Effects of alloying elements and annealing treatment on the microstructure and mechanical properties of Nb-Ta-Ti alloys fabricated by partial diffusion for biomedical applications

Journal Pre-proof Effects of alloying elements and annealing treatment on the microstructure and mechanical properties of Nb-Ta-Ti alloys fabricated by partial diffusion for biomedical applications

Jue Liu, Qiumin Yang, Jian Yin, Hailin Yang PII:

S0928-4931(19)32959-5

DOI:

https://doi.org/10.1016/j.msec.2019.110542

Reference:

MSC 110542

To appear in:

Materials Science & Engineering C

Received date:

12 August 2019

Revised date:

4 December 2019

Accepted date:

11 December 2019

Please cite this article as: J. Liu, Q. Yang, J. Yin, et al., Effects of alloying elements and annealing treatment on the microstructure and mechanical properties of Nb-Ta-Ti alloys fabricated by partial diffusion for biomedical applications, Materials Science & Engineering C (2018), https://doi.org/10.1016/j.msec.2019.110542

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© 2018 Published by Elsevier.

Journal Pre-proof Effects of alloying elements and annealing treatment on the microstructure and mechanical properties of Nb-Ta-Ti alloys fabricated by partial diffusion for biomedical applications Jue Liua, Qiumin Yangb, Jian Yina, *, Hailin Yangc,**

a. Hunan Province Key Laboratory of Engineering Rheology, Central South University of Forestry and Technology, Changsha, 410004, PR China b. Department of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou, 341000, PR China

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c. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, PR China

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* Corresponding author: Jian Yin, E-mail address: [email protected] ** Corresponding author: Hailin Yang, E-mail address: [email protected]

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Abstract

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Powder metallurgical (PM) Nb-25Ta-xTi alloys (x=5, 15, 25, 35 at. %) were

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fabricated by the elemental powder sintering technology. Effects of alloying elements and annealing treatment on the microstructural evolution and mechanical properties were investigated by conducting various tests, including X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalyses (EPMA), electron back scattered diffraction detector (EBSD), transmission electron microscopy (TEM) and tensile tests. The results indicated that the alloys showed a unique Nb-rich and Ta-rich dual structure due to the insufficient diffusion between powders. With the increase of Ti content, the β phase was always retained and the alloys exhibited a relatively high density in the range of 82.4% to 90.5%. Furthermore, owing to a higher diffusion coefficient of Ti and the strengthening effect of solid solution, the volume shrinkage and tensile strength both increased along with the increase of Ti content. After the annealing treatment was introduced, the microstructure became more homogeneous and fine equiaxed grains appeared, which induced a decrease in modulus and better ductility. The Nb-25Ta-25Ti alloys exhibited a good in vitro biocompatibility due to the chemical components and the introduce of surface pores. The PM Nb-Ta-Ti alloys were promising for biomedical applications in tissue engineering after evaluated both mechanical properties and in vitro biocompatibility. Key words: Powder metallurgy; Microstructure; Mechanical properties; Elemental powder sintering; Solid-solution strengthening.

Journal Pre-proof 1. Introduction The use of Titanium (Ti) and its alloys has been becoming an extensive concern in aerospace, chemical and biomedical industries due to the various benefits, such as superior mechanical properties and corrosion resistances [1, 2]. With the rapid development of industry, commercially pure Ti cannot satisfy the increasing market demands, so that its large-scale applications are restricted. Ti-6Al-4V is one type of alloy that has been successfully used as one biomedical material. However, the high

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concentration of the carcinogenic and allergenic V and Al is a critical concern, which

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may pose a serious threat to people’s health [3]. Nowadays, β-stabilizing alloying elements with low toxicity have gained great attention. Tantalum (Ta), Niobium (Nb),

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Zirconium (Zr) and their alloys are promising alternatives and worthy of further

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investigation as metallic biomaterials [4, 5].

