Effect of Nb and Si Content on Phase Stability, Microstructure and Mechanical Properties of Sintered Ti–Nb–Si Alloys

Abstract

The development of beta titanium alloys with biocompatible elements to replace Al and V is a subject of significant interest in the biomedical industry. This approach aims to enhance biocompatibility and mitigate potential cytotoxic effects associated with traditional alloying elements. In this work, Ti–xNb–ySi alloys were produced using powder metallurgy, with x of 35, 40, and 45 wt.%, and y of 0.10, 0.35, and 0.60% wt.%, using a $3^2$ experimental design. Milling was used to mix and disperse the powders, followed by cold pressing, sintering, and heat treatment. Nb was the main element used to stabilize the β phase, and Si was used to form Si precipitates, although Si also exhibits a β-stabilizing effect. It was found that an increase from 0.10 to 0.35 wt.% of Si improved relative density, with no benefits observed at 0.60 wt.% Si. Electron microscopy showed the presence of β phase grains, and grains with β + α intragranular structures and precipitates. Increasing Nb content resulted in a decrease in ultimate tensile strength while increasing Si content from 0.10% to 0.35 wt.% exhibited the opposite effect.

1. Introduction

Titanium and its alloys are extensively employed in the biomedical industry for implant fabrication due to their high corrosion resistance, biocompatibility, and favorable mechanical properties such as a high strength-to-weight ratio and a comparatively lower elastic modulus than steel [1,2]. However, dense α-Ti implants exhibit a considerably greater elastic modulus than human bone, which can reduce the load transferred to the bone adjacent to the implant and lead to bone resorption through a process known as stress shielding [3,4,5]. Human bone is generally categorized into two types of structures, cortical and trabecular, with cortical bone demonstrating elastic moduli within the range of 15 to 30 GPa [6]. By contrast, the Ti6Al4V alloy—widely adopted in the biomedical field due to its biocompatibility, corrosion resistance, and high strength-to-weight ratio, partly driven by its accessibility from aeronautical applications—exhibits an elastic modulus around 110 GPa [6,7,8]. In addition to the high elastic modulus mismatch, Ti6Al4V poses potential long-term biocompatibility concerns, as vanadium ions can exhibit cytotoxicity, and aluminum ions may contribute to neurotoxicity [9,10,11].

An effective strategy for reducing the elastic modulus of titanium (Ti) alloys is to increase the β phase content, as this phase generally exhibits a lower modulus of 60–80 GPa compared to the 100–120 GPa of the α phase [6,12]. Among β-stabilizing elements that are biocompatible, Zr, Nb, Mo, Hf, Si, and Ta can reduce the elastic modulus and increase strength [13], and Cr, Mn, Fe, and Co can both increase the elastic modulus and yield strength [2,9,10].

Niobium is commonly used to stabilize the β phase in Ti alloys, allowing for the development of both α + β and β phase alloys depending on its concentration. Nb contributes significantly to corrosion resistance in diverse environments, particularly in acidic and oxidative conditions [6,12,14]. It exhibits complete solubility with titanium (Ti), facilitated by the similar atomic radii of niobium (Nb) at 2.08 Å and titanium (Ti) at 2.00 Å. Additionally, Nb can inhibit the martensitic transformation from the β to ω phase when present at sufficient concentrations. Although the ω phase can form nanoscale precipitates that increase strength and hardness, it also reduces ductility and toughness [15,16]. Ehtemam-Haghighi et al. [17] demonstrated that adding up to 11 wt.% Nb in Ti–7Fe–xNb alloys enhances β phase stability and volume fraction, leading to a substantial increase in compressive yield strength, from 985 to 1847 MPa. In a study by Zhao et al. [18], Ti–xNb–yCu alloys were examined for corrosion resistance; electrochemical measurements and immersion tests revealed that higher Nb content improved corrosion resistance, attributed to the formation of Nb oxide (Nb₂O₅) on the passive layer.

