Corrosion Resistance of an Alternative Thermomechanically Processed Ti-23.6Nb-5.1Mo-6.7Zr Alloy for Biomedical Applications

Abstract

Metastable titanium alloys have been developed for biomedical use due to their lower elastic modulus, combined with high strength, good ductility, and excellent corrosion resistance. In this study, the electrochemical corrosion resistance of the alternative Ti-23.6Nb-5.1Mo-6.7Zr alloy was investigated. The alloy was initially homogenized at 1000 °C for 24 h and then tested under different processing conditions: 90% cold rolling; 90% cold rolling followed by annealing at 950 °C for 1 h and water quenching; and 90% cold rolling followed by aging at 300 °C, 400 °C, and 500 °C for 4 h each. Electrochemical behavior was assessed using anodic polarization, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) tests in a synthetic solution (Ringer’s solution) to simulate body fluid. The obtained results demonstrate the stability of the passive film formed of the conventional and modified alloys, considering long-term use in the human body, regardless of the volumetric fraction and phase distribution across the various processing routes studied as β, α, α″ and ω. The electrochemical parameters, combined with Young’s modulus and hardness of the alternative alloys, enable the definition of a multicriteria selection method of the most suitable mechanical process routes to be used. The application focused on components of functional femoral stems.

1. Introduction

Titanium and its alloys are widely used as orthopedic implants due to their excellent corrosion resistance, biocompatibility, outstanding fatigue resistance, and lower elastic moduli compared to other metallic biomaterials such as stainless steel and Co-Cr-based alloys. For a long time, the most used alloy for orthopedic applications was the α + β alloy, Ti-6Al-4V. However, studies have shown that the release of small amounts of V and Al into the human body can induce cytotoxic effects and neurological disorders, leading to the development of new β titanium alloys over the past decade [1]. Additionally, beta titanium alloys, with their lower Young’s moduli, offer significant advantages in biomedical applications.

Corrosion resistance is a critical property for biomaterials, as the fluids within the human body can create an aggressive environment for metals. Titanium exhibits excellent corrosion resistance [2]. Implants are expected to resist uniform corrosion over the long term, with a corrosion rate of less than 1 μm/year [3]. The passive film on Ti alloys is predominantly composed of TiO₂. Metastable β-Ti alloys, containing elements such as Nb, Ta, and Zr, form a passive film that incorporates Nb₂O₅, ZrO₂, and Ta₂O₅. When combined with TiO₂, this film offers improved stability and potentially provides superior protection compared to the Ti-6Al-4V alloy [4,5,6,7]. Various studies have demonstrated the good corrosion resistance of Ti-Nb-Zr [8,9,10] and Ti-Nb-Mo-Zr [11,12] alloys.

The electrochemical behavior of metastable β-Ti alloys can also be influenced by the microstructures obtained after thermomechanical treatments: single-phased β, β + α, β + ω, and β+α”. Several authors have reported that single-phase β microstructure provides the best corrosion resistance for these alloys [13,14]. The literature indicates the formation of stress-induced α” martensite during cold rolling of metastable β-Ti alloys [15,16]. According to Lin J. et al., β phase transforms into an α” phase without diffusion and, for this reason, the corrosion resistance of the alloy tends to be the same as that in the solid solution due to the close thermodynamic properties of α” and β phase [17]. The aging of metastable β-Ti alloys leads to the formation of ω or α precipitates inside the β matrix, depending on aging temperature and duration [18]. A study by Li Q. et al. showed that the corrosion resistance of the Ti-24 at% Nb-2 at% Zr alloy decreases upon precipitation of the ω phase, due to the formation of an unstable passive film [19]. On the other hand, Monteiro et al. indicated that α phase precipitates significantly decreased the corrosion resistance of a Ti-30Nb-7Zr alloy [20]. For Su B. et al., the fact that β phase is more resistant to corrosion than α phase is related to the higher Nb, Zr, and Mo content of β phase [21].

Chui et al. [11] reported results for a Ti-Zr-Nb-Mo alloy, achieving a yield strength of 834 MPa and a hardness of 486 HV. The Young’s modulus ranged from 96 GPa to 105 GPa, which is lower than that of the Ti-6Al-4V alloy. The anodic polarization and EIS revealed an increase in the electrochemical corrosion resistance in simulated body fluid at 37 °C. The best performance in terms of corrosion resistance was expressed by corrosion potential and corrosion current density equal to 0.344 ± 0.005 V and 71.9 ± 5.3 nA/cm², respectively. The performance was attributed to the formation of a more protective passive layer due to grain refinement.

