Development and Characterization of as-cast Ti-20Ta-Nb Aiming Biomedical Applications

Abstract

The present paper aims to develop and investigate Ti system alloys with the following weight compositions: Ti-20Ta, Ti-20Ta-10Nb, Ti-20Ta-20Nb, and Ti-20Ta-30Nb. These alloys are intended to contribute to the area of ​​orthopedic implants. To achieve satisfactory results, thermodynamic simulations were performed using THERMOCALC software and molecular orbital theory to predict the crystalline phases of the metals. X-ray fluorescence characterizations, energy-dispersive X-ray spectroscopy, and density measurements were performed to verify the chemical composition of the alloys. The crystal structure was verified by X-ray diffraction, optical, and scanning electron microscopy techniques. Vickers microhardness measurements were performed to verify the influence of the crystalline phases on the metal's bulk hardness. The chemical composition results showed that the produced ingots have good quality, and the density results presented close theoretical and experimental values, indicating good chemical homogeneity. The X-ray diffraction measurements showed the following phases: the Ti-20Ta alloy is of the α” type. The addition of Nb in the alloys promoted the formation of β phases, demonstrating that niobium has β-stabilizing characteristics. Regarding the microhardness values, the Ti-20Ta-10Nb and Ti-20Ta-20Nb alloys presented the same hardness values ​​(~ 298HV); the Ti-20Ta-30Nb composition has a lower hardness value (215HV) due to the formation of the β phase, which has a lower atomic packing factor.

Keywords:
Ti Alloys; Biocompatibility; Structural Characterization; Microstructural Characterization

1. Introduction

Titanium and its alloys have made significant contributions to the medical field for many years. The use of titanium alloys began in the 1940s with the first successfully developed Ti-6Al-4V alloy¹, an alloy designed for use in the aeronautical industry, considered a very promising alloy in engineering due to its properties such as high strength, high hardness and excellent corrosion resistance², and therefore caught the attention of researchers excited about the possibility of being used as a biomaterial. After the 1960s, titanium began to be used in surgical implants; 2% of the titanium produced worldwide is used in medical applications³.

According to the literature, the Ti-6Al-4V alloy is the most commonly used in orthopedics⁴. However, with the advancement of science and technology, some negative reactions that the Ti-6Al-4V alloy presents have been identified. According to Wu et al.⁵, the particles of the elements aluminum and vanadium present in the alloy are toxic and mutagenic, causing neurological problems such as Alzheimer's and osteomalacia. Additionally, according to Wu et al.⁵, vanadium can cause bone softening, anemia, and nervous disorders, while aluminum can accumulate in the brain, liver, spleen, kidneys, and thyroid, resulting in certain types of lesions. Thus, new substitutes for the Ti-6Al-4V alloy have emerged, including stainless steel, Co-Cr-Mo alloys, magnesium alloys, and Ni-Ti shape memory alloys, among others. In this scenario, titanium-based alloys were studied more intensively, and interest in their excellent mechanical, physical, and biochemical properties increased.

For several years, researchers have been searching for metastable titanium-based alloys that incorporate β-stabilizing elements (such as Ta, Nb, and Mo) to achieve properties including biocompatibility, non-toxicity, lower elastic moduli, excellent osseointegration values, and corrosion resistance⁶.

Several elements act as β-stabilizers in titanium alloys, including tantalum and niobium⁷. Both metals tend to decrease the phase transition temperature⁸ and are found together in nature⁹. They are non-toxic metals that exhibit high biocompatibility and adequate mechanical behavior, compared to other metals⁵,¹⁰,¹¹.

Tantalum has a high modulus of elasticity and high mechanical strength due to its extremely strong chemical bonds¹² and a melting point of 3273 K¹³. According to Zhou et al.¹³, in 1940, Burke used pure tantalum for bone screws, plates, and surgical implants. Black¹⁴ reported that tantalum was used as vascular clips as early as 1942.