The biocompatibility of niobium (Nb) is well documented through extensive in vitro and in vivo studies. This is attributed to the formation of stable passive oxide films on its surface, which enhance corrosion resistance and minimize adverse biological responses [6,19]. For instance, in a study by Kumar and Gautam [20] investigating the proliferation of MG63 cells, optical analysis demonstrated that Ti–Nb alloys (with 5%, 10%, and 15% Nb) exhibited a higher cell density compared to commercially pure titanium (cp-Ti). Additionally, Liu et al. [21] examined Nb–Ti–Ta porous scaffolds implanted in rabbits. After 12 weeks, the scaffolds were observed to be covered with newly formed bone tissue, confirming their capacity to support new bone generation.

Silicon is an alloying element that contributes to the stabilization of β-Ti and is relatively inexpensive, being the second most abundant element on Earth’s surface. Although Si has limited solubility in titanium, it diffuses rapidly, which enhances the sinterability of Ti alloys [22,23]. At concentrations exceeding 0.1 wt.%, however, Si-based intermetallic precipitates begin to form along grain boundaries. When Si content exceeds 0.5 wt.%, these precipitates may become coarse, leading to a reduction in the material’s plasticity [24]. Tavares et al. [19] observed that, in addition to Nb inhibiting ω phase formation at concentrations above 26 wt.%, Si also plays a role in suppressing ω phase formation, further enhancing alloy stability.

Silicon biocompatibility has been confirmed by studies like Gallardo-Moreno et al. [25], who observed a reduction in bacterial attachment with Si ion implantation on austenitic stainless steel, although without noticing any change in cell proliferation through in vitro studies of cell attachment and viability of human mesenchymal stem cells hMSCs in commercial cultures. Also, Kheradmandfard et al. [26] observed 60% of the surface of the Si/Diamond-like carbon (DLC) coating deposited in Ti–29Nb–13Ta–4.6Zr (TNTZ) samples, double the amount of the TNTZ samples. The TNTZ system shows great potential, as demonstrated in the study by Sevostyanov et al. [27]. They observed scaffolds made of Ti–30Nb–13Ta–5Zr exhibiting a cell viability rate of over 95%, with a cell density reaching nearly 1500 cells/mm².

A major limitation of the current generation of Ti–Nb-based implant materials is their poor wear resistance [1]. Wear debris generated from implants can react with bodily fluids, releasing metal ions that may lead to allergic and toxic responses [8]. Additionally, low wear resistance can contribute to implant loosening over time [28]. Due to its strong covalent bonding, in situ-formed Ti–Si precipitates offer potential for enhancing wear resistance in titanium alloys [23]. Ti–Si compounds are commonly utilized in microelectronics for their high-temperature stability and corrosion resistance [29,30]. The formation of precipitates can also improve wear resistance and mechanical properties. Xu et al. [31] used low-energy milling of a Ti–Al–Sn–Zr alloy with SiC, achieving in situ formation of TiC and Ti₅Si₃ reinforcements for high-temperature applications. Similarly, Tang et al. [32] utilized graphene to induce in situ TiC reinforcement within the pore walls of porous titanium, enhancing structural stability and wear resistance.

In this study, Nb was selected due to its β-phase-stabilizing effect, its ability to reduce the elastic modulus [15,33], its biocompatibility [6,14], and its potential to enhance corrosion resistance [34]. Additionally, small amounts of Si were introduced for its biocompatibility, its β phase stabilization properties, and its potential to improve mechanical, wear, and corrosion resistance through the formation of precipitates [23,35]. The primary objective was to develop a predominantly β phase alloy with a reduced elastic modulus and finely dispersed precipitates, aiming to enhance mechanical strength without significantly increasing the elastic modulus. A processing route involving cold pressing and sintering was chosen, due to their low energy and equipment requirements, making the process both cost-effective and environmentally friendly [36]. The resulting microstructure, phase composition, and porosity were analyzed, along with the impact of alloy content on porosity and mechanical properties, which were assessed through tensile testing and hardness measurements.

This study aims to develop and produce a low elastic modulus titanium (Ti) alloy using alloying elements identified as biocompatible through bibliographic research. The chosen processing route prioritizes minimizing energy consumption and material waste. To enhance mechanical properties, this study focuses on creating finely distributed silicon (Si) precipitates. This experiment seeks to achieve a balance, as the alloying elements used to reduce the elastic modulus may compromise mechanical strength, while Si precipitates, intended to improve mechanical strength, may inadvertently increase the elastic modulus.