This study aims to investigate the electrochemical corrosion resistance of titanium alloy Ti-23.6Nb-5.1Mo-6.7Zr after various thermomechanical treatments: cold-rolling, annealing, or aging at various temperatures following cold rolling. The microstructures obtained for these different processing routes have already been previously studied and reported by our research team [22,23]. Electrochemical behaviors were characterized using a combination of anodic polarization, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) tests in a synthetic solution (Ringer’s solution) to simulate body fluid. This integrated approach provides a more accurate and consistent assessment of corrosion resistance compared to relying solely on short immersion time anodic polarization curves. The objective of the present work is to demonstrate that all processing conditions applied to the Ti-23.6Nb-5.1Mo-6.7Zr alloy result in lower elastic modulus, a high and comparable hardness-to-Young’s modulus ratio, and excellent passive film stability, ensuring its suitability for long-term use in the human body. Ti-6Al-4V was used as reference material.

2. Materials and Methods

A 6.5 kg cylindrical ingot, with a diameter of 120 mm and a length of 125 mm, was cast in a vacuum arc remelting furnace (VAR) using a titanium bar (grade 2—ASTM B 348) to which high-purity Zr, Mo, and Nb were added. To ensure homogeneity of the resulting alloy, the ingot was remelted three times. The chemical composition of the alloy is presented in

Table 1. Chemical composition for the Ti-23.6Nb-5.1Mo-6.7Zr alloy (wt%).

Alloy	Ti	Nb	Mo	Zr	O	N
Ti-23.6Nb-5.1Mo-6.7Zr	Bal.	23.57	5.07	6.72	0.105	0.0098

. The ingot was machined into a rectangular shape and subjected to homogenization treatment at 1000 °C for 24 h in an argon atmosphere (HM), followed by cold rolling until a 90% reduction in thickness was achieved (CR90). After cold rolling, the material was either vacuum-annealed at 950 °C for 1 h followed by water quenching (CR90-ANN) or aged for 4 h at temperatures of 300 °C (CR90-AG300), 400 °C (CR90-AG400), and 500 °C (CR90-AG500), respectively, with all aged samples also water-quenched. A detailed description of the procedure can be found elsewhere [22].

Young’s moduli (YMs) were obtained using the impulse excitation technique, described in ASTM E 1876 [24]. Ten measurements were taken for each sample. Vickers microhardness values were measured with a 200 g load and a dwell time of 15 s. The mechanical properties of a Ti-6Al-4V alloy (ASTM F136/ISO 5832-4) were also assessed under the same conditions for comparison [25,26].

Electrochemical corrosion resistance was assessed by open circuit potentials measurements, anodic polarization, and electrochemical impedance spectroscopy in the presence of Ringer’s solution (NaCl 8.6g/L; CaCl₂ 0.33g/L; KCl 0.3g/L) at 37 ± 1 °C, corresponding to the body fluid and corporal temperature, respectively. Long-term tests were used to better assess the stability of the passive condition expected from the alloys and to better mimic in vivo applications.

The main experimental setup was a three-electrode electrochemical cell, attached to a multichannel digital potentiostat Multi Palm Sems 4 (10 channels) (Palmsens B.V, Houten, The Netherlands) controlled by the software MultiTrace 4.5. Double-walled glass cells were used, containing circulating water at 37 °C set by a ultratermostatic bath model Q214M2 (QUIMIS, São Paulo, Brazil). Using the multichannel configuration, six tests were carried out in parallel over 120 h. The working electrodes were manufactured from small segments of the alloys, embedded in resin, having a flat exposed area to the solution of approximately 1 cm². Platinum wires were used as counter-electrodes, and AgCl was used as reference. Surface preparation of the working electrodes included grinding until 600 mesh, ultrasonic cleaning in isopropyl alcohol, and drying with warm air, followed by storage in a drying chamber before the experiments.

The experimental work was carried out in the following sequence: initial surface characterization using smart digital microscope (Smartzoom 5, Carl Zeiss Microscopy GmbH, Oberkochen, Germany) performed at 100× and 500× magnifications, which included measurements of the exposed area; open circuit potential (OCP) monitoring over 120 h; electrochemical impedance spectroscopy (EIS) performed at intervals of 6 h up to 120 h, in the frequency range of 10⁻² to 10⁴ Hz with a 10 mV at OCP; anodic polarization to determine corrosion potential (Ecorr), conducted up to 1.5 V above Ecorr, with a scan rate of 0.167 mV/s (ASTM F2129) and a limit current of 100 μA; evaluation of the initial and final pH of the solution; and final surface characterization using Smartzoom [27].