Niobium is a beta-phase-stabilizing element that can improve the mechanical properties and biocompatibility of the alloy, reducing the modulus of elasticity¹⁵. Helth et al.¹⁶ worked on β-type Ti-40Nb alloys through various thermomechanical processes, obtaining a low modulus of elasticity of 55 GPa, which is promising for implant applications. Omran et al.¹⁷ worked on a titanium-based orthopedic alloy varying the niobium from 33.36 to 39% by weight, where the biocompatibility and corrosion resistance values were verified. The tests showed that biocompatibility increases with a higher percentage of niobium, and regarding corrosion, a very satisfactory performance was verified.

Quadros et al.¹⁸ studied the Ti-25Ta-Nb system alloys, varying the Nb of 10, 20, 30, and 40% by weight, and analyzed the effect of substitutional Nb on the structure, microstructure, hardness, and biological properties of the biomedical alloys. According to the authors, the increase in the Nb content promotes the formation of the β phase and the suppression of the orthorhombic α phase”. Due to the changes in the crystal structures, increasing the amount of Nb until the formation of β predominant Ti alloys tends to reduce the hardness value since the β phase has weaker interatomic bonds compared to the α phase, which facilitates the movement of atomic dislocations, making the alloys more susceptible to plastic deformation. Among the developed alloys, Ti-25Ta-30Nb stands out as a metastable β-Ti alloy with a low hardness value (175 HV) and promising potential for promoting cell proliferation.

Due to the mechanical incompatibility of metallic biomaterials (Co-Cr, stainless steel, CP-Ti, and Ti-6Al-4V) with human bone, the scientific community has focused on developing and producing new β-type Ti alloys with mechanical properties similar to bone, such as the modulus of elasticity, to avoid failure problems such as the well-known phenomenon of stress shielding, which causes loss of bone density. The work aims to develop and characterize a set of unpublished alloys of the Ti-20Ta-xNb system, using the following proportions of tantalum (20% by weight) and varying the niobium values in 0, 10, 20, and 30% by weight. The application of these alloys aims to utilize them as biomaterials. Tantalum and niobium were selected for this work due to their excellent properties for use with orthopedic implants and their abundance in the northern region of Brazil.

2. Materials and Methods

Molecular orbital theory was used to predict the phases of the alloys in this study. This method uses the Hartree-Fock-Slater approximation, using Slater's Xα potential. The bond order parameter, Bo, indicates the strength of the bond between titanium and its alloying elements. The energy level parameter, Md, is related to the d-layer of the alloyed transition metal and is connected to the electronegativity and metallic radius of the elements added to titanium. These two parameters are considered to predict the material's structure and its modulus of elasticity. To perform these calculations, Equations 1 and 2 were used, using the atomic percentage of each element in each alloy and the Bo and Md values ​​of the elements used, titanium, tantalum, and niobium, which are presented in Table 1:

$\overline{B_o}={{\displaystyle \sum }}_i^{}{x}_i{(B_o)}_i$ (1)

$\overline{M_d}={{\displaystyle \sum }}_i^{}{x}_i{(M_d)}_i$ (2)

where: x is the atomic percentage of each alloying element.

Table 1
Bo and Md values ​​for alloying elements¹⁹.

Element	Bo	Md (eV)
Ti	2.790	2.447
Ta	3.144	2.531
Nb	3.099	2.424

To produce 60 g of the Ti-20Ta-Nb system alloys (Nb = 0, 10, 20% by weight), precursor metals with a purity of approximately 99.7% were used. The ingots were melted five times in an electric arc furnace using a water-cooled copper crucible located in a chamber filled with inert argon gas to prevent gaseous contamination (O, N, and H)²⁰. After melting the materials, semi-quantitative chemical composition measurements were performed via XRF and EDS. The alloy powder was used for X-ray Fluorescence Spectrometry (XRF); for this, the sample was filed, and the powder removed was subjected to a magnetic field to remove the file fragments. The equipment used was a Panalytical EPSILON 3 XL, with the following parameters: a maximum voltage of 50 kV, a maximum current of 50 mA, and Helium gas (at a pressure of 10 atm/10 Kgf/cm²) was carried out at the Physical-Chemical Testing Laboratory of the Faculty of Technology, UFAM, Manaus, AM. Small pieces were used as test specimens for energy dispersive spectroscopy (EDS) measurements for each composition. To enable EDS analyses, pieces of the samples were subjected to the standard metallographic process for metals, sanding with running water (300 - 1500 mesh) and polishing in 1 μm alumina for approximately 20 minutes. The equipment used was an Oxford Instruments Inc. detector coupled to the scanning electron microscopy equipment. (SEM; JSM-6010LA, JEOL Ltd., Peabody, MA, USA).

