Material Characterization and Technological Properties of Biocompatible Ti-12Al-42Nb Spherical Powder Alloy for Additive Manufacturing of Personal Medical Implants

Abstract

The paper focuses on material characterization and technology properties of a new Ti-12Al-42Nb spherical powder alloy for additive manufacturing of personal medical implants. The electrode induction melting inert gas atomization (EIGA) method was used to produce the powder alloy. The powder sphericity coefficient (PSC) was 1.02. Image J software was used to calculate the spherical degree by processing images sets from scanning electron microscopy (SEM) and optical microscopy (OM). SEM of particles cross-sections indicated internal thermal-induced porosity (TIP) with a 2.3 μm pore diameter. Particle size distribution was in the range from 15.72 μm (d10) to 64.48 μm (d100) as measured by laser particle analyzer. It was indicated that flowability and powder bulk density were 196 sec and 2.79 g/cm³, respectively. XRD analysis confirmed the beta phase of the powder alloy with no additional phases. X-ray fluorescence spectrometry confirmed the alloyed composition. Reducing and oxidative melting methods of analysis showed a slight amount of impurities: oxygen (0.0087 wt.%), nitrogen (0.03 wt.%), hydrogen (0.0012 wt.%), sulfur (0.0016 wt.%), and carbon (0.022 wt.%). Simultaneous thermal analysis (STA) was performed to indicate weight growth and losses and thermal effects in argon, nitrogen, and air as well as the oxidation of Al₂O₃, TiO₂, and Nb₂O₅ on the surface layer of Ti-12Al-42Nb powder alloy particles. Different phase transformations of γAl₂O₃ $\to$ θAl₂O₃ $\to$ αAl₂O₃ and TiO₂ rutile $\to$ TiO₂ anatase phase transformation were detected by STA in the oxidative layer.

1. Introduction

Around 70–80% of biomedical materials are made of metallic materials due to their biological and mechanical compatibility [1]. Ti-based alloys have been rapidly expanding because they are very promising metallic materials for biomedical implants due to their high mechanical properties, low specific gravity, good corrosion resistance, and high biocompatibility [2,3,4,5,6]. Ti-based alloys are currently widely used as structural materials for artificial hip joints, dental implants, bone plates, and screws. Many Ti-based alloys have been developed using biocompatible elements such as Ti, Zr, Nb, Mo, and Ta, which have shown excellent mechanical properties, including low Young’s modulus [7]. Young’s modulus depends on the stability of the β-phase, as discussed in detail in previous studies [8,9,10,11].

Since pure Ti and Ti-based alloys have been proven to be excellent biomaterials for implants, current research is aimed at improving the characteristics of these biomaterials by modifying their compositions with suitable alloying elements to maintain the required level of biocompatibility. Metals such as Ta, Nb, Zr, and in some cases Mo meet these requirements [12].

The addition of alloying elements such as Nb, Si, Mo, Ta, W, Fe, and Cr can stabilize the β phase [13]. Nb additives improve the mechanical properties of TiAl-based alloys with an increase in the Nb content in Ti–Al–Nb alloys from 40 to 50 at.%. Al leads to the formation of γ-TiAl and σ-Nb₂Al phases in microstructure, which can improve high-temperature mechanical properties. Previous studies have shown that modification of the γ + σ phase in microstructure can improve the fracture behavior in Ti–Al–Nb alloys with a high-volume fraction of the σ-phase [14].

Other researchers [15] focused on 34 different compositions of new titanium β alloys with different amounts of niobium (Nb) and other metals such as molybdenum (Mo), tantalum (Ta), and vanadium (V), and they carried out numerous biological tests to evaluate biocompatibility of Ti-based β alloys (

Table 1. Biocompatibility of metals in Ti-based alloys: titanium (Ti), aluminum (Al), niobium (Nb), vanadium (V), zirconium (Zr), molybdenum (Mo), tantalum (Ta), iron (Fe), copper (Cu), and zinc (Zn). Adapted from [15,16,17,18].

Element	Dilatation Test	Adherence Test	Screening
Ti	Compatible	Compatible	Compatible
Al	Tolerant	Tolerant	Toxic
Nb	Compatible	Compatible	Compatible
V	Tolerant	Toxic	Tolerant
Zr	Compatible	Compatible	Compatible
Mo	Tolerant	Tolerant	Tolerant
Ta	Compatible	Compatible	Compatible
Fe	-	Compatible	Toxic
Cu	Toxic	Toxic	Toxic
Zn	Toxic	Toxic	Toxic

).

Ti-based β alloys have very low elastic moduli between 40 and 80 GPa [1], which is very close to bone elastic moduli, helping to avoid bone absorption, which results in loosening of the implant or refracture of the bone after removal of the implant.

ASTM F67-13, ASTM F1472-20, ASTM F1580-18, ASTM F3049-14, and ASTM F1295-23 cover the chemical and other requirements for Ti-based powder alloys and Ti-based alloys for biomedical implants and other applications. Table 2 shows the chemical composition of commercially available Ti-based alloys for manufacturing of implants.

TiAlNb-based alloys are much more biocompatible than TiAlV-based alloys because of their Young’s modulus, which is very compatible with the Young’s modulus of bone [15].

It was found that Ti-40Nb possess close resemblance to the Young’s modulus of human cortical bone, which lies in the range of 7–30 GPa, exhibiting suitable characteristics as a bone fixation device.