2. Materials and Methods

In order to design the alloy compositions, equilibrium diagrams were generated using the Thermo-Calc^® software 2015-b, using the SSOL5 database (for solid solutions). Molecular Orbital Method calculations were performed in conjunction with the (Bo)–(Md) diagram [37,38,39]. Those data were used to determine processing temperatures, the alloy composition, and the composition in which precipitation can start.

Ti–xNb–ySi alloys were prepared using a powder metallurgy technique, using 3 amounts of Nb (x = 35, 40, and 45 wt.%) and 3 amounts of silicon (y = 0.10, 0.35, and 0.60 wt.%). The 3k experimental design was applied with k = 2, corresponding to the two alloy elements tested. Using three levels for each factor enables the detection of both linear and quadratic (curvilinear) effects, offering a deeper insight into how these factors influence the response. This approach examines not only the individual effects of each factor but also their interactions [40,41]. A description of the powders is presented in

Table 1. Powders used in this study.

Powder	Particle Size	Purity	Supplier	Production
Ti	325 mesh	99.8%	Alfa Aesar	Ar atomization
Nb	10 µm	99.5%	SkySpring Nanomaterials	Ar atomization
Si	1~2 µm	99.9%	SkySpring Nanomaterials	Ar atomization

.

The elemental powders were weighed and placed in yttria-stabilized zirconia jars for mixing and grinding. A ball-to-powder mass ratio of 5:1 was used, with balls of the same material as the jars, having diameters of 10 and 5 mm. For each 100 g of zirconia balls, 25 g (8 balls) were 10 mm in diameter, while the remainder consisted of 5 mm diameter balls. To prevent contamination, all powder handling took place in an argon (Ar) atmosphere within a glove box, and the jars were closed inside the glove box before milling. The lids of the jars had a recess where a silicone O-ring was placed. The lids were then closed and secured using tape.

The powders were then milled in an Across International planetary ball mill, model PQ-NO4, at 350 rpm for 1 h, with 30 min interval cooling for each 30 min of grinding, changing rotation direction, with a fixed rotation-to-revolution speed ratio of 1:1.9. Following grinding, the powder mixtures were cold pressed using a tool made of steel at a pressure of 400 MPa, then sintered in a tubular alumina furnace at 1200 °C for 3 h in an Ar + 5%H₂ atmosphere. Two dies were employed: one with a diameter of 9.9 mm, and another designed to produce tensile test samples according to MPIF 2012 standard number 10—Determination of the Tensile Properties of Powder Metallurgy Materials [42]. After sintering and cooling, the alumina tube was replaced with a stainless-steel tube to enable faster cooling rates. The samples then underwent heat treatment at 850 °C for 1 h, followed by rapid cooling by removing the tube from the furnace and applying ventilation. An image of the sample is provided in Figure 1, and no polishing or machining was applied to the tensile test samples before testing.

For microstructural analysis, samples were mounted in bakelite resin, ground using SiC abrasive paper up to 1200 mesh, and polished sequentially with 0.2 µm diamond paste and 0.06 µm colloidal silica suspension. Samples were then etched with Kroll’s solution (5 vol% HF, 30 vol% HNO₃, and 65% H₂O). Microstructural examination was conducted with a Tescan Vega3 LMU scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDS) probe, from Tescan Orsay Holding based on Brno, Czech Republic. X-ray diffraction (XRD) analysis was performed using a Rigaku MiniFlex600, manufactured by Rigaku Corporation, a company headquartered in Tokyo, Japan, with pure Cu Kα radiation (λ = 1.54056 Å), employing a 2θ scan with a step size of 0.02° and a scanning speed of 2°/min.

Rietveld refinement of XRD data was conducted using Maud software version 2.9994, referencing standard files ICSD COD1532765 for Ti-α, COD9008554 for Ti-β, and ICSD652417 for Ti₃Si. XRD instrument characterization for Rietveld fit was performed using a SiC pattern and standard file ICSD2185123 for SiC. Apparent density was measured with a micrometer (0.001 mm resolution) and a precision scale (0.0001 g resolution). For each composition, three samples were prepared, with three measurements per sample, resulting in a total of 81 measurements. Relative density was subsequently calculated as a percentage of the theoretical density for each sample.