Electrochemical corrosion resistance was assessed using a multiparametric approach by integrating results obtained with different electrochemical methods. The following parameters were defined and determined: Delta OCP (∆OCP) (Open circuit potential variation), corresponding to the difference between final and initial potentials, and a positive shift during prolonged exposure to Ringer’s solution at body temperature indicating the evolution of the alloy’s passive state and improved the protective ability of the aged passive film; Ip (Anodic current density), measured at the median value within the passive range, reflecting the rate of dissolution and ion release under oxidizing conditions; and Rp (Polarization resistance), obtained via electrochemical impedance spectroscopy measurements, corresponding to the lower limit of the complex impedance value obtained at the lower frequency of the Nyquist or Bode spectra; it represents the pure resistive component of the passive film formed on the alloy surface and can be correlated with its protective ability. This value was measured at the open circuit potential (OCP) after prolonged exposure to the simulated body fluid.

Composed parameters were calculated from the three parameters described, and the results were cross-correlated to the mechanical properties parameters of the alloys. From this approach, the best configurations among the alloys were selected and compared to similar values corresponding to the Ti-6Al-4V, the reference alloy.

3. Results

In the present study, the Ti-23.6Nb-5.1Mo-6.7Zr alloy was initially homogenized at 1000 °C for 24 h and water-quenched (HM), and was subsequently investigated under five different processing conditions: (1) cold-rolled with a 90% thickness reduction (CR90); (2) cold-rolled, annealed at 950 °C for 1 h, and water-quenched (CR90-ANN); and (3–5) cold-rolled and aged for 4 h at 300 °C, 400 °C, and 500 °C, respectively (CR90-AG300), (CR90-AG400) and (CR90-AG500). These thermomechanical processing routes were strategically designed to produce distinct microstructures and, consequently, different phases, enabling a systematic evaluation of their potential impact on corrosion resistance.

3.1. Microstructural Characterization

The microstructural characterization of the alloy processed under different conditions has already been performed by our team in two previous studies [22,23] using optical microscopy (OM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). This characterization is not detailed again in this work.

Table 2. Phases observed for each processing condition. Phase identification was obtained by electron diffraction and X-ray diffraction, Adapted from Refs. [22,23].

Codification	Phases	Figure
HM	β	1a
CR90	β + α″	1b
CR90-ANN	β	1c
CR90-AG300	β + ω	1d
CR90-AG400	β + α	1e
CR90-AG500	β + α	1f

summarizes the phases present for each condition. Figure 1 shows the microstructure for each condition.

Homogenization (at 1000 °C) and the annealing (at 950 °C) heat treatments were carried out above the β-transus temperature. After water quenching, single-phase β is maintained in the microstructure for HM and CR-ANN conditions, as revealed in Figure 1a and Figure 1b, respectively. After 90% cold rolling (CR90 condition), Figure 1c shows elongated grains along the rolling direction (RD) with the formation of shear bands. The X-ray diffraction analysis revealed the presence of both β and α″ phases. After aging, nanometric precipitates were detected in the β grains. Figure 1d to Figure 1f show the microstructure after aging at 300 °C, 400 °C, and 500 °C, respectively. For the CR-AG300 condition, Figure 1d reveals very fine precipitates (≤5 nm). Electron diffraction analysis identifies these precipitates as ω phase. In the literature, ω phase precipitated during aging is mentioned as isothermal ω (ω iso). In Figure 1e, nano-sized lath-shaped α precipitates are observed in a β matrix after aging 4h at 400 °C (CR90-AG400 condition). No ω phase was detected after this heat treatment. The TEM bright-field image in Figure 1f shows α laths in a β matrix after aging for 4 h at 500 °C. These results confirm the most accepted precipitation sequence for metastable b-Ti alloys during aging: β → β + ω iso → β + ω iso + α → β + α [28].

3.2. Mechanical Properties

Figure 2 indicates the Young’s modulus for each studied processing condition. Figure 3 shows the Vickers microhardness (HV) measured for each condition. Figure 4 shows the hardness (in HV units)/Young’s modulus (in GPa) ratio. The hardness-to-Young’s-modulus ratio (HV/E) is often used as an indicator to evaluate the quality of the mechanical performances of metallic biomaterials for implants [29].