It can be useful for the aerospace industry to calculate chemical composition as well as understand the creation mechanism of ultra or nanocrystalline structures with the advanced mechanical properties of former alloys.

Typically, Ti-based alloys have temperature- and pressure-induced phase transformations [15]. Nb is an isomorphic β stabilizer of Ti-based alloys because it has a lower transition temperature (T_c) from the α phase to the β phase due to added Nb [19].

All Ti-based alloys as well as TiAlNb-based alloys are easily passivated to form protective oxide or nitride layers depending on an annealing atmosphere, which ensures its high corrosion resistance. This protective ability of the oxide depends on the method of production and the environment [20], given that various processes occur in biofluids, such as adsorption and release of ions.

Table 2. Commercidally and laboratory-available biocompatible Ti-based alloys and their chemical composition: titanium (Ti), aluminum (Al), vanadium (V), niobium (Nb), oxygen (O), and nitrogen (N).

Alloy	Concentration, wt.%						References
	Ti	Al	V	Nb	O	N
Titanium Grade 4	Balance	-	-	-	≤0.4	≤0.05	[15,21]
Ti-6Al-4V	Balance	6.70	3.71	-	0.145	0.022	[15,21]
Ti-6Al-2Nb	Balance	6.11	-	2.42	0.141	0.041	[15,21]
Ti-6Al-4Nb	Balance	5.91	-	4.84	0.095	0.012	[15,21]
Ti-6Al-6Nb	Balance	5.85	-	7.16	0.145	0.031	[15,21]
Ti-6Al-7Nb	Balance	5.93	-	8.36	0.149	0.033	[15,21]
Ti-6Al-10Nb	Balance	5.90	-	10.45	0.083	0.014	[15,21]
Ti-10Al-42 Nb	Balance	9.52	-	42.24	-	-	[22]
Ti-11Al-44Nb	Balance	11.24	-	43.80	0.110	0.003	[23]
Ti-22Al-25Nb	Balance	22.78	0.05	41.37	0.008	0.002	[24,25,26,27]

Table 2. Commercidally and laboratory-available biocompatible Ti-based alloys and their chemical composition: titanium (Ti), aluminum (Al), vanadium (V), niobium (Nb), oxygen (O), and nitrogen (N).

Alloy	Concentration, wt.%						References
	Ti	Al	V	Nb	O	N
Titanium Grade 4	Balance	-	-	-	≤0.4	≤0.05	[15,21]
Ti-6Al-4V	Balance	6.70	3.71	-	0.145	0.022	[15,21]
Ti-6Al-2Nb	Balance	6.11	-	2.42	0.141	0.041	[15,21]
Ti-6Al-4Nb	Balance	5.91	-	4.84	0.095	0.012	[15,21]
Ti-6Al-6Nb	Balance	5.85	-	7.16	0.145	0.031	[15,21]
Ti-6Al-7Nb	Balance	5.93	-	8.36	0.149	0.033	[15,21]
Ti-6Al-10Nb	Balance	5.90	-	10.45	0.083	0.014	[15,21]
Ti-10Al-42 Nb	Balance	9.52	-	42.24	-	-	[22]
Ti-11Al-44Nb	Balance	11.24	-	43.80	0.110	0.003	[23]
Ti-22Al-25Nb	Balance	22.78	0.05	41.37	0.008	0.002	[24,25,26,27]

As with other titanium aluminide alloys, the mechanical properties of TiAlNb-based alloys are related to their microstructure, which often strongly depends on the manufacturing parameters. The microstructure of TiAlNb-based alloys produced by casting technology has significant defects, such as segregation, coarse microstructure, and shrinkage cavity. Powder metallurgy methods help to avoid these defects [28], but powder properties are also very important for selective laser melting (SLM) techniques to print medical implants for personal medicine. The technological properties of Ti-based powder alloys, such as chemical composition, particle size, flowability, and tap density, are the main factors that impact the quality of products processed by SLM [29].

The aim of the current work was to perform material characterization and carry out technological properties tests on the Ti-12Al-42Nb powder alloy to assess the possibility of its application in selective laser melting (SLM) technology to print samples of biocompatible implants for personal medicine.

2. Materials and Methods

Powder alloy production: The electrode induction melting inert gas atomization (EIGA) method was used to produce the Ti-12Al-42Nb powder alloy. In this method, a pre-alloyed bar (nominal diameter/length (mm): 50/500) rotates into an induction coil for melting, which results in the metal dripping from the bottom of the bar. As the drops fall into the atomization chamber, they are hit with high-pressure gas and transformed into spherical powders.

Particle size distribution: The particle size distribution of Ti-12Al-42Nb powder alloy was measured using the laser particle analyzer Analysette 22 Microtec Plus (Fritsch GmbH, Idar-Oberstein, Germany). The program device control was MaS Control V 1.0.0.1. (Fritsch GmbH, Idar-Oberstein, Germany)The particle analyzer was calibrated before each measurement set using an external standard—corundum powder F500 (fraction 0.3–300 μm)—and it was certified according to ISO 13320. Ti-12Al-42Nb powder samples were mixed with one drop of Dusasin 901 surfactant, and 50 mL of distilled water were added to make powder slurry before the set of measurements. To avoid segregation before the set of measurements, the slurry was sonicated with a 12 mm sonotrode using the ultrasound device UIP200 (Hielscher Ultrasonic GmbH, Teltow, Germany) for 3 min and 30 W.