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Nb is a promising element for biomedical applications due to its several advantages in mechanical integrity, excellent corrosion resistance and ionic cytotoxicity. In

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particular, Nb is a highly passivating metal and the self-passivating oxide film can be formed on the surface, which can hinder the dissolution and release of metal ions. It is

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also quite stable over a wide pH range and in favor of the biocompatibility [6]. However, the sintering temperature of Nb is relatively high due to its high melting point (2477 ℃), thus impeding its widespread applications. Some invesitigations have been performed to unravel the possible applications of Nb alloys for biomedical implants and adequately study their properties. Wang et al. [7] fabricated porous Ta-xNb (x=5, 15, 25 wt.%) alloys by combining the advantages of both sponge impregnation technique and vacume sintering. The results showed that the sintering neck could grow and the particles surface became smoother with the increase of Nb content. Hussein et al. [8] fabricated a nano/sub-micro grain structured Nb-40 at.% Zr alloy by utilizing the mechanical alloy (MA) and spark-plasma sintering (SPS) technique. The results revealed that the hardness of the Nb-Zr alloy may depend on the sintering temperature and holding time, while, the relative density was not

Journal Pre-proof sensitive to these SPS parameters. Ti16Nb (wt.%) alloys containing porosity between 4.05-% and 60.79-% were produced by ErenYılmaz et al. [9] using different amounts of space holder materials. With the assistance of space holder addtions, the alloys could imitate the properties of the cortical bone for use in the production of load-bearing implants. Currently, most of the investigations are focusing on the effects of a trace of Nb addition on β-type alloys [1-3], and only little attention was given to the high

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concentration of Nb addition. In the present work, for the first time, the new

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developing Nb-25Ta-xTi (x=5, 15, 25 and 35 at.%) alloys were fabricated by adopting the elemental powder sintering technology. Firstly, Nb and Ta are strong β-stabilizers,

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which lead to a lower elastic modulus for the alloys. It is substantially of great

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importance to avoid “stress shielding effect”, which may cause bone resorption and failure of implantation, in clinical applications. On the other hand, the addition of Ti

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can contribute to lowering the sintering temperature of Nb-Ta system. Moreover, the effects of alloying elements and annealing treatment on the microstructural evolution

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and mechanical behaviors were systematically investigated, thus offering a better

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understanding of Nb-Ta-Ti alloys in biomedical applications. 2. Materials and methods

2.1. Preparation of Nb-25Ta-xTi alloys Elemental metal powders of Nb (Purity ≥ 99.5%，average particle size 40.52 μm), Ta (Purity ≥ 99.6%，average particle size 8.45 μm) and Ti (Purity ≥ 99.6%，average particle size 18.34 μm) were used as starting materials to prepare Nb-25Ta-xTi alloys with nominal compositions (x=5, 15, 25, 35 at. %). In order to produce homogeneous powder mixtures, the elementally blended powders were mixed in a three-dimensional mixer (XH-I, China) for 4 hrs by using 7 mm steel. The ball-to-powder ratio was maintained at 4:1 and rotation speed was 200 rpm. Then, the blended powders were compacted by a hydraulic press (YH41-25C, China) under 300 MPa pressure to

Journal Pre-proof obtain green compacts. Subsequently, the green compacts were sintered at 1700 ℃ with high vacuum < 10−3 Pa for 2 hrs in a vacuum furnace (JTZKG-80-2300, China). The as-sintered Nb-25Ta-xTi alloys were subjected to annealing treatment at 1000 ℃ for 3 hrs followed by furnace cooling. 2.2. Microstructure and mechanical characterization The relative densities of alloys were determined by the Archimedes' principle

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described in our previous publication [10]. The phase structures of Nb-25Ta-xTi alloys were identified by using an X-ray diffractometer (XRD, Rigaku D/MAX-2250,

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Japan) with Cu Kα radiation (λ=1.5418 Å); the 2θ was between 10° to 85° in steps of

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0.02°. The samples for metallographic observation were initially ground with sandpapers from 240 to 2000 grit, and then etched in a solution (10 vol.% HF+40

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vol.% HNO3+50 vol.% H2O) for about 30 s. SEM analysis was carried out on a

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NOVATM Nano 230 microscope. Furthermore, crystallographic observations were performed on JEOL JSM 7600F microscope equipped with Nordly’s EBSD detector.

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Results were analyzed by HKL Channel 5 software equipment. TEM specimens were first cut into 0.3 mm disk by electrical discharge machining (EDM) and then ground

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to be a thickness of about 50 μm, followed by ion-beam thinning. An electron probe microanalyses (EPMA, JXA-8530F, Japan) was used to obtain precise compositional distributions.

