraping of grinding materials. It can be seen from Fig. 9(d) that there is some flaked grinding debris, indicating that adhesive wear has also occurred to some extent. The worn surface of the pure Ti is relatively rough, and there are some plastic grooves along the slide direction (Fig. 9(b)). Additionally, as shown in Fig. 9(e), the wear debris on the pure Ti consists of flaked grinding debris. This is mainly caused by the low hardness of pure Ti. During the wear tests, it underwent plastic deformation due to the pressure of the grinding ball and the shear stress. When the plastic deformation reaches the peak of the fracture strength, the surface of alloy fractures, and then flaked grinding debris forms [60,61]. These results indicate that the wear mechanism of the pure Ti is adhesive wear. As shown in Fig. 9(c), similar to the worn surface of the Ti35Zr28Nb, the worn surface of the Ti6Al4V is also relatively flat, and there are continuous furrows. However, it can be seen in Fig. 9(f) that the wear debris on the Ti6Al4V is completely composed of particles, indicating that the wear mechanism on the Ti6Al4V is abrasive wear. As a result, the wear mechanisms of the Ti35Zr28Nb, pure Ti and Ti6Al4V are abrasive wear accompanied by adhesive wear, adhesive wear, and abrasive wear, respectively.

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Fig. 9.   Morphologies of worn surfaces generated and corresponding wear debris images of the Ti specimens after wear tests: (a) and (d) PM-fabricated Ti35Zr28Nb, (b) and (e) as-cast Ti, (c) and (f) as-cast Ti6Al4V.

4. Conclusion

In conclusion, a Ti35Zr28Nb alloy with uninform microstructure of single β phase and high relative density of 98.1 ± 1.2% can be obtained by sintering at 1550 °C from ball-milled pre-alloyed powder. The PM-fabricated Ti35Zr28Nb exhibited spontaneous passivity in SBF solution. The PM-fabricated Ti35Zr28Nb showed the highest corrosion resistance as compared to Ti6Al4V and pure Ti, including E_p (-0.22 ± 0.01 V), I_corr (57.45 ± 1.88 nA), E_p (0.05 ± 0.01 V), and E_p-E_b (1.29 ± 0.09 V) because of the formation of protective passive films containing TiO₂, Nb₂O₅, and ZrO₂. The PM-fabricated Ti35Zr28Nb showed smaller Q_b values and higher R_p values than the as-cast Ti6Al4V and pure Ti, which also indicated that the Ti35Zr28Nb has higher corrosion resistant than Ti6Al4V and pure Ti. Under the same wear conditions, the wear rate of the Ti35Zr28Nb, pure Ti and Ti6Al4V specimens were in the order: pure Ti (0.0029 ± 0.0004 mm³/m·N) > Ti35Zr28Nb (0.0021 ± 0.0002 mm³/m·N) > Ti6Al4V (0.0020 ± 0.0003 mm³/m·N), indicating that the wear resistance of the Ti35Zr28Nb is higher than that of the pure Ti and close to that of the Ti6Al4V. The wear mechanism of the Ti35Zr28Nb is mainly dominated by abrasive wear, accompanied by adhesion wear. This PM-fabricated Ti35Zr28Nb alloy can become an attractive implant material due to its excellent corrosion, satisfied wear resistance, and easy net-shape manufacturability.

Declaration of Competing Interest

None.

Acknowledgments

This research work is supported by the National Natural Science Foundation of China (51874037), 13th Five-Year Weapons Innovation Foundation of China (6141B012807) and State Key Lab of Advanced Metals and Materials, University of Science and Technology Beijing (2019-Z14). Cuie Wen acknowledges the financial support for this research by the National Health and Medical Research Council (NHMRC), Australia through project grant (GNT1087290).