Specific surface area (Ssp): The specific surface area of the Ti-12Al-42Nb powders was measured using the low-temperature argon adsorption BET method on a Tristar analyzer (Micromeretics, Norcross, GA, USA). The Autosorb system preprocesses the data obtained from measuring the volume and pressure and presents the results as a BET surface area. This method is most widely used to determine the surface area of solid materials and is based on the BET equation:

(1/W) × (P₀/P) − 1 = 1/(W_m C) + (C − 1) × P/(W_m CP₀),

(1)

where W—weight of the gas adsorbed at relative pressure; P/P₀—relative pressure; W_m—adsorbate weight. The parameter C is a constant value of the BET method, and it refers to the adsorption energy of the first layer, and therefore, the value of this parameter indicates the magnitude of the interaction between the adsorbent and the adsorbate.

True density measurements: The true density of Ti-12Al-42Nb powders was measured using an Ultrapyc 1200e gas pycnometer (Quantachrome, Boynton Beach, FL, USA) with an accuracy of ±0.03% in a high-purity helium atmosphere with constant gas flow. Cell calibration with a volume of 10 cm³ was performed using a 7.0699 cm³ stainless-steel sphere. True density was calculated using a built-in computing program based on the Archimedes displacement principle.

Electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS): The powder morphology was evaluated by SEM images obtained using a FEI Dual Beam Helios NanoLab system (FEI, Thermo Fisher, Waltham, MA, USA). Cross-sections were produced by a focused ion beam (FIB) technique, employing an FEI Dual Beam Helios NanoLab system (FEI, Thermo Fisher, Waltham, MA, USA). Elemental analysis was performed with EDAX system (USA).

Analysis of oxygen, nitrogen, hydrogen, sulfur, and carbon: The total content of oxygen, nitrogen, and hydrogen was determined using a TC–600 (LECO Corporation, St. Joseph, MI, USA) gas analyzer by the method of reducing melting in a nickel capsule in a graphite crucible in a flow of inert carrier gas, i.e., helium. Oxygen detection was carried out by the amount of CO and CO₂ by the infrared absorption method. Nitrogen detection was carried out by thermal conductivity. The total content of carbon and sulfur was determined on a CS–600 (LECO Corporation, St. Joseph, MI, USA) gas analyzer by the method of oxidative melting in a ceramic crucible in an induction furnace in the presence of a flux—a mixture of metallic tungsten, iron, and tin. Carbon and sulfur detection was by the amount of gaseous CO₂ and SO₂ released by the infrared absorption method.

X-ray fluorescence spectrometry: Elements analysis of Ti-12Al-42Nb powders were determined by X-ray fluorescence analysis using a sequential-wave dispersive X-ray fluorescence spectrometer S8 Tiger Series 2 (Bruker GmbH, Berlin, Germany). The spectrometer is equipped with an OEG 95LT X-ray tube with rhodium (Rh) anode with a maximum power of 4 kW and current up to 170 mA, crystal analyzers set, flow-proportional and scintillation detectors, collimators, as well as Al and Cu filters of various thicknesses. The spectrometer was controlled, and spectral data were processed using the Spectra Plus 5.3. (Bruker AXS Microanalysis GmbH, Berlin, Germany) software package, which includes Quant–Express software for semi-quantitative (standard-free) express analysis of samples of unknown composition.

Spherical powder degree (SPP): The spherical powder degree of the Ti-12Al-42Nb powders was determined using the ImageJ v. 1.53 program (National Institute of Health, Bethesda, MA, USA) by means of processing images from optical microscope as well as images from the scanning electron microscope. The shape particle parameter (SPP) of powder was calculated by the following formula:

SPP = P/S^0.5,

(2)

where P is the perimeter of the particle projection on the image, and S is the area of the particle projection on the image. The found parameter was compared with the parameter for an ideal circle: P_i = 3.54.

Powder sphericity coefficient (PSC): The powder sphericity coefficient (PSC) was measured as the ratio of average shape parameter of particle P_i for different powder particles to the shape parameter of an ideal circle:

PSC = SPP/(P_i)

(3)

The deviation of the powder sphericity coefficient (PSC) from 1 characterizes the deviation of the particle shape from spherical.

Powder bulk density analysis: The method for determining the bulk density of metal powders was performed according to ISO 3923-1-79. The essence of the method is that the mass of a certain amount of powder is measured in a freely poured state, which is obtained by filling a container with a funnel located above it at a certain distance and completely filling a container (glass) of a known volume. The ratio of mass to volume is the bulk density. The mass of the powder was determined with an accuracy of 0.05 g. The determination was performed on three test portions.

Bulk density is calculated by the following equation:

$$\rho =m/V$$

(4)

Flowability was quantitatively calculated by the flow time of a certain mass of powder (50 g) in seconds through a funnel with a calibrated outlet opening (2.5 mm) and an opening angle of 60° according to ISO 4490-78.