To verify the ingots' chemical stoichiometry, density measurements were performed following Archimedes' principle. A precision analytical balance from EXPLORER OHAUS was used for the measurements.

X-ray diffraction and scanning electron microscopy techniques verified the samples' crystal structures. X-ray diffraction (XRD) measurements were performed using the alloy powder. The metal powders of the alloys were obtained by sanding with iron files. Possible contamination from iron filings was removed by magnetic separation. X-ray diffraction measurements of samples Ti-20Ta-10Nb, Ti-20Ta-20Nb and Ti-20Ta-30Nb were carried out using a Rigaku, model D/Max 2100/PC, equipment with Cu-Kα radiation (λ = 1.544 Å)., current of 20 mA, the potential of 40 kV, the residence time of 1, 6 s 0.02 step size from 20° to 100° in fixed-time mode. The sample Ti-20Ta was evaluated using X-ray diffraction (XRD; Rigaku diffractometer, MiniFlex 600 model) at 40 kV and 15 mA, with Ni-filtered CuKα radiation (λ = 0.1544 nm), in fixed-time mode, a step size of 0.02°, and a collecting time of 1.6 s.

The X-ray diffraction patterns were compared with the crystallographic records of the orthorhombic (code: 1523304) and body-centered cubic (code: 9008554) phases. The records were obtained from the Crystallography Open Database.

To obtain micrographs, conventional metallographic preparations, including sanding, polishing, and chemical etching with Kroll acid solution (80% H₂O, 15% HNO₃, and 5% HF), were performed¹². The SEM used is a Jeol JSM IT500-HR located at the Multiuser Center for Analysis of Biomedical Phenomena of the State University of Amazonas—UEA Manaus/AM.

Vickers microhardness measurements were performed using a load of 0.20 kg for 10 s in each impression. For each sample, five randomly prepared impressions were performed in accordance with ASTM E92²¹ and ASTM E384²² standards. The hardness test was performed using a DIGIMESS 400310 microhardness tester.

3. Results and Discussion

The Bo and Md values ​​of the Ti–20Ta–Nb alloys produced in this work are presented in Table 2. Figure 1 shows the Bo and Md graph with the locations of the alloys studied in this work. It can be observed that the alloy without Nb (Ti-20Ta) is located in the α+β region, experimentally corroborating the findings of Zhou and Niinomi¹³. The Ti-20Ta-10Nb and Ti-20Ta-20Nb alloys are in the β/β+⍵+α” region, indicating that Nb in low amounts (10 and 20% by weight) promotes the formation of the ⍵+α”+β phases and inhibits the formation of the α phase. Finally, the Ti-20Ta-30Nb alloy is classified as a β alloy. The black arrow indicates the trend of location of titanium alloys with increasing Nb content, demonstrating a typical β-stabilizing character.

Table 2
Bo-Md parameters for Ti-20Ta-xNb alloys.

Alloys	Bo	Md (eV)
Ti-20Ta	2.812	2.452
Ti-20Ta-10Nb	2.833	2.451
Ti-20Ta-20Nb	2.857	2.450
Ti-20Ta-30Nb	2.884	2.448

Figure 1
Bo-Md diagram, with Ti-cp and the alloys of the Ti-20Ta-xNb system. Adapted from Cardoso et al.²³ and Tan et al.²⁴.

Figure 2 shows the equilibrium phase transformation temperatures (α → β) and the melting temperature (liquidus) calculated using THERMOCALC software in the Ti-20Ta-Nb system alloys. The results show that the niobium element makes the production of the alloys difficult, as it increases the melting temperature of the materials. This occurs because Nb, which has a higher melting point than Ti, acts as a substitutional element, thereby increasing the melting temperature. Another point to be highlighted is the beta-stabilizing character of Nb; the addition of Nb decreases the beta-transus temperature (768 °C → 471 °C), facilitating the formation of the β-Ti phase at low temperatures.

Figure 2
Influence of niobium content on phase transformation (α → β) and melting temperatures calculated via THERMOCALC.

