Influence of thermomechanical processing on biomechanical compatibility and electrochemical behavior of new near beta alloy, Ti-20.6Nb-13.6Zr-0.5V.

This paper presents the results for the effect of different methods of thermomechanical processing on the mechanical properties and electrochemical behavior of metastable β alloy Ti-20.6Nb-13.6Zr-0.5V (TNZV). The thermomechanical processing included hot working, solution heat treatments at different temperatures, and cooling rates in addition to aging. The thermomechanical processing conditions used in the study resulted in attainment of a wide range of microstructures with varying spatial distributions and morphologies of elongated/equiaxed α, β phases, or martensite, as a result of which several tensile properties were achieved. Aging treatment led to an increase in hardness, elastic modulus, and tensile strength and a decrease in ductility (elongation). Electrochemical tests indicated that the TNZV alloy undergoes spontaneous passivation due to spontaneous formation of an oxide film in the environment of the human body. Because the air-cooled samples possessed high hardness and also a fine grain size, they showed a lower corrosion rate than the samples treated under other conditions.

Introduction

Engineering materials used in biomedical applications must have optimal mechanical properties and biocompatibility, and should remain chemically stable under severely hostile conditions. In comparison with biomedical stainless steels and cobalt-chromium alloys, titanium (Ti)-based alloys show properties such as low specific strength and excellent resistance to corrosion, as well as a low elastic modulus close to that of bones, so they are the materials of choice for structural biomedical applications.1–4

Because of its excellent specific strength, resistance to corrosion, and biocompatible characteristics, the α+β-type Ti-6Al-4V alloy (ASTM F1108) has been used as a structural biomaterial for the manufacturing of orthopedic prostheses and dental implants.5 However, the alloying elements present in this alloy have their own adverse effects in the biomedical environment. The literature reports that the presence of vanadium (V) ions in human tissues can alter the kinetics of the enzyme activity associated with cells involved in the inflammatory response.6,7 It has been reported that aluminum (Al) increases the risk of development of Alzheimer’s disease.8 Further, the low elastic modulus of Ti alloys is a major advantage that makes them suitable implant materials for artificial joints (eg, hip, knee, shoulder). The stiffness of the implant materials must essentially be as close as possible to that of the connected bone to facilitate effective transfer of mechanical stress so as to avoid damage to bone cells.9 A large difference between the stiffness of the implant materials and bone may result in osteoporosis or poor osseointergration,10 which may consequently lead to crack nucleation and eventual failure of the implant.11 The modulus of the Ti-6Al-4V alloy is approximately 110 GPa,12 which is considerably higher than that of human bone (10–40 GPa).13 The high modulus of Ti-6Al-4V is due to an increased volume fraction of the α phase caused by a high Al content.14

In recent times, near β-type Ti alloys, containing niobium (Nb) and zirconium (Zr), have attracted considerable attention for orthopedic implant applications owing to their unique combination of good mechanical properties, low elastic modulus, and superior resistance to biocorrosion. It has been reported that the elastic modulus of these alloys can be significantly reduced by adjusting the concentration of β-stabilizing elements.15–17 In addition, this type of Ti alloys shows evidence of excellent resistance to corrosion in human body fluid. The main reason for this behavior is the formation of a protective hard and tightly adherent oxide film.18 Therefore, new β-type Ti alloys have been developed using nontoxic and nonallergic elements such as Nb, Zr, and other such elements.19 It is well known that Nb is a β-stabilizer element that forms a homogenous solid solution with Ti in all kinds of Ti alloys,20–22 whereas Zr is traditionally deemed to be a neutral element in the α-type and α+β-type alloys. On the other hand, it has been reported that Zr can be treated as a kind of β-stabilizer in the Ti-Nb-Zr alloy system because it not only inhibits α precipitation23,24 but also considerably reduces the martensitic transformation start temperature.25,26 In order to develop safe Ti-based alloys for biomedical applications, Nb and Zr are preferably added as they enhance the ability to achieve biological passivity and the ability to reduce the elastic modulus.27