Simultaneous thermal analysis (STA): Simultaneous thermal analysis (STA) in an argon atmosphere (argon flow of 80 mL/min in the furnace, argon protective gas flow of 40 mL/min in the thermogravimetric block) was performed on an STA 449 C Jupiter thermal analyzer (NETZSCH GmbH, Waldkraiburg, Germany) using a DSC+TG sensor. STA in a nitrogen and air atmosphere (nitrogen and air flow of 50 mL/min in the furnace, nitrogen protective gas flow of 20 mL/min in the thermogravimetric block) was performed on an STA 449 F3 Jupiter thermal analyzer (NETZSCH, Germany) using a DTA+TG sensor. Before measurement in the argon and nitrogen atmosphere, the furnace was evacuated three times and purged with a flow of argon and nitrogen, respectively. The sample was analyzed in an Al₂O₃ ceramic crucible with a lid. An empty crucible was used as a reference. A measurement was performed with a baseline correction. The crucible containing the sample was heated at the rate of 1 °C/min to 40 °C, and the standby mode was maintained for 20 min until the mass readings were stabilized. After that, the crucible with the sample was heated at the rate of 10 °C/min to 1200 °C. Differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermogravimetry (TG), and differential thermogravimetry (DTG) signals were recorded during the heating process. The accuracy of the signals was ensured by the appropriate calibrations. In, Sn, Bi, Zn, Al, Ag, and Au reference materials were used for the temperature and sensitivity calibration. TG calibration was performed using a 2 g standard (Sartorius AG, Goettingen, Germany) for STA 449 C Jupiter and a built-in calibration weight for STA 449 F3 Jupiter. TG calibration was verified using calcium oxalate hydrate thermal decomposition.

Metallographic research: Metallography of the Ti-12Al-42Nb powder alloy for evaluation of internal microstructure and detection of defects (porosity, non-metallic, etc.) was performed on the optical microscope Polam P215 (LOMO LLC, Saint-Petersburg, Russia). Metallographic sections were taken to study the internal structure of powder particles. Pressing was performed on an IPA 40 evo automatic press, and grinding and polishing work was performed on a Buehler Phoenix 4000 machine.

X-ray phase analysis (XRD): The X-ray phase analysis (XRD) registration of the X-ray diffraction spectra of samples was carried out on the X-ray diffractometer “UltimaIV” (Rigaku Corp, Tokyo, Japan), with a vertical goniometer and high-speed semiconductor detector “D/teX” and Ni filter on the primary beam, in CuKα radiation in the range of angles of 2 θ from 9 to 100 degrees, with a shooting step of 0.02 degrees. Phase qualitative analysis and analysis of samples were performed in the Sleve software version 2022 (ICDD, Newtown Square, PA 19073, USA) package using the ICDD database.

Hardness (H) and elastic moduli measurements (E): Hardness and elastic moduli and the ratio of elastic work to the total work of indentation (η_it) of the Ti-12Al-42Nb powder alloys were measured according to ISO 14577 Metallic materials—Instrumented indentation test for hardness and materials parameters. The Ti-12Al-42Nb powder alloy was hot-poured into epoxy resin and then polished to a roughness at least 100 nm. The surface roughness was measured using a Surtronic Neox optical profilometer (Sensofar-Tech, Terrassa, Spain). A triangle Berkovich’s pyramid was used to do nanoindentation.

3. Results

SEM analysis showed that the Ti-12Al-42Nb powder alloy had predominantly spherical shape particles with a size from 2 to 60 microns, with a slight amount of irregular particles shapes with sizes from 38 to 40 microns indicated as well (Figure 1a,b). Ti-12Al-42Nb particles had heterogeneous surface relief and an isometric grain shape (Figure 1b).

SEM analysis of Ti-12Al-42Nb particle cross-sections indicated thermally induced porosity (TIP) with a pore size of 2 µm microns in diameter (Figure 2a). The main cause of the TIP was a slight amount of argon entrapped into the Ti-12Al-42Nb powder, which is very typical for the gas-atomization process of all titanium powder grades and cannot be detected by true density techniques or other standard density measurements.

The SEM images of the Ti-12Al-42Nb particles’ cross-section showed ultra- and microcrystalline structures with grain sizes from 0.61 to 2.36 µm and an isometrical shape with clearly visible grain boundaries (Figure 2b).

Figure 3a shows the selected area of interest on the surface of the Ti-12Al-42Nb powder alloy particles for carrying out EDS. This indicated an average content of oxygen (3.92 wt.%), nitrogen (2.45 wt.%), and carbon (2.77 wt.%), with errors of 27.67%, 25.77%, and 17.60%, respectively (Figure 3b). EDS showed minimal errors determining Ti (45.18 wt.% ± 2.78) and Al (12.3 wt.% ± 2.40), but in the case of Nb content, the weight percentage was 33.40 ± 7.46%.

X-ray elemental analysis indicated that the Ti-12Al-42Nb powder alloy comprised 45.7 wt.% of titanium, 12 wt.% of aluminum, 41.3 wt.% niobium, 0.6 wt.% of silicon, and 0.4 wt.% of phosphorous as impurities (

Table 3. X-ray elemental analysis of Ti-12Al-42Nb powder alloy. Weight percentage and statistical error of titanium (Ti), aluminum (Al), niobium (Nb), silicon (Si), phosphorus (P), iron (Fe), copper (Cu), and zinc (Zn).

Element	Weight, %	Statistical Error, %
Ti	45.70	0.75
Al	12.00	3.79
Nb	41.30	0.16
Si	0.60	12.3
P	<0.26	10.8
Fe	0.089	11.3
Cu	0.029	18.5
Zn	0.026	17.8

).

