Microstructural Analysis of Sintered Ti–Al–C Composite Powder Materials

1. Introduction

Titanium and its alloys are rapidly becoming an important research object in various fields such as the automotive and aerospace industries. These materials are lightweight and have excellent properties such as high specific strength, chemical resistance and biocompatibility. The combination of these attractive properties makes the alloys suitable for structural, chemical, heat resistance, tribological, and biomedical applications [1,2].

Titanium matrix composites (TMCs) are promising metallic materials widely used in various industrial fields, such as medicine, automotive and aircraft industries. TMC materials are characterized by exceptional properties, including relatively low density, as well as high levels of specific strength and corrosion resistance [3].

The high modulus of elasticity of titanium matrix composites is the main indicator of their use in airframes, and the high relative strength has been the impetus for their use in the engine industry. For example, titanium aluminide-based TMCs at 760°C are 50% lighter compared to nickel-based superalloys for high-temperature compressors [4,5].

Titanium-based alloys and composites often contain aluminium due to their excellent properties such as low density, high elastic modulus, high mechanical strength, and high fatigue resistance, which make them widely used in the aerospace, automotive, and transportation sectors [6,7,8].

Carbon, as a component of titanium alloys and composites, is an excellent heat-resistant material. For example, when used at temperatures above 1000°C, operation from 10 to several 1000 hours is possible, and sometimes approaches 2200°C [9]. The presence of carbon in titanium alloys and composites increases the heat resistance of these materials, increases strength and improves tribological properties.

In Ti-Al-C composites, various processing processes can lead to the formation of the MAX phase, which is a three-layer metal-ceramic material with excellent metal-ceramic properties such as high strength, high hardness, self-lubrication, good electrical and thermal conductivity [10,11,12]. Ti₃AlC₂ is a typical candidate of the MAX phase and has a multi-layered morphology [13].

Titanium aluminides, such as AlTi₃, Al₃Ti or TiAl can be found in Ti-Al-C composite materials. It was determined that during heating in molten salts, the synthesis the MAX phase compounds Ti₂AlC, Ti₃AlC₂, and Ti₂AlC proceeded between 900–1000°C with the formation of auxiliary phases TiC and TiAl, which decreased in abundance with rising the synthesis temperature [14]. After growing of the synthesis temperature to 1300°C, Ti₃AlC₂ appeared together with the phases Ti₂AlC and TiC. It was established that the synthesis of Ti₂AlC allows the formation of initial TiC_1-x on graphite and titanium aluminides [14].

The researchers studied the effect of Ti₃AlC₂ content on the physical, mechanical and arc ablation resistance properties of the produced Cu-Ti₃AlC₂ composite, and found that Cu-Ti₃AlC₂ composite properties such as relative density, electrical conductivity, tensile strength and Brinell hardness vary depending on the Ti₃AlC₂ content in the composites. Samples containing 10, 15, 20, 25, 30 and 35 % of Ti₃AlC₂ sintered at 800°C were tested. Since the Ti₃AlC₂ phase has intermediate metallic and ceramic properties, increasing its content in the composite increases the mechanical properties and then starts to decrease [15,16].

In another paper [17], researchers analysed the synthesis, deformation and tribological properties of a new Ti₃AlC₂ – Ti₂AlC MAX phase metal-ceramic composite. The two-phase composition of MAX was synthesized by SPS in a vacuum environment using Ti, Al, and C primary powders. A layered composite was formed, whose deformation mechanism and tribological properties were studied by SEM, TEM, Raman spectroscopy. A detailed analysis of the worn surface showed that the Ti₃AlC₂ - Ti₂AlC two-phase MAX composite is self-lubricating.

After reviewing the literature sources, we can see that Ti-Al-C and other similar systems are widely used in various fields and are constantly researched by scientists from various countries. These materials have excellent physical and mechanical characteristics. The materials we study are characterized not only by the fact that they contain MAX phases and titanium aluminides (intermetallic compounds) in their structure, but also thanks to exposure to a high-voltage electric discharge in kerosene, carbon nanostructures such as graphene, fullerenes, and etc., which is less studied. Therefore, the novelty lies on the specific development of technology of powder treatment allowing to obtain the new composites with improved physical and mechanical properties.

This study helps us to determine the phase composition and microstructure of Ti-Al-C composite powder materials produced by SPS consolidation of the charge after the HVED processing of titanium and aluminium powders in kerosene.