In general, thermomechanical processing (TMP) is a metallurgical process through which work hardening and heat treatment processes are integrated into a single process.28 TMP plays a crucial role in altering a microstructure, leading to outstanding maaterial properties.29–31 The mechanical properties32 and corrosion behavior33 depend strongly on the alloy composition, processing history, and heat treatment conditions. Since near-β Ti alloys respond to thermal treatment and TMP, various microstructural constituents, like size, shape, and amount of the various phases, can be modified by varying the TMP parameters. However, there have been very few reports of the influence of thermal treatment and TMP on the microstructural features of the as-cast Ti-Nb-Zr alloy system, and in turn, of its mechanical and electrochemical behavior. Geetha et al33 investigated the effect of heat treatment on the corrosion behavior of Ti-13Zr-13Nb alloy in Ringer’s solution and found that water-quenched α+β alloys have superior resistance to corrosion owing to a homogeneous distribution of the alloying elements. Studies pertaining to the effect of TMP on the microstructure and mechanical properties of Ti-13Zr-13Nb alloy34 and the corrosion behavior of this alloy in simulated body fluid35 report that major results are achieved due to the cooling rate after solution treatment.

In view of the potential advantages offered by biocompatible Ti alloys, a lot of dedicated focused work needs to be carried out by engineers and materials scientists to develop novel Ti alloys with a low elastic modulus and superior electrochemical behavior for biomedical applications. With this in mind, we investigated the effect of TMP on the micro-structure, mechanical properties, and corrosion behavior of a novel Ti-20.6Nb-13.6Zr-0.5V (TNZV) alloy.

Materials and methods

The alloy used in the present investigation was prepared by casting of a mixture of sponge Ti, niobium powder, and Zr chips. The TNZV alloy was prepared using the nonconsumable vacuum arc melting technique and supplied in the form of 600 g pancakes. The pancakes were remelted three times to obtain compositional homogeneity. The composition of the alloy was analyzed, and the chemical composition in wt% is given in Table 1.

Table 1

Chemical composition (wt%) of Ti-20.6Nb-13.6Zr-0.5V

Ti (wt%)	Nb (wt%)	Zr (wt%)	V (wt%)	Fe (wt%)
Balance	20.6	13.6	0.5	0.14

Abbreviations: Nb, niobium; Ti, titanium; Zr, zirconium; V, vanadium; Fe, iron.

The as-cast TNZV alloy was thermally treated at 1,000°C for 1 hour for homogenization and then water-cooled. Subsequently, the homogenized samples were subjected to a 10% by forging above the β transition temperature (850°C) and directly subjected to a 25% reduction by rolling at the same temperature and then air-cooled (AC) to room temperature. After complete plastic deformation, the alloy was found to be free from any metal working defects, indicating that the entire metal working process was performed successfully.

The hot deformed TNZV samples were solution-treated at 850°C (above β transus temperature) for 1 hour in a dynamic argon atmosphere and then either water-quenched (WQ), AC, or furnace-cooled (FC). Aging treatment for the WQ samples was done at 500°C for 5 hours. The TMP route for the TNZV alloy is shown schematically in Figure 1.

Figure 1

Schematic diagram of the thermomechanical processing of Ti-20.6Nb-13.6Zr-0.5V alloy.

Abbreviations: WQ, water-quenched; AC, air-cooled; FC, furnace-cooled.

The major and trace elements were determined using an X-Strata Isis 1559 X-ray fluorescent spectrometer (Oxford Instruments, Abingdon, UK). Analysis of the microstructure of the thermally treated samples was carried out using an optical microscope and a Nova Nano 450 field emission scanning electron microscope (FEI Company, Eindhoven, The Netherlands) at 2 kV. For microstructural analysis, the metallographic samples were prepared using standard techniques employed for Ti and its alloys.36 The samples were ground with 1,200 grit silicon carbide, followed by polishing to a mirror finish using 0.5 µm diamond paste. The samples were etched with Kroll’s reagent (10 vol% hydrogen fluoride and 5 vol% HNO3 in water). X-ray diffraction analysis was carried out at room temperature using a PW1830 X-ray diffractometer (Philips, Eindhoven, The Netherlands) with Cu Kα radiation at a wavelength of 1.54056°A at 40 kV and 30 mA. The scanning rate was kept at 3 degrees to 2θ/60 seconds.