The tensile tests of the alloys were conducted using an Instron 3369 machine at a cross-head speed of 1mm·s-1 at room temperature. During the test, ultimate tensile strength and tensile strain curves were recorded. Moreover, the slope of the initial linear portion of the stress-strain curve was used to define the elastic modulus. Fracture morphology was obtained by SEM after tensile testing. Vichers hardness of the specimens was measured at a load of 1000 g for 10 s on HVS-5 hardness testing machine. To ensure the reliability, tests were conducted 5 times for each sample.

Journal Pre-proof 2.3. In vitro experiments The in vitro biocompatibility of as-sintered/annealed Nb-25Ta-25Ti alloys were examined by Human osteoblast-like MG-63 cells. MG-63 cells were cultured in medium containing 10% (v/v) fetal bovine serum (FBS, Thero), 1% antibiotics and 0.85 mM ascorbic acid-2 phosphate. Before the test, the specimens were machined into a size of Φ 10 mm × 2 mm and polished with 1500＃ sand paper. Then, samples were sterilized with autoclave at 120 ℃ for 30 min. Subsequently, MG-63 cells were

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seeded onto the surface of the alloys with an initial cell density of 1 × 105 cell per

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wall. The samples were placed in 24-well plates and transformed to incubator (in a humidified 5% CO2 at 37 ℃). After 7 days’ culture, the media was removed. The

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samples were washed by phoaphate buffered saline (PBS) for three times, and then

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pre-fixed in 3% glutaraldehyde for 30 min. After gradient ethanol dehydration and subsequent gradient hexamethl-disilazane dehydration were carrried out, specimens

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were Au-coated and then observed by SEM. The cellular proliferation was assessed by Alamar Blue Assay for different time (1, 3, 5 and 7 day) as discribed in our

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previous publication [10]. At least five specimens were tested for each data and

3. Results

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statistical procedures were analyzed by SPSS 16.0.

3.1. Microstructural evolution and mechanical properties of as-sintered Nb-25Ta-xTi alloys

As shown in Fig. 1(a) and (b), two regions could be observed in Nb-25Ta-xTi alloys with Ti content lower than 15 at.%. When the content of Ti increased to 25 at.%, the grain boundary was clear and the two regions were invisible, as shown in Fig. 1(c). Grain boundaries could be seen and the grain size was in the range of 10-30 μm. To observe the detailed microsturcture, Fig.1 (c) presented some refined phase precitating on the grain. Furthermore, as the Ti content increased to 35 at.%, the gain boundaries became more obvious and the density of precitated phase got higher (Fig. 1(d)).

Journal Pre-proof Furthermore, as shown in Fig. 2, EDS was performed on some specific areas in order to figure out the effect of Ti content on phase distribution and microstructure. As indicated in point “a” in Nb-25Ta-5Ti alloy (Fig.1(a)), segregation could be seen in Fig. 2(a) and a high content of Ta resulted in a bright area; while as indicated in point “b” in Nb-25Ta-5Ti alloy, a high content of Nb resulted in a slightly dark area. On the other hand, the EDS result of point “c” in Fig. 1(c) revealed that the compositions were close to the design compositions for Nb-25Ta-25Ti alloy. In order to further verify the phase composition, a map scanning was performed for element distribution

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via EPMA (Fig. 3). The bright region was mainly composed by Ta and the dark region

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was Nb-rich. The XRD spectra of Nb-25Ta-xTi alloys are shown in Fig. 4. It could be observed that only typical peaks of β phase were evident in all alloys, which

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associated with the (110), (200), (211) and (220) diffraction planes. Fig. 5 (a) shows the relative density/volume shrinkage of as-sintered Nb-Ta-Ti alloys.

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As Ti content (low melt point consitute) increased from 5 at.% to 35 at.%, the relative density increased from 82.4% to 90.5%. Accordingly, the volume shrinkage ranged

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from 7.86% to 16.36%. On the other hand, the hardness values, as indicated in Fig. 5(b), were 228.4 ± 4.7 HV, 235.8 ± 6.5 HV, 247.0 ± 6.5 HV and 280.5 ± 3.2 HV,