2. Materials and Methods

The samples were prepared from the powder with such composition: 70-75% of titanium and 15% of aluminium. The prepared powder was processed with HVED in liquid kerosene (method is described in detail in [18]). The mass relation of 100 g of hard phase and 1.5 l of kerosene was set to be 1:12. The parameters of HVED are as mentioned here: the stored energy E₁ = 1 kJ and the integral (total) energy E_sum = 25 MJ kg⁻¹ that is released during the processing of the disperse system. The processing schema is presented in Figure 1.

The samples were sintered in vacuum under the pressure of 50-60 MPa in a graphitic die between the graphitic electrodes (Figure 1b). The sintering duration was 5 minutes for all samples. The temperature and the current used are presented in Table 1. The samples were designated according to the temperature of sintering and the powder composition after HVED. The experimental plan was made, and the research was performed according to it.

After sintering of the powder, the samples (Figure 1c) were mounted on plastic, ground, and polished with the different suspensions containing 1μ, 0.25μ, and 0.05μ abrasive particles (supplied by Advanced Abrasives Corporation) followed by proper washing. They were explored using ZEISS EVO MA10 scanning electron microscope equipped with the Bruker XFlash 6/10 EDX detector.

The dimensions of cylindrical sintered samples were obtained as Ø8 mm × 5 mm.

Table 1. Data SPS of the powder samples under HVED.

Designation of specimens	Parameters of SPS			Composition of the powder after HVED treatment
	T, °C	I, A	t, min
G13	950	840	5	75% Ti, 15% Al3Ti, 10% Ti3AlC2
G14	960	770	5	70% Ti, 15% Al3Ti, 15% Ti3AlC2
13	985	995	5	75% Ti, 15% Al3Ti, 10% Ti3AlC2
14	1020	915	5	70% Ti, 15% Al3Ti, 15% Ti3AlC2

Table 1. Data SPS of the powder samples under HVED.

Designation of specimens	Parameters of SPS			Composition of the powder after HVED treatment
	T, °C	I, A	t, min
G13	950	840	5	75% Ti, 15% Al3Ti, 10% Ti3AlC2
G14	960	770	5	70% Ti, 15% Al3Ti, 15% Ti3AlC2
13	985	995	5	75% Ti, 15% Al3Ti, 10% Ti3AlC2
14	1020	915	5	70% Ti, 15% Al3Ti, 15% Ti3AlC2

An inVia Raman microscope (Renishaw) was used for Raman scattering measurements. The excitation beam was focused on the sample using a 50× objective. Silicon was used for the calibration of Raman setup. Here the parameters of laser:

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532 nm wavelength.

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Laser power at the sample surface - 3.5 mW.

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Integration time - 10 s.

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Signal was accumulated 10 times.

The Raman Stokes signal was dispersed with a diffraction grating (2400 grooves/mm) and data was recorded using a Peltier cooled charge-coupled device (CCD) detector (1024×256 pixels). This system yields a spectral resolution of about 1 cm^-1.

The XRD analysis was carried out on the D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany). Here are the parameters of the XRD setup:

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The tube voltage of 40 kV.

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The tube current of 40 mA.

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The X-ray beam was filtered with Ni 0.02 mm filter to select the CuKα wavelength (λ = 1.5406 Ǻ).

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Diffraction patterns were recorded using a fast-counting detector Bruker LynxEye based on silicon strip technology.

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Scanning over the range 2θ = 20–90° at a scanning speed of 6°/min using a coupled two theta/theta scan type.

The XRD data of the samples were analysed, and the quantitative determination of the phases was carried out with the Bruker AXS TOPAS 4.1 [19,20] software program using the Rietveld crystal structure refinement method.

3. Results and Discussion

The SEM analysis of the sintered Ti-Al-C powder composites showed that all the samples consisted of a metal matrix and reinforcing particles like carbides and MAX-phases (Figure 2a–c).

It is visible from the micrographs (Figure 2), that the Ti-Al-C powder composites with a heterogenous microstructure contain a huge number of reinforcing particles in the shape of spheres, plates, and needles. The lowest sintering temperature (950°C) gave the biggest number of micro-pores (Figure 2a). With the increased temperature to 960°C and more, the composite became denser with a much lower number of micro-pores (Figure 2b–d).

Figure 2. Metal matrix and reinforcing particles (carbides, and intermetallic phases) by SEM analysis: a) sample G13, sintered at 950°C; b) sample G14, sintered at 960°C; c) 13, sintered at 985°C; d) sample 14, sintered at 1020°C.