Vickers microhardness (HV) measurements were performed on a computer-controlled precision microhardness tester (MicroWhizHard, Mitutoyo Corporation, Kanagawa, Japan) where an indentation load of 300 gf and a dwell time of 5 seconds were used for each of the indents. The HV test was carried out on the polished specimens to obtain a clean and flat surface. Final polishing was carried out using 0.5 µm diamond paste. Ten indentations were taken for each specimen and the average was considered.

Conventional tensile testing was carried out using a computerized UTE-60 universal testing machine (Fuel Instruments & Engineers Pvt Ltd, Kolhapur, India) at a constant cross-head speed of 1 mm per minute in air at room temperature. Mechanical testing, as per ASTM E8M, was performed to determine the ultimate tensile strength (UTS), 0.2% off-set yield strength (YS), and elongation (e%). The elastic modulus was obtained by measuring the slope of the linear part of the stress–strain response. Dog bone-shaped tensile specimens with dimensions as shown in Figure 2 were precisely machined using a wire electrical discharge machine. After machining, tensile specimens were polished using waterproof silicon carbide papers of up to 2,500 grit and the gauge length of the specimens was mechanically polished using 0.5 µm diamond paste.

Figure 2

Dimensions of tensile specimen (ASTM E-8 standard).

The corrosion behavior of the TNZV alloy was studied using a potentiostat, comprising a three-electrode cell with Ag/AgCl (KCl saturated) as the reference electrode (all the potential measurements were made with reference to this) and platinum foil as the counter electrode (cathode). Test specimens with dimensions of 10 mm ×10 mm ×2 mm were used as the working electrode (anode). Anodic polarization was carried using a computer-interfaced WPG100e corrosion measuring system and Sequencer version 5 software. The open circuit potential (OCP) and passive current density were used to evaluate the corrosion characteristics of the thermomechanically treated samples of Ti alloy.

The surface area exposed to the electrolyte was 0.126 cm2. For each experiment, the specimens were prepared by sequential grinding with waterproof emery paper with up to 2,000 grit silicon carbide, followed by polishing with 0.5 µm alumina to obtain a high mirror surface finish, and then cleaning in an ultrasonic bath three times. Freshly prepared Ringer’s solution was used as the electrolyte to simulate a body fluid environment.37–40 This solution when dissolved in 1 L of distilled water had the following chemical composition: 9.00 g NaCl, 0.43 g KCl, 0.20 g NaHCO3, and 0.24 g CaCl2. The pH of the solution was maintained at 7.4. The solution was naturally aerated and kept at 37°C±1°C. The OCP was plotted as a function of time until a stable value was reached. Next, the corrosion potential (Ecorr) and passive current density (Icorr) of the alloy were determined from the potential versus current density polarization curve. The polarization curves were obtained using a scan rate of 0.166 mV per second in the range of 750–2,500 mV (Ag/AgCl). The polarization tests were repeated at least three times for each specimen. The Ecorr and Icorr were determined from the registered curves by the extrapolation method.

Results and discussion

Microstructure and X-ray diffraction analysis

Microstructure and its analysis are important given that several vital characteristics of materials, such as their mechanical and corrosion properties, are strongly influenced by microstructural features. Thermal treatment and TMP are the two main techniques used to achieve the desired microstructure. Microstructure observations of the as-cast sample (Figure 3Aa and Ba) indicated a fine needle-like (acicular) α in the β matrix with segregation of the α phase on grain boundaries. The X-ray diffraction profiles of the as-cast TNZV alloy is shown in Figure 4A, which indicates that only peaks corresponding to the α and β phase are present in the alloy.