Table 3 shows the weight percentage of the elements detected in the alloys via X-ray fluorescence spectrometry (XRF). It can be observed that the percentage deviation of the element Nb detected in the Ti-20Ta-10Nb, Ti-20Ta-20Nb, and Ti-20Ta-30Nb alloys is approximately 17%, 12%, and 13%, respectively. A large discrepancy is observed for the Ti-20Ta alloy, with an error of approximately 40% for the amount of Ta. However, it is worth noting that Ta is an element with a low beta-stabilizing character; high concentrations of this element are required to modify the crystalline structures (70% by weight of Ta is needed to retain the beta phase at low temperatures completely). Thus, the Ti-20Ta and Ti-28Ta alloys have the same phase characteristics and mechanical properties. Ti-20Ta-Nb alloys form a new class of Ti alloys, where an ASTM standard has not yet been produced to ensure what deviation is permitted in the chemical compositions of the Ta and Nb elements. In the Ti-Mo system, the ASTM F2066-13²⁵ standard establishes that the permitted difference between the experimental and nominal values is up to 1% Mo in the Ti-15Mo alloy, where Mo is a strongly beta-stabilizing element. For strongly beta-stabilizing elements, this error tends to be smaller for classifying the alloys, since a small fluctuation in experimental values ​​of chemical composition tends to significantly alter the crystalline phases and mechanical properties, which is not the case for the Ta and Nb elements.

Table 3
XRF’s analysis of the Ti-20Ta-xNb alloys x = 0, 10, 20 and 30 wt%. Conditions after melting.

Chemical element	Ti-20Ta (wt%)	Ti-20Ta-10Nb (wt%)	Ti-20Ta-20Nb (wt%)	Ti-20Ta-30Nb (wt%)
Se	0.162	0.135	0.126	0.117
Fe	0.261	0.161	0.508	0.198
Ta	28.01	23.55	23.34	23.77
Nb	-	11.69	22.33	33.79
Ti	Balance	Balance	Balance	Balance

Table 4 presents the percentage of alloying elements detected using energy-dispersive spectroscopy (EDS). For the element Nb, in the Ti-20Ta-10Nb alloy, the relative deviation was 3%, while for the Ti-20Ta-20Nb and Ti-20Ta-30Nb alloys, the deviations were 1 and 4%, respectively. The EDS result showed a percentage deviation of approximately 47% of Ta in the Ti-20Ta-10Nb alloy.

Table 4
Results of EDS of the Ti-20Ta-xNb alloys x = 0, 10, 20, and 30 wt%. Conditions after melting.

Chemical element	Ti-20Ta (wt%)	Ti-20Ta-10Nb (wt%)	Ti-20Ta-20Nb (wt%)	Ti-20Ta-30Nb (wt%)
Ta	15.91	29.44	25.89	24.66
Nb	-	10.28	19.86	28.86
Ti	Balance	Balance	Balance	Balance

The results obtained with each technique are discrepant, as these characterizations are semi-quantitative and subject to large fluctuations in chemical composition.

Table 5 shows the density values ​​of the Ti-20Ta, Ti-20Ta-10Nb, Ti-20Ta-20Nb, and Ti-20Ta-30Nb alloys. It can be observed that the theoretical and experimental values ​​are very close, with deviations of less than 1%, demonstrating that the difference between the theoretical value and the experimental value is within the experimental error. Another point to be highlighted is the gradual increase in density values with the increase in Nb content in the alloys, due to the density of substitutional niobium (8.57 g/cm³)²⁶ being greater than that of titanium (4.57 g/cm³).

Table 5
Results of density of the Ti-20Ta-xNb alloys x = 0, 10, 20, and 30 wt. Conditions sample after fusion.