3.3. Electrochemical Corrosion Results

Results obtained with open-circuit potential are shown in Figure 5. In general, all processing conditions studied for this alternative alloy exhibited higher positive variations in OCP than Ti-6Al-4V alloy in the Ringer’s solution. The test duration was 120 h. The minimum OCP measured after the immersion of the alloys was −0.55V_Ag/Cl, and the higher value was about −0.10V _Ag/Cl after 120 h immersion.

Results obtained by EIS are shown in Figure 6. Nyquist plots were obtained at 6 h and 120 h immersion time in Ringer’s solution at OCP. All processing conditions and the reference alloy exhibited a single capacitive semicircle within the frequency range adopted. It can be observed that an increment of the impedance with time was registered for all alloys. The electrical circuit represented in the Nyquist plots in Figure 6 was modeled using ZView software (version 4.0h, Scribner Associates, Inc., Southern Pines, NC, USA) and fitted to the experimental data. The parameters obtained from the fitting analysis are presented in

Table 3. Parameters obtained by fitting equivalent circuit model using ZView to the EIS data.

Codification	Rs (Ω·cm2)	CPE 1 (Sn Ω−1·cm−2)	Rp (MΩ·cm2)	n (0 < n < 1)	Chi-Squared
CR90	12.4	3.09 × 10−5	0.28	0.83	2.09 × 10−5
CR90-ANN	16.8	4.65 × 10−5	1.19	0.93	2.26 × 10−4
CR90-AG300	19.6	4.14 × 10−5	1.01	0.90	6.02 × 10−4
CR90-AG400	18.5	3.22 × 10−5	1.37	0.94	9.01 × 10−4
CR90-AG500	22.1	4.00 × 10−5	1.49	0.93	1.29 × 10−5
Ti-6Al-4V	18.0	2.71 × 10−5	1.42	0.92	7.669 × 10−5

. In the obtained equivalent circuit, Rs represents the solution resistance, CPE1 corresponds to the capacitance modeled as a constant phase element, and Rp denotes the resistance of the oxide film of the alloy. The exponent “n” of the constant phase element showed values close to 1, indicating a non-ideal capacitor associated with a heterogeneous or non-equilibrium distribution of the oxide film.

Results of anodic polarization curves are shown in Figure 7. All alloys exhibited a fully passive response, without any incidence of localized corrosion in Ringer’s solution, after 120 h immersion. The obtained parameters from electrochemical corrosion results, with standard deviations, are in

Table 4. Parameters obtained from electrochemical corrosion results.

Codification	Ei (V)	Ef (V)	ΔOCP (V)	Rp (Moh·cm2)	Ip (mA/cm2)
CR90	−0.403 ± 0.10	−0.121 ± 0.01	0.281 ± 0.11	0.282 ± 0.04	1.620 ± 0.03
CR90-ANN	−0.460 ± 0.06	−0.89 ± 0.00	0.371 ± 0.06	1.191 ± 0.02	1.677 ± 0.22
CR90-AG300	−0.454 ± 0.01	−0.114 ± 0.00	0.341 ± 0.01	1.012 ± 0.04	1.342 ± 0.08
CR90-AG400	−0.517 ± 0.14	−0.182 ± 0.09	0.335 ± 0.04	1.374 ± 0.03	1.553 ± 0.30
CR90-AG500	−0.378 ± 0.07	−0.080 ± 0.03	0.318 ± 0.12	1.489 ± 0.01	1.425 ± 0.22
Ti-6Al-4V	−0.272 ± 0.15	−0.040 ± 0.03	0.232 ± 0.12	1.417 ± 0.04	0.902 ± 0.01

. No indication of localized corrosion was registered on the samples after anodic polarization tests.

Surface characterization of the samples was conducted both prior to and following the electrochemical corrosion tests to assess morphological changes. No variations in surface morphology were observed, and neither pits nor other forms of corrosion were detected, in agreement with the electrochemical results presented. As an example of the surface characterization, the obtained images before and after electrochemical measurements for (CR90-AG500), (CR90), and Ti-6Al-4V, are presented in Figure 8.