To determine the elastic modulus, ultimate tensile strength, and elongation, tensile test specimens were produced according to MPIF 2012 standard 10—Determination of the Tensile Properties of Powder Metallurgy Materials [42]—using a dedicated die. The dimensions are shown in Figure 1. Testing was conducted at a constant crosshead speed of 1.524 mm/min, selected to achieve a quasi-static strain rate of approximately 0.001 s⁻¹. A universal testing machine, MTS Criterion Model 45, equipped with a clip-on extensometer, was used to measure strain. The elastic modulus was calculated for each specimen as the slope of the linear portion of the stress–strain curve. For each of the nine compositions, six valid tests were conducted, yielding a total of 54 data points. Contour plots were generated to facilitate visualization of the results, and marginal means plots (weighted) were created to analyze the individual effects of each element.

3. Results

3.1. Calculations and Diagrams

The prediction of the niobium (Nb) content necessary to form a predominantly β phase titanium alloy was conducted using the Molecular Orbital Method. This method indicates that β phase stability is enhanced by an increase in the average bond order $\overline{{\mathrm{B}}_{\mathrm{o}}}$ or a decrease in the average metallic d-orbital energy level $\overline{{\mathrm{M}}_{\mathrm{d}}}$ [37,38,39].

Table 2. Calculations of β stability of the studied alloys.

Alloy Nominal Composition	β Stability Indicators
	$\overline{{\mathbf{B}}_{\mathbf{o}}}$	$\overline{{\mathbf{M}}_{\mathbf{d}}}$
TiSi0.10Nb35	2.858	2.441
TiSi0.10Nb40	2.870	2.441
TiSi0.10Nb45	2.882	2.440
TiSi0.35Nb35	2.856	2.440
TiSi0.35Nb40	2.868	2.439
TiSi0.35Nb45	2.881	2.438
TiSi0.60Nb35	2.855	2.439
TiSi0.60Nb40	2.867	2.438
TiSi0.60Nb45	2.879	2.437

presents the results showing that silicon (Si) addition leads to a slight decrease in both $\overline{{\mathrm{B}}_{\mathrm{o}}}$ and $\overline{{\mathrm{M}}_{\mathrm{d}}}$. By contrast, Nb addition was observed to increase $\overline{{\mathrm{B}}_{\mathrm{o}}}$ and slightly reduce $\overline{{\mathrm{M}}_{\mathrm{d}}}$, thereby improving the stability of the β phase. Alloys containing 35 wt.% Nb were found to lie within the β/β + ω phase region. As Nb content increased, the alloys shifted into the β phase region, suggesting enhanced phase stability. A higher Nb content helps suppress the formation of the ω phase, which is characterized by a high elastic modulus and hardness, thereby enhancing the alloy’s mechanical resistance [43].

Thermo-Calc^® software calculations were employed to identify an appropriate sintering temperature for the alloy. Figure 2 shows the phase diagrams calculated for an alloy with 0.35 wt.% Si. At 1200 °C, complete solubilization of the alloy occurs, resulting in a single-phase β structure across the entire composition range analyzed. At 850 °C, however, the equilibrium state comprises a β matrix with Ti₃Si precipitates.

Table 3. Calculated amount of phases at room temperature.

	Calculated Phase Amount
	α wt.%	β wt.%	Ti3Si wt.%
Ti.010Si35Nb	64.61	35.01	0.38
Ti0.10Si45Nb	54.52	45.06	0.42
Ti0.60Si35Nb	61.53	35.01	3.46
Ti0.60Si45Nb	51.44	45.07	3.49

further predicts that, under thermodynamic equilibrium, the α phase predominates, with varying amounts of the β phase and Ti₃Si also present. Consequently, a heat treatment is necessary to stabilize the maximum possible β phase in the alloy [19,44,45].

3.2. Powder Characterization

The scanning electron microscopy (SEM) analysis of the powders reveals an irregular and angular morphology prior to milling, as illustrated in Figure 3a–c. Post-milling, depicted in Figure 3d, the powders exhibit a more granular structure resulting from the milling process, while retaining their irregular morphology. This retained irregularity enhances mechanical interlocking during cold pressing, thereby increasing the green strength of the compact and improving the effectiveness of the subsequent sintering process [33]. Figure 4 shows a typical X-ray diffraction pattern observed after grinding, with mainly Ti and Nb peaks observed.