Figure 2. Metal matrix and reinforcing particles (carbides, and intermetallic phases) by SEM analysis: a) sample G13, sintered at 950°C; b) sample G14, sintered at 960°C; c) 13, sintered at 985°C; d) sample 14, sintered at 1020°C.

The X-Ray analysis results from the previous research work showed that strengthening particles were TiC, Ti₃Al and Al₄C₃ when the composition of the as received powder was 80% Ti, 15% Al₃Ti and 5% Ti₃AlC₂ [18]. For this research we did the elemental mapping of the microstructures with different compositions and sintered at different temperatures of SPS to reveal the distribution of every element in the microstructure and to get data about possible existing reinforcing particles. The mapping micrographs show that the needle-like structure contains more aluminium, and the spherical particles are rich with titanium and carbon (Figure 3). Therefore, we can conclude that the main reinforcing particles can be the carbides of titanium.

EDS analysis was performed by analysing separate reinforcing particles: spherical and needle-like fractions (Figure 4), and dispersed unknown elements detected at the grain boundaries – possible titanium carbides, located at the grain boundaries [21]. It was determined that the reinforcing particles mainly consist of titanium, however the peaks of carbon and aluminium are visible, especially when the elongated crystal plates where investigated. These crystal plates could be phases of titanium and aluminium [22].

Some micrographs show contamination with iron, possibly due to the presence of steel electrode (Figure 4a).

Figure 4. The EDS analysis of the samples: a) sample G13, sintered at 950°C; b) sample G13, sintered 960°C; c) sample 13, sintered at 985°C; c) sample 14, sintered at 1020°C.

Figure 4. The EDS analysis of the samples: a) sample G13, sintered at 950°C; b) sample G13, sintered 960°C; c) sample 13, sintered at 985°C; c) sample 14, sintered at 1020°C.

Raman spectroscopy was performed on the dark carbide particles, dark big areas, and light grains of the matrix. Raman spectroscopy analysis of the results was made by comparing the values of the intensity of the peaks obtained and the ones found in the literature resources. The review is presented in Table 2.

The results of Raman spectroscopy analysing the Ti-Al-C powder composites are presented in Figure 5.

The results of Raman spectroscopy showed that the dark areas of the Ti-Al-C powder composites revealed the peaks corresponding to the carbon nanostructures like fullerenes C₆₀ and C₇₀, graphene or crystalline graphite (Figure 5a,b). The strong peak at about 1580cm^-1 was detected in all samples, but it is hard to indicate the nature of the carbon nanostructure since some literature resource states that this peak corresponds to the graphene [32,34] and the others – to the crystalline graphite [27,35]. The literature resource [31] proves the existence of graphene which presents peak at about 1555 cm^-1. Graphene is believed to be one of the most promising materials that reinforce the metal matrix composites due to its strengthening of the sintered powder materials [24,26,32,34,37].

The areas with small reinforcing particles show different characters of the Raman curves [Figure 5c,d]. The figures present strong peaks at 1555 cm-1 and 1570 cm-1., which could be obtained due to the graphene [28,30,31].

Table 2. The identification of the peaks observed in Raman spectra.

	The position of the peak or the group of the peaks, cm-1	The identification of the peak	The references
	125	Ti3AlC2	[23]
1	148-154	Al-C alloy	[24], [25], [26]
	150	Ti2AlC	[23]
	183, 201	Ti3AlC2	[23]
2	208-213	Al(Ti)-C alloy	[24], [25], [26]
3	256-267	Al(Ti)-C alloy	[24], [25], [26]
	262, 268	Ti2AlC	[23]
4	271-273	C60	[27]
	270	Ti3AlC2	[23]
	365	Ti2AlC	[23]
5	415-431	Al(Ti)-C alloy	[24], [25], [26], [27]
6	488-497	C70	[27], [28]
7	608-613	Al(Ti)-C alloy	[24], [25], [26], [29]
	623	Ti3AlC2	[23]
8	632-633	Ti3AlC2	[22]
	663	Ti3AlC2	[23]
9	771-773	C60	[27], [30]
10	820-840	Al4C3	[24]
11	1234-1252	C60	[27], [30]
12	1331	Graphene (D band)	[31]
13	1340	Graphene (D band)	[30]
14	1350	Graphene (D band)	[32], [33], [34]
15	1355-1360	C60	[26]
16	1419-1422	C60	[27]
17	1462-1469	C60	[27], [28]
18	1555	Graphene (G band)	[31]
19	1575-1576	Graphene (G band)	[30], [28]
20	1580-1583	Crystalline graphite	[27], [35]
21	1580-1600	Graphene (G band)	[32], [34]
22	1584-1590	C70	[29], [33]
23	2700-2720	The overtone of the line at 1355 cm-1	[36], [34]
24	2930-2950	Ti-C alloy	[36]
25	2320	Ti-C alloy	[36]
26	2690-2700	Graphene (2D band)	[30], [32], [35]

Table 2. The identification of the peaks observed in Raman spectra.