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respectively, which also showed an increasing trend with the increase of Ti content. Fig.5 (c) presents the ultimate tensile strength (UTS) and elastic moduli of Nb-25Ta-xTi alloys at the ambient condition. The UTS initially showed a slight increase. However, when Ti content was 35 at.%, the UTS exhibited a rapid increase. Moreover, the elastic moduli showed a rapid decrease with increasing Ti content, which decreased from 94.3 ± 1.1 GPa to 84.5 ± 2.5 GPa. Fig. 6 (a) shows the tensile stress-strain curves of as-sintered Nb-25Ta-xTi alloys (x=5 and 35 at.%) at room temperature. As shown in Fig. 6(a), as-sintered samples were brittle during the tensile test, thus brittle failure occurred at the maximum load, where the rupture strength, tensile strength and yield strength were regarded as the same. The fracture surfaces of the as-sintered Nb-Ta-Ti alloys in Fig. 6(b)-(e) were typically brittle, which were rather flat and mainly consisted of large-area cleavage planes and even river patterns.

Journal Pre-proof The fracture feature could be explained by the fact that under the specific tensile strength, stress concentration may occur at Nb-rich/Ta-rich regions, leading to the brittle featuring of the as-sintered Nb-Ta-Ti alloys. 3.2. Microstructural evolution and mechanical properties of annealed Nb-25Ta-xTi alloys The microstructure of Nb-25Ta-xTi alloys after annealing treatment is shown in Fig. 7.

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It can be found in Fig. 7(a) and (b) that the phase distribution was more homogeneous. The bright/dark regions could almost not be observed due to a sufficient diffusion

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during annealing treatment. Grain boundaries could be easily distinguished. Pores

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with a size of 10 μm were found, which distributed along the grain boundaries. A small amount of α phase was identified in large equiaxed β phase of about 58.4 μm in

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diameter for Nb-25Ta-5Ti alloy. Similarly, there was a significant change in Fig.

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7(c)-(d), the bcc β phase gradually transferred to α + β type when Ti content increased from 15 at.% to 35 at.%. As reported in literatures, α phase was described as spiculate

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shape with short clusters (Fig. 7 (e)). As shown in Fig. 8, EDS elemental mapping was performed on an area of annealed Nb-25Ta-5Ti alloy. The bright/dark regions

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disappeared and the distribution of Nb/Ta/Ti elements was further proven to be more homogeneous. Moreover, XRD spectra in Fig. 9 for Nb-25Ta-35Ti alloy could identify the diffraction peaks of orthorhombic α’’ phase, which was distinguished by the peak around 2θ = 38° and low density peak at 2θ = 41° [11]. Differently, the diffraction density of α’’ phase was much weaker than that of β phase, or even can be neglected in other alloys. Furthermore, Fig. 10 shows the β phase and the corresponding selected area electron diffraction pattern (SADP) of Nb-25Ta-35Ti alloy after annealing treatment [12, 13]. A ladder-like phase could be observed and identified as α’’-martensite phase, which further confirmed the result in Fig. 9. The relative density in Fig. 11(a) showed a slight increase after annealing treatment with different Ti contents. The highest relative density, which was approximately 94% , was obtained when Ti content was 35 at.%. In comparasion, Vickers hardness

Journal Pre-proof values of Nb-25Ta-xTi alloys after annealing treatment were 220.9 ± 3.5 HV, 228.7 ± 4.6 HV, 242.5 ± 5.4 HV and 254.6 ± 7.3 HV, respectively. Although the Vickers hardness values displayed a slight decrease after annealing treatment, these values were higher than that of CP-Ti and 316L SS [14]. Importantly, the minimum elastic modulus was achieved at 35 at.% Ti content (79.5 GPa), which was much lower than that of Ti-6Al-4V (124 GPa) [15], CP-Ti/Nb (103 GPa) and Ta (185 GPa) [16-18]. As shown in Fig. 12 (a), the annealed samples exhibited a much better ductility. Moreover, small dimples, stretched dimples and tear ridges were observed in annealed

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Nb-25Ta-35Ti alloy (Fig. 12(b)). As expected, the frature surface was evolved from

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the cleavage fracture into the ductile fracture when annealing treatment was

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introduced [19, 20].