3.3. Phase Analysis and Microstructure of the Sintered and Treated Samples

X-ray diffraction (XRD) analysis confirmed that the primary phase in all samples was β-Ti, with minor amounts of α-Ti, as exemplified in Figure 5. In the sample containing 0.10 wt.% Si, only β and α phases were detected. Conversely, in the sample with 0.60 wt.% Si, additional smaller peaks were observed, which are likely associated with Si precipitates. Notably, no discernible peaks corresponding to α″, α′, or ω phases were identified.

These XRD findings were further supported by scanning electron microscopy (SEM) analysis. As shown in Figure 6, the microstructure of the 0.10 wt.% Si sample consists of a β matrix interspersed with α lamellae. For the sample with 0.60 wt.% Si, SEM imaging revealed the presence of precipitates distributed within the β matrix. Energy-dispersive X-ray spectroscopy (EDS) analysis of the Ti–0.60Si–45Nb alloy, presented in Figure 7, highlights the compositional variation between the α and β regions. The α regions were enriched in titanium (Ti), whereas the β regions exhibited higher concentrations of niobium (Nb). Quantitative spot analysis indicated that the β regions consisted of 72.6 ± 1.3 wt.% Ti, 26.7 ± 1.4 wt.% Nb, and 0.7 ± 0.1 wt.% Si. In comparison, the α regions contained 91.2 ± 6.4 wt.% Ti, 8.2 ± 5.5 wt.% Nb, and 0.7 ± 0.9 wt.% Si.

Rietveld refinement was performed to quantify the phases present and to further investigate the effects of alloying elements on phase composition. The results, presented in

Table 4. Rietveld refinement performed for samples Ti0.10Si35Nb, Ti0.10Si45Nb, Ti0.60Si35Nb, and Ti0.60Si45Nb.

	σ	Rwp (%)	α wt.%	α Cell				β Phase wt.%	β Cell		(Ti,Nb)3Si wt.%	(Ti,Nb)3Si Cell
Alloy				Calc.		CIF			Calc.	CIF		Calc.		CIF
				a	c	a	c		a	a		a	c	a	c
Ti0.10Si35Nb	2.23	7.76	9.24	2.96	4.76	2.91	4.67	90.76	3.30	3.31	---	---	---	---	---
Ti0.10Si45Nb	3.45	13.55	3.96	2.97	4.78	2.91	4.67	96.04	3.30	3.31	---	---	---	---	---
Ti0.60Si35Nb	1.77	6.84	4.90	2.96	4.77	2.91	4.67	89.55	3.30	3.31	5.55	10.21	5.03	10.21	5.07
Ti0.60Si45Nb	2.06	5.84	8.02	2.98	4.78	2.91	4.67	89.11	3.31	3.31	2.87	10.20	5.14	10.21	5.07

, show that for samples containing 0.10 wt.% Si, the (Ti,Nb)₃Si phase was not detected, and the β phase content increased significantly with 45 wt.% Nb. For samples with 0.60 wt.% Si, the β phase content was similar across both compositions. However, the Ti–0.60Si–45Nb sample exhibited twice the α phase content of Ti–0.60Si–35Nb, with values of 8.015 wt.% and 4.899 wt.%, respectively. By contrast, the Ti–0.60Si–35Nb sample displayed twice the amount of the (Ti,Nb)₃Si phase compared to Ti–0.60Si–45Nb.

Attempts to analyze the solid solution behavior through lattice parameter measurements were inconclusive, as the atomic radii of titanium (2.00 Å) and niobium (2.08 Å) are very similar, resulting in negligible lattice distortion [15,16].

A comparison of the experimental phase compositions (Table 4) with the predicted values (Table 3) reveals a higher β phase content than anticipated, demonstrating the effectiveness of the processing route in stabilizing the β phase within the alloys. For instance, in the case of the Ti0.10Si35Nb alloy, the predicted β phase content was 64.61 wt.% (Table 3), whereas the processing route achieved 90.76 wt.% β phase.

3.4. Porosity and Relative Density

Figure 8 presents SEM micrographs of the as-sintered samples, revealing a highly porous surface morphology. The impact of Nb and Si additions on the relative density is also depicted in Figure 8. The analysis indicates that Nb reduces the relative density from 77.4 ± 0.6 to 73.1 ± 0.5% of the theoretical density. Conversely, Si exhibits a minor positive effect on the relative density, with values increasing slightly from 74.6 ± 0.9% at 0.60 wt.% Si to 76.1 ± 0.9% at 0.35 wt.% Si.