Analysis of Ti-12Al-42Nb powder alloy by reducing melting method showed very small amounts of impurities, such as oxygen 0.0087 ± 0.0018 wt.%, nitrogen 0.0360 ± 0.004 wt.%, and hydrogen impurities 0.0012 wt. ± 0.0002% (

Table 4. Analysis of oxygen (O), nitrogen (N), and hydrogen (H) by reducing melting method and analysis of sulfur (S) and carbon (C) content by oxidative melting method of Ti-12Al-42Nb powder alloy. Weight percentage and standard deviation.

Element	Weight, %	Standard Deviation
Oxygen	0.0087	0.0018
Nitrogen	0.0360	0.0040
Hydrogen	0.0012	0.0002
Sulfur	0.0016	0.0010
Carbon	0.0220	0.0003

).

Infrared absorption of CO₂ and SO₂ gases from Ti-12Al-42Nb powder alloy during oxidative melting indicated small impurities of sulfur 0.0016 ± 0.001 wt.% and carbon 0.022 ± 0.0003 wt.% (Table 4).

Figure 4a presents the XRD analysis showing that the Ti-12Al-42Nb powder alloy had hexagonal titanium beta phase-stabilized niobium, as verified by the COD 9008554 reference pattern. The Ti-12Al-42Nb powder alloy had average particle size distribution ranging from 15.72 (d10) to 64.48 (d100) microns (Figure 4b,

Table 5. Technological properties of Ti-12Al-42Nb powder alloy: average diameter, particle size distribution, fluidity of powder, powder bulk density, true density, spherical degree, and specific surface area (BET and calculated).

Parameter	Measurement Result	Measurement Units
Average diameter of particles with percentage fraction distribution	2–5 (0.28%)	µm
	5–10 (2.49%)
	10–20 (14.56%)
	20–45 (62.40%)
	45–75 (20.27%)
Particle size distribution	d3—10.28	µm
	d10—15.72
	d25—24.08
	d50—34.12
	d90—50.59
	d97—57.67
	d100—64.48
Fluidity of powder	196	sec
Powder bulk density	2.79	[g/c${\mathrm{m}}^3$]
True density	5.34	[g/c${\mathrm{m}}^3$]
Powder sphericity coefficient	1.02	-
Specific surface area (BET)	Not indicated	-
Specific surface area (calculated)	2209.79	[cm2/cm3]

).

Table 5 considers all technological properties of the Ti-12Al-42Nb powder alloy. The powder flow density meter indicated that fluidity was 196 sec. Powder bulk density was 2.79 g/cm³, and true density was 5.34 g/cm³.

The spherical degree of the powder was calculated and equaled 1.02. Specific surface area was not indicated by BET analysis due to the low value of the specific surface area of Ti-12Al-42Nb powder alloys, but it was calculated as 2209.79 cm²/cm³ using all measurement data from the laser particle analyzer (Table 5).

Analysis of optical microscopy microphotographs by means of Image J showed a triangle print of Berkovich’s pyramid on each Ti-12Al-42Nb spherical particle surface after a set of indentations. The particle surface roughness of the Ti-12Al-42Nb powder alloy was 100 nm before measurement by optical profilometer before running indentations. Nanoindentation showed that the Ti-12Al-42Nb powder alloy had a hardness (H) of 3.4 ± 1.1 GPa, elastic modulus (E) of 67 ± 19 GPa, indentation of maximum depth of 970 ± 150 nm, and ratio of elastic work to the total work of indentation (η_it) of 54 ± 18% (Figure 5,

Table 6. Nanoindentation results: maximum depth, hardness (H), elastic modulus (E), and ratio of elastic work to the total work of indentation (η_it).

Indentation	Hardness, GPa	Elastic Modulus, GPa	MaxDepth, nm	ηit, %
1	3.37	53.37	968.43	47.52
2	4.04	73.94	860.85	45.66
3	5.11	100.17	757.46	38.00
4	2.03	52.19	1175.07	83.59
5	2.77	58.10	1027.66	76.49
6	2.35	50.06	1115.79	51.44
7	3.89	80.73	862.75	37.81
Mean value	3.40 ± 1.10	67 ± 19	970 ± 150	54 ± 18

).

Figure 6 presents the result of simultaneous thermal analyses (STA) of Ti-12Al-42Nb powder alloy in argon, nitrogen, and air atmospheres.

STA analysis of the Ti-12Al-42Nb powder alloy in an argon atmosphere indicated exothermic peaks in the temperature range from 89.8 °C to 680 °C, with extremum temperatures at 412.0 °C and 633.1 °C. Slight endothermic effects were detected from 633.2 °C to 973.6 °C; then, exothermic effects were noted from 973.6 °C to 1200.0 °C, with maximum peaks at 1055.8 °C and 1147.0 °C. A small exothermic peak was indicated at 1147.0 °C as well. The sample weight was stable until 380 °C, and then, it gradually increased from 100 mass. % to 101.26 mass. % at 1200 °C (Figure 3a). Different peaks on the DSC curve were indicated in the temperature range from 400 °C to 700 °C, related to the particle surface’s slight oxidation of the Ti-12Al-42Nb powder alloy, with formation of TiO₂ and Al₂O₃. A γAl₂O₃ $\to$ θAl₂O₃ phase transformation occurred in a temperature range of 900–1100 °C as well as a θAl₂O₃ $\to$ αAl₂O₃ phase transformation in the temperature range of 1100–1200 °C.