Alloys	Ti-20Ta	Ti-20Ta-10Nb	Ti-20Ta-20Nb	Ti-20Ta-30Nb
Theoretical density (g/cm3 )	5.39	5.59	5.94	6.33
Experimental density (g/cm3)	5.40 ± 0.01	5.60 ± 0.01	5.96 ± 0.01	6.37 ± 0.01

Figure 3 shows the X-ray diffractograms of the Ti-20Ta, Ti-20Ta-10Nb, Ti-20Ta-20Nb, and Ti-20Ta-30Nb alloys. Figure 3a reveals an orthorhombic martensite structure (α”) in the Ti-20Ta alloy. Figure 3b shows that the Ti-20Ta-10Nb alloy has a predominance of α”. The presence of peaks corresponding to the β phase at approximately 40 ° and 70° can also be observed, indicating a body-centered cubic structure for this phase. Figure 3c also shows that the Ti-20Ta-20Nb alloy is of the α” + β type. Finally, Figure 3d shows the diffractogram of the Ti-20Ta-30Nb alloy with characteristic peaks α” and β, but only one peak, α”, was detected at 39°.

Figure 3
Results of XRD patterns of the Ti-20Ta-xNb alloys x = 0, 10, 20, and 30 wt%. Conditions after melting.

As the amount of niobium increased, the β phase prevailed, especially in the Ti-20Ta-30Nb alloy, demonstrating that niobium acts as a β-stabilizing element. According to Myslyvchenko et al.²⁷, the addition of β-stabilizers increases the precipitation and nucleation of the β phase. The martensitic phase (α”) generally has moderate mechanical properties and obtains desirable properties through the decomposition of martensite into α and β phases²⁸. However, beta-type alloys attract the attention of the scientific community due to their low elastic modulus value⁶.

Using Equation 3²⁹, each alloy's percentage of the α" and β” phases was calculated, and the results are presented in Figure 4. The equation considers the sum of the intensities of the peaks of each phase and the sum of the intensities of all peaks. A gradual increase in the amount of the β phase to the detriment of the α" phase was observed due to the increase in the concentration of niobium, a β-stabilizing element.

Figure 4
Phases quantification of the alloys as a function of niobium content.

$\%phase=\frac{\sum phase}{\sum allphases}$ (3)

Using the diffractogram data, the lattice parameters (a) of the β phase of the alloys were calculated. For this purpose, Equation 4 was used, adapted from Bragg's Law for a cubic crystal structure (a = b = c), where n is the order of reflection (integer), λ = 1.544 Å is the X-ray wavelength, (hkl) are the Miller indices, and θ is the angle. For each diffractogram, the lattice parameter for each β-phase peak was calculated, and the results are presented in Figure 5 as the average of these values.

Figure 5
Lattice parameter of the β phase as a function of the niobium content.

Since the atomic radius of niobium (1.56 Å) is larger than the atomic radius of titanium (1.48 Å), the lattice parameter of the β phase increases when titanium is replaced by niobium in the alloy. This result suggests that the alloy with a higher Nb content may have a lower hardness value due to a decrease in the atomic packing factor.

$a=\frac{\lambda \sqrt{h^2+k^2+l^2}}{2sin\theta }$ (4)

Figure 6 shows the scanning electron micrographs (SEM) of Ti-20Ta, Ti-20Ta-10Nb, Ti-20Ta-20Nb, and Ti-20Ta-30Nb alloys after melting. The cooling of the alloys took place inside the furnace. The formation of phases in titanium-based alloys depends on various factors, including heating rate, cooling rate, alloy composition, and type of thermomechanical treatment³⁰,³¹.

Figure 6
Results of SEM micrographs (a), (b), (c), and (d), of the Ti-20Ta, Ti-20Ta-10Nb, Ti-20Ta-20Nb, and Ti-20Ta-30Nb, respectively.

Figure 6a shows a Ti-20Ta alloy with the formation of primary plates without defined boundaries, showing coarse lamellar structure in different directions characteristic of the orthorhombic phase (α”). Zhou et al.³² presented similar results with the Ti-20Ta alloy. Figure 6b shows the micrograph of the Ti-20Ta-10Nb alloy with three-grain boundaries characteristic of the β phase and α” precipitates. Figures 6c and 6d show that in the Ti-20Ta-20Nb and Ti-20Ta-30Nb alloys, there is only a smooth matrix delimited by grain boundaries. This morphology is characteristic of β-type titanium alloys.

Figure 7 shows the variation of Vickers microhardness with the niobium concentration. The alloys have higher hardness values than pure titanium (167 HV)¹², indicating an increase in mechanical resistance due to solid solution hardening. The hardness values ​​demonstrate that these alloys are sensitive to the addition of niobium, and the microhardness also depends on the microstructures present in each sample²⁶.