4. Discussion

Several works are available in the scientific literature aiming at the development of alternative Ti alloys. The main objectives have been the suppression of Al and V, due to biocompatibility requirements, and an improved mechanical response, aiming to provide a better compliance with biological tissues and biological structures. Within the frame of this development, it is essential to preserve high corrosion resistance, a mandatory requirement for biomaterials. The scientific literature points out the fact that Zr, Ta, and Nb oxides improve the protection ability of the passive films formed in modified Ti alloys [5]. Zr increases the corrosion resistance monotonically. The mechanism would be related to grain refinement. This conclusion is based on corrosion current density measurements, improved polarization resistance, and reduced corrosion rate. Some authors concluded that the corrosion morphology changes from uniform corrosion to mixed pitting and uniform corrosion when the Zr content is up to 40 wt% [21].

In the present work, the adopted approach aimed to integrate the two objectives and perform an in-depth analysis of the electrochemical corrosion resistance of alternative alloys obtained by different processing routes. The physiological solution selected for evaluation was Ringer’s solution, widely used in the literature. The evaluations were carried out under a longer exposure time, 120 h, necessary to reach a stationary condition of interaction between metallic materials and potentially corrosive media. The electrochemical parameters were determined in independent tests for each material, using triplicate working electrodes. The use of integrated indices, determined by mechanical and electrochemical responses, is the most relevant contribution of the adopted approach, besides the definition of the best processing route. The Ti-6Al-4V alloy was used as a reference or control material.

Figure 2 reveals that the Young’s modulus of the five processed alloys is much lower than the Young’s modulus of Ti-6Al-4V. A low Young’s modulus is important for alloys used in orthopedic implants. This requirement is mandatory to prevent the stress shielding that occurs due to the differences in Young’s moduli between the implant and the cortical bone. On the other hand, the hardness of Ti-6Al-4V is intermediate compared to the hardness of the processed alloys, as shown in Figure 3. Combining these two mechanical properties, Figure 4 indicates that the HV/E ratio is much higher for all the processed alloys compared to the HV/E ratio of Ti-6Al-4V. These higher HV/E values express a better compliance with biological tissues and structures. Therefore, the five processed alloys have superior mechanical performances than that of Ti-6Al-4V for orthopedic implants.

The open circuit potential results, as shown in Figure 5 for all processing conditions, exhibited in general higher positive variations in OCP than Ti-6Al-4V alloy in the Ringer’s solution. The ranking of the results as described above indicates better performance of (CR90-ANN), followed by the (CR90-AG300), (CR90-AG500), (CR90-AG400), and (CR90) processing conditions. The lowest ∆OCP was registered for the Ti-6Al-4V alloy. This parameter expresses the aging of the passive films with immersion time towards a more protective condition, as expected, and suggests that the modifications introduced are positive in terms of electrochemical corrosion resistance. These data were used to calculate and integrate electrochemical parameters. The trend of a more positive OCP with time in simulated body fluid of Ti and Ti alloys is reported in the literature by [4,7,20,30,31].

Results obtained by EIS are shown in Figure 6. All materials exhibited a pure capacitive response containing a single capacitive loop within the frequency range used. It can be observed that an increment in the impedance with time was registered for all conditions. This trend can be associated with the positive evolution of the passive conditions towards higher corrosion resistance. Only the initial branch of the capacitive loop was obtained, as commonly reported for passive materials containing a semiconductor passive film at the metal/solution interface. The extraction of the pure resistive component of impedance at lower frequency, or Rp—polarization resistance—was possible by numerical fit of the data using the Zview software. These values correspond to the polarization resistance of the passive film acting as a semiconductor. Higher Rp values suggest higher protection abilities of the passive films. The results obtained after 6 h and 120 h comparatively show the increment of Rp for all materials with immersion time. Considering the absolute values after 120 h, summarized in Figure 6c, it is possible to observe that the values of the Ti-6Al-4V are intermediate compared to those of the group of alloys under investigation. The order of magnitude for Ti-6Al-4V alloy and (CR90 AG400), (CR90-AG300), (CR90-ANN) processing conditions was 10⁶ Ω·cm². Similar values obtained for alloys (CR90-AG500) were significantly lower, having 10⁵ as order of magnitude. Capacitive passive layers of dielectric characteristics of high-niobium Ti alloys in saline solution are reported [32]. The same capacitive response of passive Bo-modified Ti alloys in Hanks Balanced Salt Solution (HBSS). Increment of Z with immersion time was reported, up to 168 h, and the Rp values were in the range 0.5 to 1.0 MΩ·cm² [33]. Regarding the structure and composition of the passive films, new knowledge is obtained by using Mott–Schottky and EIS analysis. Detrimental effects of Nb in Ti-Zr alloys have also been reported. When Nb is higher than 20%, the corrosion resistance increases. The authors mention localized corrosion incidence [34].