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3.3. In vitro behavior

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In vitro behavior of as-sintered/annealed Nb-25Ta-25Ti alloy was evaluated by cell test. The morphologies of MG-63 cells grown on alloy surface after 7 d are shown in

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Fig. 13(a)(b). All the cells adhered and spread well on the samples, showing flatter or irregular morphology with a size of 10-20 μm. Fig. 13 (c) also showed that cells

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clustered and were abounded with pseudopodia protruding/anchoring in a pore. It should be noted that pores could provide mechanical anchoring to cells, thus facilitating the ingrowth to bone tisssue in vivo [21, 22] . Fig. 13(d) exhibits the proliferation results of both as-sintered and annealed Nb-25Ta-25Ti after 1 d, 3 d, 5 d and 7 d culture of MG-63 cell. The adhesion and proliferation of MG-63 cells on two groups of samples were quick, as expected, the annealing process showed no signs of possible negative effects to in vitro biocompatibility. 4. Discussion 4.1. Effect of alloying element and annealing treatment on microstructural

Journal Pre-proof evolution In this study, Nb-25Ta-xTi alloys were fabricated by powder metallurgy with blended elemental powders, which was actually a physical diffusion process sensitive to the chemical compositions. The sintering temperatures of Nb and Ta are extremely high (2468 ℃ for Nb and 3017 ℃ for Ta), therefore the addition of Ti tends to lower the sintering temperature. At a low Ti content, two distinct zones (Nb-rich zones and Ta-rich zones) with different contrasts could be observed. The difference was caused

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by the insufficient solid-state interdiffusion. When Ti content increased to 25 at.%, the

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diffusion between the particles seemed to be easier and the phase became more homogeneous. Nb and Ta belong to the same group in the periodic table of elements

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and are chemically similar. Therefore, the structures of Nb-rich/Ta-rich zones were continuous, and no obvious difference could be observed in the XRD results (Fig. 4).

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the sintering process.

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Although Nb and Ta are strong β stablizers, α/α’’ phase could also be formed during

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The pores in alloys can be categorized into two major types: i.e. residual pores between the starting elemental powders and pores caused by Kirkendall effect [18].

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Owing to the big difference in diffusion coefficients between Nb, Ta and Ti, partial diffusion could occur where atoms transformed unbalancedly, resulting in porosity and vacancies. With the increase of Ti content or the introduction of annealing treatment, diffusion became more sufficient and accelerated the densification. For Nb-25Ta-5Ti alloy, the partial diffusion was evident, and thus, presenting the lowest relative density. Furthermore, the partial diffusion could be weakened due to a more sufficient diffusion after annealing treatment, leading to a higher relative density of annealed alloys. Moreover, the annealing treatment also played a significant role in the pore shape. The EBSD analysis of the as-sintered and annealed Nb-25Ta-35Ti alloys is presented in Fig. 14. The annealed specimen exhibited fine equiaxed grains, which were larger than that of as-sintered alloy. As the annealing treatment continued, the residual porosity was reduced as pores were eliminated by bulk diffusion to grain

Journal Pre-proof boundaries. The pores in annealed specimens were fewer. In addition, the angular and irregular pores became smooth, and the shape gradually changed to perfect spheres. 4.2. Effect of alloying element and annealing treatment on mechanical properties Generally, the alloying elements and annealing treatment could influence the microstructural evolution, and the phase/microstructural transformation would, in turn, affect the mechanical properties. As mentioned in Section 3.1, the tensile strength of

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the as-sintered Nb-Ta-Ti alloys increased with increasing Ti content. The strengthening mechanism is very complicated. However, large quantitative studies

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showed that the main influence factors of strengthening effects could be summarized

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as solid-solution strengthening, boundary strengthening, precipitating strengthening and strain hardening. Fleischer model and Labusch model were introduced in various

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works to analyze the solid-solution strengthening [23]. Solid-solution strengthening

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occurred when other alloying elements were alloyed as solute atoms, which were different from the matrix atoms in atomic size. The atomic radius of Ta (0.149 nm) is

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about 1.4% larger than that of Nb (0.147 nm) and Ti (0.147 nm) [24]. Thus, Ta dissolves in the lattice of the Nb-Ti matrix, which contributes to the lattice distortion