Figure 3

Microstructural investigations by (A) optical microscopy and (B) scanning electron microscopy of the Ti-20.6Nb-13.6Zr-0.5V alloy samples: (a) as-cast, and deformed at 850°C and solution-treated for 1 hour at same temperature followed by (b) furnace-cooling, (c) air-cooling, (d) water-quenching, and (e) aging of water-quenching samples at 500°C for 5 hours.

Figure 4

X-ray diffraction profiles of (A) as-cast TNZV alloy and (B) TNZV alloy samples deformed at 850°C and solution treated for 1 hour at the same temperature followed by WQ, AC, FC, and aging of water-quenched samples at 500°C for 5 hours.

Abbreviations: TNZV, Ti-20.6Nb-13.6Zr-0.5V; AC, air-cooling; FC, furnace-cooling; WQ, water-quenching.

The microstructure of the TNZV alloy depends essentially on the plastic deformation process and heat treatment sequences used. Hence, hot working, ie, 10% hot forging plus 25% hot rolling at 850°C (above β transus temperature), is an effective method for generating dynamic recrystallization with an equiaxed structure. It has been observed that treating solution above β transus temperature dissolves the entire α phase developed during hot plastic deformation, leading to complete transformation to β phase in the TNZV alloy. It can be seen from the micrographs in Figure 3Ab, Bb, Ac, and Bc) that hot working and solution treatment at 850°C (above the β transus temperature) led to transformation of a part of β phase to α phase for both FC and AC samples.

In line with the findings of others,34,41 the microstructure of FC or AC samples consisted of Widmanstätten α-laths (in a basket-weave arrangement) that were composed of different variations of α plates within pre-existing β-grains. The volume fraction of the α phase in the FC samples was high due to the lower cooling rate. The thickness of the α plates in FC samples was higher than that of the AC samples, because the size depended mainly on the cooling rate. Thus, the microstructure of the AC samples shows the presence of fine α-β structures within pre-existing β-grains.

A nonequilibrium metastable martensite phase and a retained β phase were found in the samples which were WQ from solution treatment at 850°C (Figure 3Ad and Bd). A large number of papers have reported the presence of a martensite structure in Ti materials if the solution is treated at a high temperature (above β phase field) with a sufficiently high cooling rate.20,34,41–46 Thus, rapid cooling from the above temperature resulted in a martensite phase in the microstructure of the WQ samples. Two types of metastable martensites, α′ and α″, which are hexagonal and orthorhombic structures, respectively, are found in Ti alloys quenched from the β phase based on the concentration of the beta alloying elements.20 In a Ti-Nb binary system, the hexagonal α′ is observed when the Ti alloys contain Nb <13 wt%, whereas the orthorhombic α″ is seen at a higher Nb content.23,47,48 In the present research, the Nb content in the TNZV was 20.6 wt%, which specifies the formation of orthorhombic α″ martensite after WQ from 850°C. It is reported that the orthorhombic structure of the α″ phase is obtained because of the whole correspondence between the atoms of Ti and Nb associated with the phase change during WQ could result in formation of the orthorhombic α″ phase.49

Aging of WQ samples at 500°C for 5 hours is a diffusion-controlled process. During aging, the marten-site phase decomposes into the α+β phase structure and generates a fine distribution of small globular α along the pre-existing martensite plates (Figure 3Ae and Be). It is reported that the orthorhombic martensite α″ phase is metastable and transforms to an α+β phase structure if the martensitic transformation start temperature is more than room temperature.50

To obtain an indepth understanding, the above phase constituents in the microstructures of the thermally treated TNZV samples were identified using X-ray diffraction spectra as seen in Figure 4B, which shows the presence of peaks of α and β phases in FC, AC, and aged TNZV samples, whereas only martensite is found in WQ samples.