STA analysis of the Ti-12Al-42Nb powder alloy in a nitrogen atmosphere showed slight visible exothermic peaks in the temperature range from 83.5 °C to 625.9 °C and an endothermic peak in the temperature range of 800–1200 °C, with the minimum at 973.8 °C, showing an endothermic effect from 625.9 °C until 935.6 °C. The DTA curve illustrated different exothermic peaks in the temperature range of 200–650 °C; then, it decreased sharply with a slight endothermic effect until 935.6 °C. The sample weight was stable until 900 °C in nitrogen, and it gradually increased from 100 mass. % to 106.3 mass. % at 1200 °C, and the DTA curve increased at a steady rate as well (Figure 3b). The nitrogen adsorbed at the surface diffused into the Ti-12Al-42Nb powder alloy with the formation of an interstitial solution of nitrogen on the surface of the particles until 500 °C. In the temperature range of 500–900 °C, the concentration of nitrogen on the gas/particle interface became greater, and a new Ti₂N phase formed. The phase transformation of Ti₂N into TiN began at 935.6 °C and continued until 1200 °C.

STA analysis of Ti-12Al-42Nb powder alloy in air illustrated exothermic peaks with extremum temperatures at 531.2 °C, 654.5 °C, 1000.4 °C, and 1084.7 °C. The sample oxidation started at 300 °C, and the sample’s weight increased sharply to 122.14 mass. % at 1200 °C (Figure 6c). The DTA curve indicated a peak at 531.32 °C related to the Ti-12Al-42Nb powder alloy’s oxidation and the formation of a titanium oxide (TiO₂) layer on the particle surface, mainly composed of anatase TiO₂. The next peak at 654.5 °C indicated the formation of aluminum oxide γAl₂O₃ as an additional intermediate layer on the Ti-12Al-42Nb particles. The DTA curve indicated a phase transformation of Al₂O₃ from γAl₂O₃ to θAl₂O₃ in the temperature range from 900.0 °C to 1041.4 °C, with the extremum at 1000.4 °C, as well as a phase transformation of TiO₂ from anatase to rutile in the temperature range from 1000 °C to 1200 °C. A phase transformation from θAl₂O₃ to αAl₂O₃ was indicated at the same temperatures, with the extremum at 1084.7 °C. The DTA curve showed a peak at 870.7 °C related to the formation of Nb₂O₅ during STA in air as well as NbO and NbO₂ in the temperature range from 500 °C to 800 °C (Figure 6c).

4. Discussion

SEM analysis confirmed the spherical shape of Ti-12Al-42Nb particles with a particle size from 2 to 60 µm, and this indicated thermal-induced porosity (TIP), as mentioned by previous authors [30].

EDS elemental quantification of light elements did not work correctly because of the inaccuracy of the absorption correction, and we detected large errors (Figure 3). Thus, a better way was using X-ray elemental analysis (Table 3) and the reducing/oxidative melting method to characterize the chemical composition of the Ti-12Al-42Nb powder alloy (Table 4).

There are some requirements for commercially available Ti-based powder alloys and TiAlNb-based wrought to use for medical applications. On the one hand, the ASTM F1580-18 standard specification has chemical requirements for Ti-6Al-4V powder alloys, such as impurities of oxygen (0.2 wt.% ± 0.02), carbon (0.08 wt.% ± 0.02), nitrogen (0.05 wt.% ± 0.02), hydrogen (0.015 wt.% ± 0.002), copper (0.10 wt.% ± 0.05), and iron (0.30 wt.% ±0.10). This is used for manufacturing by the plasma rotating electrode process, inert gas atomization, hydride–dihydride, or other methods capable of producing powder for further use in fabricating coatings on titanium alloy implants. On the other hand, the ASTM 1295-23 standard specification does not have many different chemical requirements for TiAlNb-based alloys, such as impurities of oxygen (0.2 wt.% ± 0.02), carbon (0.08 wt.% ± 0.02), nitrogen (0.05 wt.% ± 0.02), hydrogen (0.009 wt.% ± 0.002), iron (0.25 wt.% ± 0.10), and cobalt (0.10 wt.% ± 0.02), and other elements need not be reported unless the concentration level is greater than 0.1% each or 0.4% total. This standard is used for wrought annealed, cold-worked, or hot-worked Ti-6Al-7Nb alloy bar, wire, sheet, strip, and plate to be used in the manufacture of surgical implants. According to X-ray elemental analysis and the reduction/oxidative melting method, the Ti-12Al-42Nb powder alloy produced by EIGA met the chemical composition requirements of the standard specifications ASTM F1580-18 and ASTM 1295-23 (Table 4 and Table 5).

XRD analysis indicated the hexagonal titanium beta phase (Figure 4a), which was confirmed by other authors for very close chemical composition of TiAlNb-based alloys [31,32].

Table 6 focuses on all the technological properties of Ti-12Al-42Nb powder alloy particles, such as particle size, flowability, true density, and other main factors that impact the quality of products processed by SLM [29].

The laser particle analyzer showed normal distributions of Ti-12Al-42Nb powder alloy particle size: d10—15.72 µm, d50—34.12 µm, and d90—50.59 µm (Figure 4b, Table 6). Other authors [24,25,27] carried out the SLM of a Ti-22Al-22Nb powder alloy with only slightly different particle size distributions: d10—15.9 µm, d50—32.5 µm, and d90—58.2 µm.

It was confirmed that the wide range of fine and coarse particles effects powder bulk and powder packing density, but it reduces the powder flowability as a result of the effect of powder cohesion and inter-particle forces [33].