A contour plot further reveals that the maximum relative density of 78.7 ± 0.8% is achieved at 0.35 wt.% Si and 35 wt.% Nb, suggesting that this combination represents the most favorable composition for sinterability. The minimum relative density was achieved at 0.6 Si and 45 wt.% Nb at 72.0 ± 0.5%.

3.5. Mechanical Properties

Figure 9 illustrates the typical behavior observed during tensile tests, characterized by minimal elongation prior to failure.

The tensile strength test results are summarized in Figure 10, illustrating the influence of Si and Nb on the elastic modulus and ultimate tensile strength (UTS). Marginal means plots are employed to delineate the individual effects of each element. Regarding the elastic modulus, no significant variation is observed with an increase in Si content from 0.10 wt.% to 0.35 wt.%; however, a slight reduction is noted as Si content increases to 0.60 wt.% (Figure 10b). Conversely, an increase in Nb content results in a notable decrease in the elastic modulus, as shown in Figure 10c. The contour plot (Figure 10a) further reveals that the minimum elastic modulus values occur at approximately 45 wt.% Nb and at Si levels below 0.10 wt.% (55.7 ± 4.1 GPa) or above 0.60 wt.% (58.3 ± 3.6 GPa).

For UTS, the marginal means plots indicate no significant change with Si content up to 0.35 wt.%; however, UTS seems to decrease for 0.60 wt.% Si (Figure 10e). Similarly, an increase in Nb content leads to a decline in UTS, as illustrated in Figure 10f. The contour plot indicates that the maximum UTS is achieved at approximately 35 wt.% Nb and Si content below 0.60 wt.%.

The mechanical testing revealed a very low elongation, approximately 1%, across all conditions tested. This result suggests limited ductility in the material. However, scanning electron microscopy (SEM) analysis revealed the presence of fracture dimples typical of ductile fracture, as shown in Figure 11.

4. Discussion

The microstructural analysis conducted after milling revealed the distribution of Ti and Nb particles within the material. Despite this initial heterogeneity, the subsequent sintering process successfully dissolved Nb and Si into the Ti matrix, as confirmed by the absence of elemental Nb and Si peaks in X-ray diffraction (XRD) analysis. Additionally, the production route employed in this study, combined with the specific Nb and Si contents, facilitated the stabilization of a high percentage of the β phase, reaching up to 96 wt.% β-Ti, as presented in Table 4. As shown in Figure 2, at 1200 °C, a large β phase field is present, enabling alloy homogenization. Yilmaz et al. [46] utilized this effect to produce β + α Ti–Nb alloys through the powder metal injection technique, employing sintering temperatures of 1500 °C for 4 h and Nb content ranging from 0 to 40 wt.%. They achieved up to 82.2 wt.% β phase in a Ti–40Nb alloy.

Martensitic phases, specifically α″ and ω, were not detected in either XRD or scanning electron microscopy (SEM) analyses. This absence can be attributed to the Nb content used, which effectively suppressed the formation of these phases. As noted by Talbot et al. [47], Nb has the ability to inhibit the formation of α″ and ω phases in Ti–Nb alloys.

The absence of the martensitic α″ and ω phases further indicates that the applied cooling rate was carefully controlled to achieve an optimal balance. It was sufficiently rapid to stabilize a high proportion of β-Ti yet slow enough to prevent martensitic transformations. Cooling was conducted using forced ventilation inside a stainless-steel tube within a controlled atmosphere to prevent the oxidation of the porous material, which would have occurred with water or air cooling. This approach aligns with observations by Ehtemam-Haghighi et al. [17] who reported the β → α″ martensitic transformation in β phase Ti–Fe–Nb alloys during air cooling after casting. Similarly, Mehjabeen et al. [48] highlighted the strong dependence of martensitic transformations (β → α″ and β → ω) on the cooling rate, particularly during water quenching. Lower cooling rates, used in studies by Tavares et al. [19] and Dewangan and Singhal [44], were associated with the formation of a β matrix alongside the α phase.