Mechanical properties

Microhardness

The effect of thermal treatment on the microhardness of the TNZV samples is presented in Figure 5A, which shows that the AC samples have a higher HV than the FC and WQ samples because of the formation of fine phases. In contrast, the WQ sample showed a significant decrease in hardness (HV 220±3), as reported by others.44 Consequently, it is expected that the presence of martensite with an insufficient amount of α in the microstructure would decrease the HV after WQ from 850°C. However, aging treatment of the WQ samples increased the HV significantly, and this thermal treatment provided the most hardness (HV 292±5). Many authors8,34,51,52 have mentioned that substantial enhancement in hardness can be achieved after aging treatment. This heat treatment process has the ability to precipitate a fine α phase in the matrix by decomposition of martensite and retained β phases. These fine precipitates of α in the matrix increases the HV of the TNZV alloy. Thus, the HV of the aged samples was higher when fine α-grains were crystallized in the β-matrix.

Figure 5

(A) Hardness and elastic modulus and (B) tensile results of TNZV alloy deformed at 850°C and solution treated at 850°C for 1 hour followed by WQ, AC, FC, and aging of water-quenched samples at 500°C for 5 hours.

Abbreviations: TNZV, Ti-20.6Nb-13.6Zr-0.5V; AC, air-cooling; FC, furnace-cooling; WQ, water-quenching; UTS, ultimate tensile strength; YS, off-set yield strength.

Elastic modulus

The elastic modulus is strongly influenced by the bonding force among atoms, the phase/crystal structure,11 and the chemical composition of a Ti material.20,53 Therefore, in any multiphase Ti alloy, the specific modulus of the phases and their volume fractions are key parameters for developing the modulus. It is well known that all these parameters depend mainly on the history of thermal and TMP.54 The literature reports that the elastic modulus of the phases in Ti alloys increases in the sequence Eβ < Eα″ < Eα < Eω.11,20,44,55 It is inferred that the microstructure of Ti materials has a significant effect in reducing the modulus, because a mixture of β and α″ phases lowers the modulus whereas the α phase increases it.20,56 The modulus of TNZV subjected to the different thermal treatment conditions is shown in Figure 5A, which indicates that the modulus of the thermal-treated TNZV samples varies from 59 to 82 GPa. It can be seen that the modulus decreases significantly with increasing cooling rate. After solutionizing at 850°C, the WQ samples developed a microstructure consisting of martensite and retained β phase (Figure 3Ad and Bd). Given that both these phases afford a lower elastic modulus as compared with α phase, the WQ samples had the lowest modulus value (59±1.9 GPa) when compared with the FC and AC samples. On the other hand, compared with the AC samples, the FC samples had a higher amount of α because of the slower cooling rate. Therefore, the high modulus value of the FC samples is associated with the maximum volume fraction of α phase in the microstructure. It was observed that aging treatment resulted in decomposition of the low modulus martensite and retained β phase into high modulus α. The modulus of the aged samples increased to 70±2.3 GPa.

Tensile properties

Tensile test results were obtained for the TNZV samples deformed at 850°C and solution treated at the same temperature followed by WQ, AC, and FC. The YS, UTS, and fracture plasticity (e%) of the samples is shown in Figure 5B, which demonstrate that the UTS, YS, and e% varied from 670 to 696 MPa, 516 to 570 MPa, and 13% to 21%, respectively. It can be seen that the strength of the AC samples is higher than that of the FC or WQ samples. The superior strength of the AC samples is a result of the fine microstructure produced by relatively rapid cooling. In addition, the YS of the FC and AC samples were analogous and higher than that of the WQ samples. On the other hand, the WQ samples showed remarkable ductility (21%) compared with FC or AC samples. The solid solution-hardening effect after WQ is the main factor of the reasonable strength. However, formation of soft β and α″ martensite phases in the microstructure of WQ samples played an important role in producing a lower YS and better e%. Hao et al44 pointed out that the martensite structure has lower strength and HV but better e%, which is in good agreement with the present tensile strength result of WQ samples.