Ti-12Al-42Nb powder alloy was characterized next by its technological properties: flowability—196 sec, bulk density—2.79 g/c${\mathrm{m}}^3$, and true density—5.34 g/c${\mathrm{m}}^3$. Other authors [33,34,35] considered the same set of properties for SLM powders.

The powder sphericity coefficient (PSC) is a key factor for additive manufacturing techniques [36,37] because irregular or elongated shapes of particles reduce powder flowability and the homogeneity of the powder layer distribution. This influences the quality and density of the fabricated parts [33]. Image analysis indicated a 1.02 powder sphericity coefficient (PSC) of the Ti-12Al-42Nb powder alloy particles, which meets all requirements for SLM.

The microhardness (H) of Ti-12Al-42Nb powder alloy particles was 3.40 GPa ± 1.10, with an average maximal depth of indentation of 970 nm ± 150. The microhardness value corelates with that found in other works [32,38]. Previous authors [1,7,39,40] found the elastic moduli of different Ti-based alloys, and it was also considered that nanocrystalline hot-rolled biocompatible single-phase β-state TiNbZr-based alloy had an elastic modulus of 70 GPa [39], which was very close to the average elastic modulus (E) of the Ti-12Al-42Nb powder alloy—67 GPa ± 19.

It is well known that the different inert or reactive atmospheres and gas mixtures significantly influence on high-temperature oxidation intensity, microstructure, and phase composition of TiAlNb-based powder alloys produced by electrode induction melting inert gas atomization (EIGA) and other techniques. Thus, it is very important to know the thermal stability of the Ti-12Al-42Nb powder alloy in different atmospheres before selective laser sintering (SLM) and further annealing of personal medical implants samples. It is very significant to highlight that there is a great need for bioactive implants with antimicrobial properties for the treatment of patients with orthopedic pathology complicated by infection [41], which is why the oxidation process of the Ti-12Al-42Nb powder alloy with TiO₂ thin antibacterial layer formation has to be studied due to the antibacterial properties of Ti-based oxides.

STA in argon, nitrogen, and air atmospheres showed that all peaks on the DSC/DTA curves below 150 °C were related to free-bonded water and physically adsorbed water (Figure 6). STA in the air atmosphere indicated two peaks on the DTA curve at 80.5 °C and 143.1 °C, with the slight visible maximum at 126.8 °C (Figure 6c). The reason is that the free-bonded water evaporated until reaching 126.8 °C, and then, the physically absorbed water evaporated until 200 °C, which has been confirmed by other authors as well [42,43,44].

STA in the argon atmosphere, shown in Figure 6a, indicated slight oxidation of the Ti-12Al-42Nb powder alloy. Practically, the main reason is the slight oxygen content in argon inert gas, with a purity of 99.99993 wt.%. This is the reason for the small oxidation effect on the DSC curves and the formation of Ti-, Al-, and Nb-based oxide films on the particle surface, with mass increasing from 100 mass. % to 101.6 mass. % in the temperature range of 380–1200 °C.

STA in the nitrogen atmosphere, shown in Figure 6b, showed the same DTA effects discussed in other articles. Nitrogen and aluminum are strong stabilizers for titanium alloys. Aluminum was indicated to be primarily responsible for the formation of the elongated nitride grains [45]. Other authors [46,47] have carried out nitridation experiments with pure titanium to determine the phase composition and found that the phase transitions of the sample surface during nitriding can be written as αTi $\to$ α(N)Ti $\to$ Ti₂N $\to$ TiN. The nitrogen adsorbed at the surface diffuses into the Ti-12Al-42Nb powder alloy with the formation of an interstitial solution of nitrogen on the surface of particles until 500 °C [48]. In the temperature range of 500–900 °C, the concentration of nitrogen on the gas/particle interface became greater, a new Ti₂N phase formed, and further phase transformation of Ti₂N into TiN started from 935.6 °C and continued until 1200 °C [45,46,47,48,49,50,51].

In the case of Ti–Al–Nb alloys oxidation, TiO₂ was the predominant phase, while aluminum was probably only in the amorphous form. This result contradicts the beneficial effect of niobium in aluminum formation as described in the literature. It can be assumed that the addition of niobium can also improve the oxidation resistance of titanium alloys with a lower aluminum content [52,53].

Different peaks on the DSC/DTA curves, shown in Figure 6, were indicated in the temperature range from 400 °C to 700 °C and are related to the particle surface’s slight oxidation of the Ti-12Al-42Nb powder alloy with the formation of TiO₂ and Al₂O₃. A γAl₂O₃ $\to$ θAl₂O₃ phase transformation occurred in temperature range of 900–1100 °C as well as a θAl₂O₃ $\to$ αAl₂O₃ phase transformation in the temperature range of 1100–1200 °C, which has been confirmed by other researchers as well [54,55,56,57,58,59,60,61,62].

It is also well known that the oxidation of pure Nb and Nb-based alloys go through the next transformations chain: NbO_x $\to$ NbO_y $\to$ NbO $\to$ NbO₂ $\to$ Nb₂O₅. This is the reason that different Nb-based suboxides and NbO, NbO₂, and Nb₂O₅ formed during the oxidation in air of the Ti-12Al-42Nb powder alloy in the temperature range from 270 to 910 °C and higher temperatures. Non-stochiometric NbO_x and NbO_y formed in the temperature range from 270 to 500 °C and between 330 and 500 °C, respectively. NbO formation between 500 °C and 700 °C as well as NbO₂ formation between 650 °C and 810 °C were also indicated. NbO₂ suffers a reversible second-order phase transition between 797 °C and 808 °C, together with a change of the crystal structure into a regular rutile lattice. Nb₂O₅ formed from 800 °C onward [63,64,65,66,67].