Figure 7
Results of microhardness values ​​of the alloys Ti-20Ta, Ti-20Ta-10Nb, Ti-20Ta-20Nb and Ti-20Ta-30Nb. Conditions sample after melting.

The Ti-20Ta alloy presented a hardness value of 312 ± 63 HV. Zhou et al.³³ demonstrated that Ti-Ta alloys with Ta contents ranging from 20 to 25% by weight are at the chemical composition limit for the α’ → α” crystalline phase transition to occur and can form alloys with a low modulus of elasticity. The Ti-20Ta-10Nb and Ti-20Ta-20Nb alloys have hardness values ​​of 299 ± 21 and 298 ± 15 HV, respectively. It is observed that among the Ti-20Ta-Nb alloys (Nb = 0, 10, and 20% by weight), there are no changes in the hardness values; only the alloys have higher values ​​compared to commercially pure titanium. These higher values can be justified due to the martensitic microstructures present in these compositions, along with the beta phase, which hinders the movement of atomic dislocations and contributes to solid solution hardening. The Ti-20Ta-30Nb alloy exhibited a hardness value of 215 ± 13 HV, corroborating with previous studies that have shown a decrease in hardness with increasing niobium content, attributed to the formation of the beta phase, which has the lowest atomic packing factor³⁴.

Quadros et al.¹⁸ also studied the Ti-Ta-Nb system alloys, producing alloys with 25% by weight of Ta, varying the Nb concentration between 0, 10, 20, and 30% by weight. According to the authors, when Nb is added to the alloys, the β phase is formed due to its beta-stabilizing action. As the β phase stabilizes, there is a reduction in the hardness value since single-phase beta alloys tend to have a lower hardness value compared to two-phase α”+β alloys. The results highlighted by these authors corroborate the data obtained in this work, indicating that beta alloys tend to have lower hardness values. In this study, it was found that the Ti-20Ta-30Nb alloy has a lower hardness value of 215 HV. Quadros et al.¹⁸ highlighted in their study that the Ti-25Ta-30Nb alloy has approximately 180 HV of hardness. It is noted that adding only 5% by weight of Ta further decreased the hardness value, as Ta is also a beta-stabilizing element, which further decreases the hardness value of the alloys.

4. Conclusions

The results obtained allowed us to reach the following conclusions:

• - As for the molecular orbital theory, it shows that the alloys pass through the regions α+β (Ti-20Ta), β/β+⍵+α” (Ti-20Ta-10Nb, Ti-20Ta-20Nb) up to the β region (Ti-20Ta-30Nb).

• - For the chemical analyses of XRF and EDS, the percentage of the starting materials was respected within an acceptable margin of error. XRF showed that quantifications (0, 10, 20, and 30% by weight of Nb and 20% of Ta) of the starting materials were respected within an acceptable margin of error.

• - In the density measurements of the studied compositions, it was observed that increasing the niobium content significantly increased the density of the alloys

• - The density values ​​obtained experimentally are very close to the theoretical values, indicating the good quality of the starting materials.

• - For the microstructural analyses, the alloys presented peaks of the α” and β phases. The Ti-20Ta alloy presented a 100% α” phase, while the Ti-20Ta-30Nb alloy presented a 96% β phase.

• - Scanning electron microscopy (SEM) showed images with characteristics of the α” phase (orthorhombic), that is, lamellar structures in different directions, and also of the β phase, which showed images with uniform surfaces.

• - For the hardness values, it is noted that when niobium is added, the hardness values ​​decrease. The Ti-20Ta alloy showed a higher hardness value, while the Ti-20Ta-30Nb alloy showed a lower hardness value.

5. Acknowledgments

The authors would like to thank Brazilian agencies CNPq Project Grants #314.810/2021-8 and #421.677/2023-6), and FINEP Project Grant #0122/21, for their financial support.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Edited by

• Associate Editor:
  Rodrigo Orefice.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Publication Dates

• Publication in this collection
  28 July 2025
• Date of issue
  2025

History

• Received
  24 Jan 2025
• Reviewed
  15 Apr 2025
• Accepted
  16 May 2025