The UTS, YS, and e% of the WQ samples after aging treatment were found to be 880 MPa, 769 MPa, and 9%, respectively. These values suggest that the strength (UTS and YS) of the aged samples was significantly higher than the strength of other the heat-treated samples but the ductility was substantially lower. Thus, the extensive increase in strength after aging can be related to the precipitation of fine α phase in the matrix as a result of decomposition of martensite and retained β phases. On the other hand, the lowered e% of the aged samples is related to the precipitation of fine α particles along the pre-existing martensite plates, which act as effective sites for nucleation of the cracks, and in turn, decrease the e% of the alloy.

Electrochemical properties

Open circuit potential

Figure 6 shows the variation in OCP of TNZV alloy in different conditions as a function of immersion time in naturally aerated Ringer’s solution at 37°C, until its variation with time became negligible. The steady-state OCP of TNZV alloy varies with heat treatment conditions. By comparing the results depicted in Figure 6, it can be observed that the AC samples have more positive corrosion potential values (nobler behavior) than the FC, WQ, and aged samples. In this research, of all the thermally treated samples, the FC samples showed a greatest decrease in OCP. In addition, the OCP curve for the WQ and aged samples shows fluctuations in the initial stages and then becomes stable, whereas the AC and FC samples showed a tendency for spontaneous formation of surface oxides and a continuous shift in a noble (positive) direction when they were in contact with Ringer’s solution. The time profiles for OCP are characteristic of passive film formation, especially in the stabilized region, on the alloy surfaces immersed in aerated solutions under different heat treatment conditions.57 AC samples, however, showed the greatest tendency for spontaneous formation of oxide film in Ringer’s solution. The OCP values for all heat treatment conditions show an overall increase with the time duration of 1 hour, indicating that their resistance to corrosion increases with time and reaches a relatively stable value. In other words, the OCP shifts in the positive direction, indicating passive formation of a protective oxide film on the surface of the TNZV alloy.

Figure 6

Open-circuit potential versus time curves of the TNZV alloy in Ringer’s solution at 37°C deformed at 850°C and solution treated at the same temperature for 1 hour followed by WQ, AC, FC, and aging of water-quenched samples at 500°C for 5 hours.

Abbreviations: TNZV, Ti-20.6Nb-13.6Zr-0.5V; AC, air-cooling; FC, furnace-cooling; WQ, water-quenching.

The protective film forms rapidly and acts as a barrier to metal dissolution, leading to a decrease in the corrosion rate. The thickness of the spontaneously formed protective oxide film on the surfaces of Ti and its alloys has been reported to be 1–4 nm under open circuit conditions.58 It is well known that the superior corrosion behavior of Ti and its alloys is attributable to spontaneous creation of a tightly adherent protective oxide film on their surface, even in solutions with a low oxygen content.59 Oliveira et al60 reported creation of an oxide film on Ti-13Zr-13Nb and Ti-50Zr alloys in aerated solution. It has been reported in the literature that X-ray photoelectron spectroscopy reveals that the amorphous oxide film formed comprises three types of oxides, ie, TiO, Ti2O3, and TiO2.18,61,62 It has also been found that oxides of Ti, TiO, and Ti2O3, transform to more stable TiO2 and come out on the electrode/electrolyte interface after direct contact between Ti material and the electrolyte.26 TiO2 is an n-type semiconductor,63 and the corrosion of Ti is controlled kinetically by migration of oxygen vacancies through this film.64 Hence, the corrosion behavior reached a relatively stable state as the resistance to corrosion of the electrode improved. The decrease in anodic dissolution current of the Ti alloy and shifting the OCP gradually in the positive direction are normal results of an increase in corrosion resistance.