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by the atomic size mismatch during the diffusion process and the dislocation glide becomes more difficult. The higher the proportion of solute atoms, the reinforcement becomes more obvious. Thereby, the increase in tensile strength could be observed in both as-sintered and annealed alloys with the increase of Ti content, especially for the alloys with a large amount of Ti content. Moreover, the Nb-rich and Ta-rich particles embedded by incomplete diffusion also played a role through precipitation strengthening for the strength reinforcement. However, the slight decrease of tensile strength after annealing treatment could be attributed to the reduction and/or disappearance of the Nb-rich/Ta-rich zones and the big-sized grains. It is worth noting that the tensile strengths of the as-sintered Nb-25Ta-5Ti and Nb-25Ta-15Ti alloys were very close. It is speculated that the precipitation strengthening of Nb-rich/Ta-rich particles was the dominant strengthening mechanism at a low Ti content, and then

Journal Pre-proof changed into solid-solution strengthening when Ti content was higher than 15 at.%. As presented in Fig. 5(c), the modulus of the as-sintered Nb-Ta-Ti alloys was relatively high due to the retained Ta particles. However, the reduction of elastic modulus after annealing treatment was remarkable. The modulus of as-sintered Nb-Ta-Ti alloys with 5 at.% and 15 at.% content of Ti decreased by ~10.6% and ~9.6% after annealing treatment, respectively. A larger reduction in modulus of annealed Nb-25Ta-35Ti alloy is attributed to the more homogeneous phase [1]. As

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well known, tensile strength, elastic modulus and ratio of strength-to-modulus are

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important factors for biomedical materials which can indicate the mechanical performance and determine the applications. Therefore, a literature of review was

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conducted to compile a brief list of the mechanical properties of some widely used

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metallic biomaterials, together with the annealed Nb-Ta-Ti alloys in this work (Table 1). As shown in Table 1, more attention was mainly focused on the studies of β-type

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Ti alloys with low concentrations of Nb and Ta. Therefore, the present work plays a great concern on the influence of a wide range of Nb and Ta additions on

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microstructures and mechanical properties. It shows that compared with alloys currently used in biomedical field, the Nb-Ta-Ti alloys exhibit a lower elastic modulus,

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a higher tensile strength and a larger strength-to-modulus ratio. 4.3. Biocompatibility

The chemical components have a significant effect on biocompatibility of implant alloys. Nb-Ta-Ti alloys were fabricated by solid solution of pure powders, thus maintaining a superior biocompatibility, good corrosion resistance and low cytotoxity. The oxides formed on the surface of substrates were composed of TiO2, Nb2O5 and Ta2O5, which could provide a bio-inert layer in the environment of body liquid [28]. For a long term, the enrichment of TiO2/Nb2O5/Ta2O5 oxides on the surface could suppress the dissolution of ions, which is of great importance in determining the interaction between implant materials and surrounding tissues. Moreover, the surface characteristics of materials such as surface topography plays an important role in the

Journal Pre-proof adhesion of cells to surface of biomaterials. The introduce of pores can provide better surface interaction and mechanical anchoring sites, thus facilitating the attachment and promoting the growth of cells. Furthermore, cell adhesion will in turn affect on various cellular physiological behavior (cell growth, defferentiation and migration) by the expression of specific proteins [29]. PM fabrication process and annealing treatment can control porosity of alloys by adjusting chemical compositions and fabrication parameters. Therefore, the PM Nb-Ta-Ti alloys are suitable for cell

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attachment and promising for biomedical applications in tissue engineering.

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Acknowledgements

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The authors would like to acknowledge the National Natural Science Foundation of China (No. 51904357), the Youth Scientific Research Foundation of the Central South of

Forestry

and

Technology

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University

(No.QJ2018003A)

and

the

Conclusions

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support.

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National Key R&D Program of China (No. 2016YFC0700801-01-01) for financial

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This study investigated the effects of alloying elements and annealing treatment on the microstructural evolution and mechanical properties of Nb-Ta-Ti alloys. According to the results of this study, following conclusions can be drawn: (1) Nb-25Ta-xTi alloys with a dual structure were fabricated using elemental powders by the incomplete diffusion. The alloys, which consisted of Nb-rich and Ta-rich zones, were not homogeneous after sintered at 1700 ℃. (2) After annealing treatment at 1000 ℃ for 3 hrs, the phase distribution was more homogeneous and exhibited with fine equiaxed grains. Because of the more sufficient diffusion with the increase of Ti content, the fine equixed grains were larger than those in the as-sintered alloys. The hexagonal α phase and orthorhombic α’’ phase could be detected in annealed Nb-25Ta-35Ti alloy.