Anodic polarization curves are shown in Figure 7. All conditions exhibited a fully passive response, without any incidence of localized corrosion. This result is expected for Ti alloys in body fluids, being commonly reported in the literature [4,7,9,12,20,30,31,35]. Different profiles in the anodic polarization curves are related to different OCPs and slight variations among the passive current densities. The former values were used to estimate an additional parameter, Ip, the anodic current density registered at a potential corresponding to 50% of the passive range. Ti-6Al-4V exhibited the lowest value, followed by processing conditions (CR90-AG300), (CR90-AG500), (CR90-AG400), (CR90), and (CR90-ANN). Ip is related to corrosion rate, at almost zero, but deserves special attention if long-term exposures can lead to release of adverse ions during in vivo applications [5,31,36,37]. Anodic polarization results of modified Ti alloys that formed a stable protective passive layer within the tested potential range, with no incidence of localized corrosion, are reported [12,34,35] and used to rank the corrosion resistance.

Surface analysis indicated no traces of localized corrosion attack for all processing conditions after anodic polarization. This result confirms the stable passive condition achieved in the Ringer’s solution after 120 h immersion.

The corrosion test results demonstrated that no direct correlation could be established between the microstructure, with its different phases, and the observed corrosion resistance. However, this correlation is reported by some authors, who advocate higher corrosion resistance for certain microstructural configurations, especially martensitic Ti-6Al-4V extra-low interstitial alloy [10], and reduction of the resistance promoted by preferential dissolution at α/β phase boundaries during long-term exposure in 5 M HCl, unlike the neutral Ringer’s solution used as physiological environment in the present work [4].

In order to establish a classification of the corrosion resistance for the different processing conditions, the electrochemical parameters were merged on a unique parameter, Elindex, composed of ΔOCP, Rp, and Ip, calculated through the ratio (Rp x ΔOCP)/Ip [38]. This formulation is based on the positive and negative effects of increasing values on electrochemical corrosion resistance. The higher the ELindex, the higher the corrosion resistance. Based on the results shown in

Table 5. Electrochemical parameters of the processing conditions and the Ti-6Al-4V reference alloy.

Codification	Rp (MΩ·cm2)	∆OCP (mV)	Ip (μA/cm2)	ELindex
CR90	0.28	281	1.62	48
CR90-ANN	1.19	371	1.68	263
CR90-AG300	1.01	340	1.34	256
CR90-AG400	1.37	305	1.55	269
CR90-AG500	1.49	318	1.43	331
Ti-6Al-4V	1.42	231	0.90	364

, it can be concluded that the alternative alloy (CR90-AG500) exhibited the best performance, however, slightly below the response obtained with the reference alloy Ti-6Al-4V. The integrated electrochemical index permits a more accurate analysis of the corrosion resistance of the Ti alloys in Ringer’s solution.

As mentioned above, the β-Ti alloy in the CR90-AG500 condition has a much higher HV/E ratio, associated with a lower elastic modulus, compared to the reference Ti-6Al-4V alloy and, therefore, a better compliance with biological tissues and structures. Considering the results obtained in the present work, the β-Ti alloy in the CR90-AG500 condition stands out as a superior alternative to the Ti-6Al-4V alloy, offering comparable corrosion resistance and a markedly improved mechanical response, making it a more advantageous choice for biomedical applications.

5. Conclusions

The adopted methodology permitted an integrated assessment, demonstrating that the modified Ti alloys, such as Ti-23.6Nb-5.1Mo-6.7Zr, not only meet the critical requirement of corrosion resistance for biomedical applications, but also suggest their potential for enhanced biocompatibility owing to its improved mechanical properties. This is particularly relevant, considering the ongoing need for materials that can provide long-term stability and compatibility within the human body.

All alloys under development exhibited stable passive conditions in Ringer’s solution for long-term exposure, fulfilling the basic requirements for biomedical applications. The incidence of localized corrosion was not detected by electrochemical measurements followed by digital surface analysis.

The alloy annealed at 950 °C for 1 h, water-quenched after 90% cold rolling, and aged at 500 °C for 4 h after 90% cold rolling (CR90-AG500) was the best option to replace the Ti-6Al-4V alloy commonly used. Thus, the potentially harmful components, Al and V, can be suppressed, leading to higher biocompatibility.