It is important to mention that many researchers57,65–67 have found that the film formed on the surface of Ti and its alloys in various physiological solutions shows a two-layered structure comprising a dense inner layer and a porous outer layer. The excellent corrosion behavior of the Ti-Nb-Zr alloy system is essentially because of the barrier provided by the inner layer, which offers high resistance.57,65

Many authors61,68–71 have reported that the passive film in the Ti-Nb-Zr alloy system consists principally of TiO2 with trace quantities of Nb2O5 and ZrO2. The presence of Nb2O5 or ZrO2 with the main passive TiO2 layer develops the structural integrity of the oxide film and increases its resistance to dissolution.61,72 For example, the presence of Nb cations enhances the passivation properties of the surface film by decreasing the concentration of anion vacancies present on a Ti oxide film. These anion vacancies are generated by the presence of lower Ti oxidation states.40,61,73

Potentiodynamic polarization

Figure 7 shows the potentiodynamic anodic polarization curves for different heat-treated TNZV samples as measured with respect to a saturated calomel electrode in naturally aerated Ringer’s solution at 37°C. The continuity, stability, and intensity of the passive Ti oxide film are analyzed by this technique. As can be seen from Figure 7, the polarization curves obtained for the investigated TNZV alloy samples in all heat treatment conditions show a typical active-passive characterization, a rising anodic current with increasing potential, and then transforming directly into the passive area from the Tafel curves. The average Ecorr values can be estimated from these curves as −181.18, −191.81, −125.81, −209.23 mV (versus the saturated calomel electrode) for the FC, AC, WQ, and aged samples, respectively. The corrosion potentials determined from the polarization curves are significantly lower than those obtained from the open circuit potential measurements. This is expected, as the polarization tests were started at a cathodic potential relative to the corrosion potential, so the surface passive oxide film was at least partially removed owing to the highly reducing initial potentials.

Figure 7

Cyclic polarization of the TNZV alloy in a Ringer’s solution at 37°C deformed at 850°C and solution treated at same temperature for 1 hour followed by WQ, AC, FC, and aging of WQ samples at 500°C for 5 hours.

Abbreviations: TNZV, Ti-20.6Nb-13.6Zr-0.5V; AC, air-cooling; FC, furnace-cooling; WQ, water-quenching.

The mean Icorr and corrosion rates were obtained for the TNZV alloy by the Tafel extrapolation analysis method using both anodic and cathodic branches of the polarization curves (Figure 8). The corresponding corrosion data, which include the mean corrosion current densities (Icorr), corrosion potentials (Ecorr), and corrosion rates of TNZV alloy under different thermal treatment conditions, are given in Table 2.

Figure 8

Tafel plots obtained for furnace-cooled sample in Ringer’s solution at 37°C.

Table 2

Results of electrochemical tests via anodic polarization curves for the heat-treated Ti-20.6Nb-13.6Zr-0.5V alloy samples in Ringer’s solution at 37°C

Cooling conditions	Ecorr (mV)	Icorr (µA/cm2)	Corrosion rate (mils/year)
WQ	−125.81	2.2246	1.81
AC	−191.81	1.0324	0.84
FC	−181.18	4.508	3.67
Aging	−209.23	5.7645	4.51

Abbreviations: Ecorr, corrosion potential; Icorr, passive current density; WQ, water-quenched; AC, air-cooled; FC, furnace-cooled.

From the corrosion test results presented in Table 2, it can be observed that in most of the heat treatment conditions, the Ecorr varies roughly within a narrow range. It can be seen that the FC samples and AC samples show almost comparable Ecorr values. On the other hand, aging of the WQ samples results in a further decrease in the Ecorr value, which is lower than that of the FC, AC, or WQ samples. Comparing the four anodic curves, however, it can be noticed that the FC samples and aged samples exhibit similar anodic polarization behaviors, including almost comparable current densities of 4.508 and 5.7645 µA/cm2, respectively, at approximately 2,500 mV (Ag/AgCl). As mentioned in the OCP analysis, as soon as the TNZV samples are rinsed in Ringer’s solution, a protective surface oxide film is formed on the surface of the samples and they become passivated, which is reproduced in the anodic polarization curves (Figure 7).