Journal Pre-proof (3) The relative density and volume shrinkage of as-sintered/annealed Nb-Ta-Ti alloys exhibited a close correlation to the Ti content. All the as-sintered alloys showed a relative density higher than 80%. The content increase of Ti could increase the tensile strength due to the solid solution strengthening mechanism. Annealing treatment was helpful to refine the grain size and improve the ductility. (4) The Nb-25Ta-35Ti alloys exhibited a good in vitro biocompatibility due to the

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chemical components and the introduce of surface pores.

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[15] S. Guo, J. Zhang, X. Cheng, X. Zhao, A metastable β-type Ti-Nb binary alloy with low modulus and high strength, Journal of Alloys and Compounds, 644 (2015) 411-415. [16] K. A. Nazari, A. Nouri, T. Hilditch, Mechanical properties and microstructure of powder metallurgy Ti-xNb-yMo alloys for implant materials, Materials & Design, 88 (2015) 1164-1174. [17] H. Li, J. Xu, MRI compatible Nb-Ta-Zr alloys used for vascular stents: Optimization for mechanical properties, Journal of the Mechanical Behavior of Biomedical Materials, 32 (2014) 166-176. [18] Y. Liu, K. Li, H. Wu, M. Song, W. Wang, N. Li, H. Tang, Synthesis of Ti-Ta alloys with dual

Journal Pre-proof structure by incomplete diffusion between elemental powders, Journal of the Mechanical Behavior of Biomedical Materials, 51 (2015) 302 - 312. [19] H. Özkan Gülsoy, N. Gülsoy, R. Calısıcı, Particle morphology influence on mechanical and biocompatibility properties of injection molded Ti alloy powder, Bio-Medical Materials and Engineering, 24 (2014) 1861-1873. [20] P. S. Nnamchi, C. S. Obayi, I. Todd, M. W. Rainforth, Mechanical and electrochemical characterisation of new Ti-Mo-Nb-Zr alloys for biomedical applications, Journal of the Mechanical Behavior of Biomedical Materials, 60 (2016) 68-77. [21] H. H. Huang , C. P. Wu , Y. S. Sun , W. E. Yang , M. C. Lin, T. H. Lee, Surface nanoporosity

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of β-type Ti-25Nb-25Zr alloy for the enhancement of protein adsorption and cell response, Surface and Coatings Technology, 259 (2014) 206-212.

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[22] J. Chen, E. Paetzell, J. Zhou, L. Lyons, W. Soboyejo, Osteoblast-like cell ingrowth, adhesion

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[25] Y. H. Hon, J. Y. Wang, Y. N. Pan, Influence of hafnium content on mechanical behaviors of Ti-40Nb-xHf alloys, Materials Letter, 58 (2004) 3182-3186. [26] D. Kuroda, M. Niinomi, M. Morinaga, Y. Kato, T. Yashiro, Design and mechanical properties of new β type titanium alloys for implant materials, Materials Science and Engineering A, 243 (1998) 244-249.

[27] M. A. Khan, R. L. Williams, D. F. Williams, The corrosion behaviour of Ti-6Al-4V, Ti-6Al-7Nb and Ti-13Nb-13Zr in protein solutions, Biomaterials, 20 (1999) 631-637. [28] E. Yılmaz, A. Gokçe, F. Findik, H. Gulsoy, Metallurgical properties and biomimetic HA deposition performance of Ti-Nb PIM alloys, Journal of Alloys and Compounds, 746 (2018) 301-313. [29] E. Yılmaz, B. Çakıroğlu, A. Gökçe, F. Findik, H. O. Gulsoy, N. Gulsoy, Ö. Mutlu, M. Özacar, Novel hydroxyapatite/graphene oxide/collagen bioactive composite coating on Ti16Nb alloys by electrodeposition, Materials Science & Engineering C, 101 (2019) 292–305.

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Figure/Table Captions Fig. 1 SEM micrographs showing the microstructure of the Nb-25Ta-xTi alloys sintered at 1700 ℃. Fig. 2 EDS results of the corresponding zones in Fig. 1. Fig. 3 EPMA mapping showing the element distribution of as-sintered Nb-25Ta-5Ti alloy.

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Fig. 4 XRD pattern of Nb-25Ta-xTi alloys sintered at 1700 ℃.