	The position of the peak or the group of the peaks, cm-1	The identification of the peak	The references
	125	Ti3AlC2	[23]
1	148-154	Al-C alloy	[24], [25], [26]
	150	Ti2AlC	[23]
	183, 201	Ti3AlC2	[23]
2	208-213	Al(Ti)-C alloy	[24], [25], [26]
3	256-267	Al(Ti)-C alloy	[24], [25], [26]
	262, 268	Ti2AlC	[23]
4	271-273	C60	[27]
	270	Ti3AlC2	[23]
	365	Ti2AlC	[23]
5	415-431	Al(Ti)-C alloy	[24], [25], [26], [27]
6	488-497	C70	[27], [28]
7	608-613	Al(Ti)-C alloy	[24], [25], [26], [29]
	623	Ti3AlC2	[23]
8	632-633	Ti3AlC2	[22]
	663	Ti3AlC2	[23]
9	771-773	C60	[27], [30]
10	820-840	Al4C3	[24]
11	1234-1252	C60	[27], [30]
12	1331	Graphene (D band)	[31]
13	1340	Graphene (D band)	[30]
14	1350	Graphene (D band)	[32], [33], [34]
15	1355-1360	C60	[26]
16	1419-1422	C60	[27]
17	1462-1469	C60	[27], [28]
18	1555	Graphene (G band)	[31]
19	1575-1576	Graphene (G band)	[30], [28]
20	1580-1583	Crystalline graphite	[27], [35]
21	1580-1600	Graphene (G band)	[32], [34]
22	1584-1590	C70	[29], [33]
23	2700-2720	The overtone of the line at 1355 cm-1	[36], [34]
24	2930-2950	Ti-C alloy	[36]
25	2320	Ti-C alloy	[36]
26	2690-2700	Graphene (2D band)	[30], [32], [35]

The peaks observed in the region between 2300cm^-1 and 3000cm^-1 refer, probably, to the Ti-C alloy with the exception the peak at about 2713cm^-1, which is the overtone of the peak at 1355 (in our case 1346cm^-1 and 1351cm^-1) [36]. One sample showed the peak at about 632 cm^-1 (Figure 5c) and it could refer to Ti₃AlC₂ [22]. Furthermore, it is determined, that the MAX phase Ti₂AlC, which occurs below 900°C, transforms into MAX phase Ti₃AlC₂ when the temperature increases above 900°C [23]. In our case, sintering temperature is from 950°C to 1020°C, so, the existence of Ti₃AlC₂ could be possible.

Comparing the obtained results with the data found from scientific resources we can conclude that the proposed method of HVED treatment in the kerosene including SPS enables to obtain the metal matrix composites reinforced with the carbon nanostructures that develop the mechanical, electrical, thermal properties, such as heat wear resistance [32,34,37].

Figure 5. The Raman analysis of the samples with a composition: a) 75% Ti, 15% Al₃Ti, 10% Ti₃AlC₂ sintered at 950°C; b) 70% Ti, 15% Al₃Ti, 15% Ti₃AlC₂ sintered at 950°C; c) 75% Ti, 15% Al₃Ti, 10% Ti₃AlC₂ sintered at 985°C; d) 70% Ti, 15% Al₃Ti, 15% Ti₃AlC₂ sintered at 1020°C.

Figure 5. The Raman analysis of the samples with a composition: a) 75% Ti, 15% Al₃Ti, 10% Ti₃AlC₂ sintered at 950°C; b) 70% Ti, 15% Al₃Ti, 15% Ti₃AlC₂ sintered at 950°C; c) 75% Ti, 15% Al₃Ti, 10% Ti₃AlC₂ sintered at 985°C; d) 70% Ti, 15% Al₃Ti, 15% Ti₃AlC₂ sintered at 1020°C.