The corrosion current density of the WQ samples lies between the values for the AC and FC samples. Moreover, the anodic current densities are lower for the AC samples than for the FC, WQ, and aged samples. This indicates that, of all the samples, the minimum Icorr is for the AC samples due to the more compact and protective nature of the surface film formed on the AC samples. This implies that the anodic current density of TNZV under all thermal treatment conditions increases with an increase in the potential, but this increase is always larger for the FC samples and aged samples. The rise in Icorr with the potential is likely to be because of the unsatisfactory increase in oxide film thickness, to recompensate the increase in potential. It is reported that this increase in current could be associated with the oxidation of TiO and Ti2O3 to TiO2.61 Thus, it appears that the surface oxide films become thick to compensate for the increase in potential where the current does not change with potential.

In the current research, it is expected that the fine (α+β) structure in the matrix of the AC samples increases the α/β interface area and then accelerates the galvanic corrosion of the alloy. Regardless of this, the AC samples exhibited a lower Icorr and a lower corrosion rate than the FC or aged samples. It has been found that the corrosion behavior of cp-Ti (single α phase) is lower than that of Ti-6Al-7Nb alloy (α+β).74 Therefore, the most likely reason for the better corrosion behavior of the AC samples is the presence of a small amount of less noble α phase in their microstructures. During the diffusionless martensitic β→α″ phase transformation in the WQ samples, the solute partitioning does not occur and, consequently, the WQ samples have improved corrosion stability (lower Icorr) as a result of the formation of a more stable passive film than the FC or aged samples. This result agrees well with the result reported in the literature66 and confirms that the β ST WQ Ti-13Nb-13Zr alloy remains passive in Ringer’s solution at positive potentials as 2.5 V (saturated calomel electrode). On the other hand, formation of marten-site occurred in the WQ samples via a shear mechanism of phase transformation, that involves an invariant plane strain shape deformation with a large shear component. This entails a high amount of strain energy because the shape change must be accommodated in the parent phase.31,75 Therefore, in spite of the absence of any α/β interface in the WQ samples, their corrosion behavior was found to be less stable than that of the AC samples (Figures 6 and 7) due to the presence of a strained and highly faulted substructure.

Aging of the WQ samples created higher amounts of fine precipitates of less noble α phase in the microstructure, as shown in Figure 3Ae and Be. Thus, in the same way, the α/β interface increased with the increase in α phase volume friction, which in turn led to a decrease in Ecorr (−209.23 mV) and an increase in Icorr (5.7645 µA/cm2) despite relieving the strain energy associated with martensite.

Conclusion

The effect of TMP on the microstructure, mechanical properties, and electrochemical behavior of metastable β Ti-20.6Zr-13.6Nb-0.5 V alloy for biomedical applications has been investigated. Based on the results of the present study, the following conclusions are drawn. A wide range of microstructures with varying distributions and morphologies of elongated/equiaxed α, β phases, or martensite is attained depending upon the TMP conditions. AC samples show greater hardness (HV) and strength (UTS, YS) as compared with FC or WQ samples. WQ samples offer reasonable strength and higher plasticity (21%) compared with other heat-treated samples. WQ samples introduce a lower Young’s modulus (59±1.9 GPa) compared with other heat-treated samples due to the martensitic transformations. Aging treatment of WQ samples causes an increase in the HV, strength, and elastic modulus, and also a decrease in the e%, owing to decomposition of martensite and retained β in α phase. Corrosion tests indicated that the TNZV alloy undergoes spontaneous passivation owing to spontaneously formed oxide film in the environment of the human body. The AC samples had a lower corrosion rate owing to their fine grain size and higher HV than the samples treated under other conditions. The AC samples, which had optimum results for strength and resistance to corrosion as well as reasonable results for e% and modulus, can be considered as a potential candidate for biomedical application after further investigations on in vitro and in vivo behavior. The TNZV alloy developed in the present study has superior mechanical properties and corrosion behavior, but further studies on the tribological and biocompatibility behavior of the alloy are needed to assess its overall potential for biomedical application.