In the Ti₃Si structure, titanium (Ti) and niobium (Nb) share very similar properties, allowing their atoms to be interchanged [19]. For this reason, the notation (Ti,Nb)₃Si is used throughout most of this work. (Ti,Nb)₃Si has a tetragonal structure and is characterized by high hardness, thermal stability, and a high melting point. When dispersed in Ti alloys, it acts as a barrier to dislocation movement, thereby strengthening the alloy and enhancing both its mechanical strength and creep resistance. However, despite its beneficial effects on strength, its brittle nature can negatively impact the ductility of the alloy [49,50].

Thermo-Calc^® simulations accurately predicted the presence of Ti₃Si precipitates at room temperature and during heat treatment at 850 °C, as shown in Figure 2. The observed increase in α phase content, particularly at 0.60 wt.% Si, can be explained by the decomposition reaction Ti-β → Ti-α + Ti₃Si/Nb₃Si, shown in Table 4. This reaction occurs near 865 °C in the Ti–Si system and is considered a slow process. In Ti alloys, Si acts as a β stabilizer under rapid cooling conditions such as water quenching. However, above the solubility limit of approximately 0.09 wt.% Si, heterogeneous Si precipitates form, depleting β stabilizers in the matrix and promoting α phase formation [49,50]. Studies by Kartamyshev et al. [51] and Zhao et al. [24] have shown that higher Si concentrations lead to increased precipitate growth, further supporting these observations. Tavares et al. [19] compared the effect of cooling rates on Ti–35Nb–xSi alloys produced by casting and hot rolling. They observed that higher cooling rates stabilized the β phase, and in the case of Ti–35Nb–0.55Si, both water quenching and air cooling prevented the formation of the α phase.

The relative density of the material plays a critical role in its mechanical properties, with even small increases in density having significant effects. As highlighted by German [52], a 2–3% increase in relative density can enhance UTS by up to 20%. However, the diffusion behavior of alloying elements affects sintering efficiency. Niobium (Nb), which diffuses slowly in titanium (Ti), negatively impacts sintering and reduces relative density, as observed in Figure 8d [53,54,55]. By contrast, silicon (Si) diffuses rapidly in Ti, facilitating the sintering process and slightly increasing relative density, as noted at 0.35 wt.% Si in Figure 8c. This behavior aligns with findings by Hayat et al. [22], who identified Si as a sintering aid for Ti due to its higher diffusion rate compared to Ti-β self-diffusion, thus improving sinterability [56].

The observed drop in relative density at 0.60 wt.% in Figure 8c could be a consequence of (Ti,Nb)₃Si formation. As Table 3 shows, the increase in Si content from 0.10 to 0.60 wt.% caused an almost tenfold increase in the predicted Ti3Si content. Also, the increase in Si could lead to Si precipitate growth, as observed by Tavares et al. [19]. In fact, the Si content of cast Ti alloys is typically lower than 0.5 wt.% to avoid precipitates coarsening [24]. Therefore, it can be theorized that the reduction in relative density is a consequence of Si precipitates coarsening and acting as sintering barriers.

The reduction in ultimate tensile strength (UTS) observed at Si levels exceeding 0.35 wt.% (Figure 10d,e) can be attributed to precipitate coarsening. Studies by Kartamyshev et al. [51] and Goi et al. [10] confirm that Si concentrations above 0.5 wt.% promote the growth of Si precipitates in Ti alloys, which detrimentally affects mechanical properties.

Porosity further influences the mechanical performance, particularly elongation, as evidenced by the brittle fracture behavior shown in Figure 11c,d. Although dimples, indicative of ductile fracture, are observed in Figure 11a,b, high levels of porosity likely dominated the fracture behavior. Kan et al. [57] noted that beyond a certain threshold, porosity effects outweigh microstructural contributions to ductility. Similarly, Laursen et al. [58], in their study of 176 AlSi10Mg samples, demonstrated a clear relationship between increased porosity and reduced ductility. These observations highlight the interplay between microstructure, alloying elements, and porosity in determining the mechanical properties of Ti-based alloys.