Thus, material characterization and technological properties testing was carried out for the Ti-12Al-42Nb powder alloy. It was confirmed that all data have been proven by other researchers, and the Ti-12Al-42Nb powder alloy meets the requirements of ASTM F1472-20, ASTM F67-13, and ASTM F3049-14 for use in the additive manufacturing (SLM) of personal medical implants.

5. Conclusions

• XRD analysis showed that the Ti-12Al-42Nb powder alloy had hexagonal titanium beta phase-stabilized niobium, as verified by COD 9008554 reference pattern. X-ray elemental analysis indicated that the Ti-12Al-42Nb powder alloy contained 45.7 wt.% of titanium, 12 wt.% of aluminum, 41.3 wt.% niobium, 0.6 wt.% of silicon, and 0.4 wt.% of phosphorous as impurities;
• Thermal-induced porosity (TIP) with a pore size of 2 µm in diameter was indicated by means of scanning electron microscopy of particle cross-sections in the Ti-12Al-42Nb powder alloy produced by electrode induction melting inert gas atomization (EIGA). SEM indicated ultra- and microcrystalline structures with grain sizes from 0.61 to 2.36 µm and an isometrical shape with clearly visible grain boundaries;
• The Ti-12Al-42Nb powder alloy has the following technological properties: particle size distributions of d10—15.72 µm, d50—38.28 µm, and d100—64.48 µm; powder flow density meter—196 sec, powder bulk density—2.79 g/cm³; true density—5.34 g/cm³; powder sphericity coefficient—1.02; and calculated specific surface area—2209.79 cm²/cm³;
• Reducing melting method showed that the Ti-12Al-42Nb powder alloy had a very small amount of impurities, such as oxygen 0.0087 ± 0.0018 wt.%, nitrogen 0.0360 ± 0.004 wt.%, and hydrogen impurities 0.0012 wt. ± 0.0002%. Infrared absorption of CO₂ and SO₂ gases from the Ti-12Al-42Nb powder alloy during oxidative melting indicated small impurities of sulfur 0.0016 ± 0.001 wt.% and carbon 0.022 ± 0.0003 wt.%;
• Nanoindentation showed that the Ti-12Al-42Nb powder alloy had a microhardness (H) of 3.4 ± 1.1 GPa, elastic modulus (E) of 67 ± 19 GPa, indentation of maximum depth of 970 ± 150 nm, and ratio of elastic work to the total work of indentation (η_it) of 54 ± 18%;
• STA in argon showed that the Ti-12Al-42Nb powder alloy’s weight was stable until 380 °C, and then, it gradually increased from 100 mass. % to 101.26 mass. % at 1200 °C (Figure 3a). Different peaks on the DSC curve were indicated in the temperature range from 400 °C to 700 °C and were related to the particle surface’s slight oxidation of the Ti-12Al-42Nb powder alloy, with the formation of TiO₂ and Al₂O₃. A γAl₂O₃ $\to$ θAl₂O₃ phase transformation occurred in temperature range of 900–1100 °C as well as a θAl₂O₃ $\to$ αAl₂O₃ phase transformation in the temperature range of 1100–1200 °C;
• STA in nitrogen illustrated that the Ti-12Al-42Nb powder alloy weight’s was stable until 900 °C in nitrogen, and it gradually increased from 100 mass. % to 106.3 mass. % at 1200 °C, and the DTA curve increased at a steady rate as well (Figure 3b). The nitrogen adsorbed at the surface diffused into the Ti-12Al-42Nb powder alloy, with the formation of an interstitial solution of nitrogen on the surface of particles until 500 °C. In the temperature range of 500–900 °C, the concentration of nitrogen on the gas/particle interface became greater, and a new Ti₂N phase formed. The phase transformation of Ti₂N into TiN started at 935.6 °C and continued until 1200 °C;
• STA in air indicated the Ti-12Al-42Nb powder alloy’s oxidation started at 300 °C, and the sample’s weight increased sharply to 122.14 mass. % at 1200 °C (Figure 6c). The DTA curve indicated a peak at 531.32 °C related to the Ti-12Al-42Nb powder alloy’s oxidation and the formation of a titanium oxide (TiO₂) layer on the particle surface, mainly composed of anatase TiO₂. The next peak at 654.5 °C indicated the formation of aluminum oxide γAl₂O₃ as an additional intermediate layer on the Ti-12Al-42Nb particles. There was also formation of Nb-based suboxides NbO_x and NbO_y between the temperatures 270 °C and 500 °C and 330 °C and 500 °C, respectively, as well as Nb-based oxides such as NbO in the temperature range from 500 to 700 °C, NbO₂ in the temperature range of 650–810 °C, and Nb₂O₅ at 870 °C. Also, the DTA curve indicated a phase transformation of Al₂O₃ from γAl₂O₃ to θAl₂O₃ in the temperature range from 900 °C to 1041.4 °C, with the extremum at 1000.4 °C, as well as a phase transformation of TiO₂ from anatase to rutile in the temperature range from 1000 °C to 1200 °C. Also, a phase transformation from θAl₂O₃ to αAl₂O₃ was indicated at the same temperatures, with the extremum at 1084.7 °C.