The detailed phase composition was obtained by X-Ray diffraction (Figure 6). It is worth noting that graphene cannot be detected by X-ray diffraction. Graphene is composed of a single layer (monolayer) of carbon atoms tightly bound together in a hexagonal lattice [38]. This is an allotropy of carbon. When graphene layers stack on top of each other, graphite is formed. Separated graphene layers in graphite are retained by Van der Waals forces, which can be broken up during exfoliation. Then you get graphene. At least two parallel planes of atoms are required for X-ray diffraction to occur. Therefore, graphene is almost undetectable in the X-ray diffractogram. So, where graphite is identified by XRD, it is graphite. Because the main most intense maximum at 2TH=26.40 degrees shows that the graphene layers are already packed into the graphite lattice.

Considering the carbon nanostructures detected in the Ti-Al-C powder composites, we can conclude that different allotropies of carbon were found in the microstructure – graphene, fullerenes, and graphite.

Two types of the intermetallic phases and one type of carbide were detected existing in the microstructure: AlTi₃, Al₃Ti, and TiC, respectively.

Figure 6. The XRD analysis of the samples: a) sample G13, sintered at 950°C; b) sample G14, sintered at 950°C; c) sample 13, sintered at 985°C; d) sample 14, sintered at 1020°C.

Figure 6. The XRD analysis of the samples: a) sample G13, sintered at 950°C; b) sample G14, sintered at 950°C; c) sample 13, sintered at 985°C; d) sample 14, sintered at 1020°C.

The samples G13 and G14 showed a lot of graphite formed in the composition of the material, then the hardness will decrease, as it did not react with titanium. This means that when processing the powders, the specific energy was higher than necessary and excess graphite appeared. It is necessary to regulate the amount of energy during processing so that there is no excess graphite, and it can all react during processing, and the small amount that remains during sintering. Therefore, the parameters of the technology of HVED+SPS should be improved.

Despite the high content of the graphite, the microhardness measured in the previous work [18] was obtained from 400 to 600 HV. This means that the graphite is equally distributed and does not decrease the mechanical properties of the material.

XRD analysis of the samples showed that they are composed of different amounts of intermetallics like Al₃Ti and AlTi₃ (Figure 6). These materials have high melting points, high mechanical strength, oxidation resistance and low densities [39]. The phase composition of the different samples treated under different technological parameters obtained from XRD results are presented in Figure 7. Comparing the contents of Al₃Ti and AlTi₃ the AlTi₃ is dominant, which is less stable than the Al₃Ti.

5. Conclusions

This research was focused on the dependence of SPS parameters on the final phase composition and microstructure of the Ti-Al-C powder materials. This study investigated the Ti-Al-C composite powder materials produced by SPS consolidation of the powder mixture after the HVED processing of titanium and aluminium powders in kerosene. An experimental plan was developed which allowed us to establish the relationship between experimental parameters and results.

The SEM analysis showed the microstructure with a big number of reinforcing particles - shape of globular particles and elongated needles distributed randomly in clusters all over the surface as a matrix. Samples have few pores on their surfaces. The lower temperatures gave more pores. The elemental mapping of the Ti-Al-C composite powder materials allowed to determine the distribution of Ti, Al, and C in the microstructure. It was observed that the spherical particles consisted mainly of titanium and carbon since the elongated grains revealed mostly aluminium and less titanium. Here, the line intensity of carbon was obtained to be low. Probably, the phases of TiC and Ti₃AlC₂ were detected, carbide and MAX-phase, respectively.

Raman analysis of the composites revealed the existence of carbon nanostructures in the microstructure. The dominant carbon nanostructure was graphene and fullerenes C₆₀ and C₇₀. These nanostructures increase strength and tribological properties of the material. Some detected peaks were not determined and will be investigated in further study.

XRD analysis showed that samples contain such phases like TiC, AlTi₃, Al₃Ti and that strengthen material and increase heat resistance of composites. The samples show the formed different amount of graphite depending on the sintering temperature – increasing temperature decreases the excess graphite.

Considering the carbon nanostructures detected in the Ti-Al-C powder composites, we can conclude that different allotropies of carbon were found in the microstructure – graphene, fullerenes, and graphite. The previous investigation showed that the microhardness was sufficient – 400-600 HV, it means that the graphite is distributed equally and does not decrease the mechanical properties of the material. Despite of that, the technological parameters of the HVED+SPS procedure should be improved.

Further investigation of Ti-Al-C composites will be performed according to the tasks of the joint project of Lithuania and Ukraine.