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Fig. 5 (a) Relative density and volume shrinkage of as-sintered Nb-25Ta-xTi alloys;

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(b) Hardness of as-sintered Nb-25Ta-xTi alloys with different Ti contents; (c) Ultimate tensile strength and elastic modulus as a function of the Ti content of

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as-sintered Nb-25Ta-xTi alloys.

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Fig. 6 (a) Tensile stress-strain curves of as-sintered Nb-25Ta-xTi (x = 5 and 35 at.%); (b)-(e) SEM micrographs showing the fracture morphology after tensile tests of

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as-sintered Nb-25Ta-xTi alloys.

Fig. 7 SEM micrographs showing the microstructure of the Nb-25Ta-xTi alloys after

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annealing treatment at 1000 ℃ for 3 h. Fig. 8 Fig. 7 EDS mapping of annealed Nb-25Ta-5Ti alloys. Fig. 9 XRD patterns of Nb-25Ta-xTi alloys after annealing treatment at 1000 ℃ for 3h. Fig. 10 (a) Bright field TEM showing β phase and α’’ phase precipitating in β phase; (b) the corresponding SADP of β and α’’ phases. Fig. 11 (a) Relative density of annealed Nb-25Ta-xTi alloys; (b) Hardness of annealed Nb-25Ta-xTi alloys with different Ti contents in comparison to those of commonly used metallic biomaterials; (c) Ultimate tensile strength and elastic modulus as a function of Ti content of annealed Nb-25Ta-xTi alloys.

Journal Pre-proof Fig. 12 (a) Tensile stress-strain curves of annealed Nb-25Ta-xTi (x = 5 and 35 at.%); (b) SEM micrographs showing the fracture morphology after tensile tests of annealed Nb-25Ta-35Ti alloy. Fig. 13 SEM micrographs showing the morphologies of MG-63 cells after 7 d culture of (a) as-sintered Nb-25Ta-25Ti alloy and (b) annealed Nb-25Ta-25Ti alloy; (c) SEM micrographs showing the cell growth in the surface pore; (d) Alamar Blue assay of MG-63 cells cultured on as-sintered/annealed Nb-25Ta-25Ti alloys for different time.

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Fig. 14 EBSD orientation map using IPF colouring for the grains of (a) as-sintered

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and (b) annealed Nb-25Ta-25Ti alloy.

Table 1 Comparison of tensile properties of typical biomaterials annealed Nb-Ta-Ti

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alloys.

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Author Contribution Statement Jue Liu performed the experiments, analyzed the data and wrote the paper. Hailin Yang conceived/designed the experiments and provided financial support. Qiumin Yang and Jian Yin reviewed and edited the manuscript. All authors read and approved

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the final version of the manuscript.

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The authors declared that they have no conflicts of interest to this work.

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Table 1 Comparison of tensile properties of typical biomaterials and annealed Nb-Ta-Ti alloys. Tensile strength (MPa)

Elastic modulus (GPa)

Ratio of strength-to-modul us 2.33-5.34 7.23 2.98-3.14 / / 2.98-3.14 4.3 / / 4.3

Ref.

CP-Ti Ti-6Al-4V Co-Cr-Mo Nb Ta 316L SS Ti-50Ta Ti-10Mo Ti-14Nb Ti-12Mo-3Nb (sol.) Ti-29Nb-13Ta Ti-16Nb-13Ta-4M o Ti-4Al-7Nb (sol.) Nb-25Ta-5Ti(anne aled) Nb-25Ta-15Ti (annealed) Nb-25Ta-25Ti (annealed) Nb-25Ta-35Ti (annealed)

250-550 896 655-690 / / 965 380 / / 450

103 124 220 103 185 379 88 95 98 105 103 98

/ /

[26] [27]

110 84.3

/ 4.92

[27] This work

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5.12

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459

82.8

5.54

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79.5

6.68

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110-1050 415

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428

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Alloys

[19] [18] [18] [16, 17] [16, 17] [17] [20] [24] [25] [24]

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Highlights (1) Nb-25Ta-xTi alloys with a dual structure were fabricated by partial diffusion. (2) Effects of alloying elements and annealing treatment on the microstructure and mechanical properties of Nb-Ta-Ti alloys were investigated.

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(3) Nb-Ta-Ti alloys are promising as a new candidate of biomedical applications.

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