For comparison, Tavares et al. [19] studied a family of alloys of Ti–35Nb–(x)Si, processed through casting followed by hot rolling and water quenching, and observed elongation of 29 ± 4 to 36 ± 6% for the alloys with 0–0.35Si. To achieve an elastic modulus of 60–72 GPa, Tavares et al. [19] employed a processing route that included casting, homogenization at 1000 °C for 8 h, hot rolling at 1000 °C, water quenching, and subsequent machining to prepare samples for tensile strength tests. The manufacturing process used in this study generates residual porosity, which further reduces material rigidity and helps mitigate stress shielding in implants. Additionally, the powder metallurgy processing route eliminated steps like the machining and hot rolling steps.

An increase in Nb content led to a reduction in the elastic modulus from an average of 68 GPa to 58 GPa. This reduction is primarily attributed to the stabilization of the β phase and the suppression of the ω phase, as reported in previous studies [6,39,59].

Borborema et al. [43], in their study on Ti–12Mo–xNb alloys, observed that a low Nb content of approximately 2 wt.% increased the elastic modulus (E) due to the suppression of martensitic α″ phases. However, as the Nb content exceeded 14 wt.%, the E began to decrease, coinciding with the increased stabilization of the β phase.

The decrease in mechanical properties, including the elastic modulus and ultimate tensile strength, can be explained by changes in the β phase crystal lattice. As Nb content increases, the bond forces within the β phase lattice are weakened, resulting in reduced alloy strength. This phenomenon has been corroborated by Yilmaz et al. [46], who linked the reduction in bond strength to decreases in both the elastic modulus and ultimate tensile strength.

Table 5. Comparison of cortical bone, alloys of this study, and commercially used alloys for implants.

Material Designation	UTS (MPa)	EL (%)	E (Gpa)
Cortical bone [6]	70–150	0.55–0.94	15–30
Ti0.10Si35Nb	412 ± 44	1.2 ± 0.6	68.6 ± 4.6
Ti0.10Si45Nb	350 ± 21	1.1 ± 0.3	54.4 ± 3.1
Ti0.35Si40Nb	396 ± 18	0.8 ± 0.5	64.0 ± 6.3
Ti0.60Si35Nb	392 ± 20	0.8 ± 0.2	66.6 ± 3.9
Ti0.60Si45Nb	361 ± 29	0.9 ± 0.2	59.5 ± 2.6
CP-Ti, grade 1 [60]	240	24	115
CP-Ti, grade 4 [60]	620	19	115
Ti6Al4V ELI, annealed [60]	930	16	114

UTS—ultimate tensile strength; EL—elongation at break; E—elastic modulus.

shows a comparison of mechanical properties of the studied materials, bone, and alloys used in implants. The alloys studied show values of E almost half of those of CP-Ti and Ti6Al4V and are much closer to the range of E values for cortical bone. The values for UTS compare favorably with CP-Ti grade 1 but are less than half of those of Ti6Al4V.

5. Conclusions

• A predominantly β phase alloy was successfully produced through pressing, sintering, and heat treatment, offering an alternative route to casting and rolling. Also, a lower sintering temperature, when compared to the work of Yilmaz et al. [46], of 1200 °C was used.
• The formation of Si precipitates and minor amounts of the α phase, as anticipated, was confirmed through SEM and XRD analyses.
• The influence of precipitate growth, predicted in the literature, was observed as a change in mechanical behavior at 0.35 wt.% Si during tensile testing.
• Further reductions in the elastic modulus may be achievable through the addition of other alloying elements like Sn and Ta.
• From the results analyzed, it can be concluded that complete diffusion occurred, with no elemental Ti, Nb, or Si observed. XRD analysis showed that Si was in solid solution for 0.10 wt.% and, at 0.6 wt.%, it formed precipitates of (Ti,Nb)₃Si.
• The results of the relative density anslysis showed that contents above 0.25 wt.% reduced relative density as the precipitates grew, also reducing the ultimate tensile strength and elastic modulus.
• The best result for relative density was 78.7 ± 0.8% and occurred at 0.35 wt.% Si and 35 wt.% Nb.
• The highest UTSs observed were at 35 wt.%Nb for both 0.10 and 0.35 wt.% Si.
• The lowest elastic moduli observed were for Nb at 45 wt.%, at 0.10 and 0.60 wt.% Si.
• SEM of the fractured surfaces showed a mainly ductile surface, but tensile strength tests showed very low elongation. This behavior was attributed to the highly porous structure, with porosity being the dominant effect.