1 Introduction

Nowadays, to ensure required operating life of parts and products in hard operating conditions, there is a need to use high-strength and oxidation-resistant materials like advanced ceramic materials (silicon carbide, silicon nitride, alumina, zirconia, etc.) and various composites. Owing to their high-temperature strength and chemical inertness, these materials can be operated under high pressure or vacuum, high temperature, radiation, corrosion, etc. [1,2,3,4,5,6,7,8,9,10].

In contrast to conventional titanium alloys, an operating temperature of which is limited by 300–500 °C, advanced Ti-based composites possess high fracture toughness and strength under bend and tensile loading in the temperature range of 20–650 °C. Owing to their high strength-to-weight ratio, these composites are promising for applications in components of modern engines (aircraft, rocket, and internal combustion ones) as well as other power equipment (compressors, gas turbines, fuel cells, etc.) [11,12,13,14,15,16,17,18,19,20,21,22,23]. However, there is a need to increase their operating temperature range up to 700–800 °C. Under such conditions, these materials must meet the requirements on high strength and crack growth resistance as well as corrosion resistance [11, 24,25,26]. This should be taken into account while developing new materials and improving microstructure and mechanical properties of already existing ceramics and composites [27,28,29,30,31,32,33,34]. The substantiation of chemical composition and processing and treatment modes are crucial issues in improving the phase compositions, microstructure, and mechanical properties of the developed materials [35,36,37,38,39,40,41,42,43,44].

Depending on the chemical composition, Ti and Cr-based composites may comprise some amount of high-temperature phases, namely, silicides, aluminides, MAX phases, etc. [45,46,47,48,49,50,51,52,53,54,55,56,57]. In particular, MAX phases are formed as quite distinct regions along the boundaries of titanium or chromium grains in Ti and Cr based composites [53, 58, 59]. It was shown in a number of works that MAX phases are of great practical interest in terms of creating materials for use in mechanical engineering, aerospace and nuclear industries [2, 59, 60]. MAX phases are ternary layered compounds corresponding to the conditional formula Mn+1AXn (n = 1, 2, 3 …), where M is a transition metal of the d-group (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta), A is an element of the p-group (Si, Ge, Al, Ga, S, P, Sn, As, Cd, I, Tl, Pb), and X is carbon or nitrogen [61].

Among a variety of MAX phases, Ti3SiC2 MAX phase is not an ideal self-healing material. Its self-healing performance can be improved somewhat by partially replacing Si with Al due to the faster diffusion and oxidation of Al and a good oxidation resistance of Al2O3. By partially replacing the A atoms of Ti3SiC2 with Al, Ti3Si1-xAlxC2 solid solutions are formed [18, 58, 62] that improves oxidation resistance of the material. Besides, in Ti2AlC MAX phase, the oxidation temperature of Al could be lowered to 900 °C by partially replacing Al with Sn [18, 55, 58, 63]. At temperatures below 600 °C, SnO2 can be already formed in Ti2Al1-xSnxC. This may serve as an attractive crack filling potential for Ti2Al1-xSnxC at lower temperatures. However, no significant increase in the mechanical strength was found in such a composite due to the poor mechanical strength of the SnO2 and the poor bond strength [58, 63].

It is known that MAX phases belong to the hexagonal crystal system. Similarly to other materials belonging to the hexagonal crystal system [57, 58, 61, 62], there exists a possibility to fabricate textured bulk MAX phases. Pressure-assisted sintering, e.g., HP and SPS, are the most widely used techniques to prepare textured bulk MAX phases. As a result, the material consists of plate-like grains preferentially oriented under external conditions (e.g., a magnetic field or a uniaxial pressure). In contrast to the mentioned fabrication techniques, spark plasma sintering or direct hot pressing of MAX phases does not allow formation of highly textured microstructures, but may result in formation comparatively distinct MAX phase regions along the boundaries of titanium grains in Ti and Cr-based composites [13, 58, 59, 64,65,66,67].

It is also known that textured bulk MAX phases have anisotropic properties due to the lamellar crystal structure [58, 61]. This is a reason of high mechanical strength and crack growth resistance of these materials along specific directions and make them applicable to more harsh service environments.

Strength and wear resistance tests of ceramic and composite materials are widely used to estimate the bearing capacity of the corresponding products [68,69,70]. However, to prevent the degradation of microstructure of materials in environmentally assisted harsh conditions [71, 72], there is a need to obtain material resistive to microstructural changes in such conditions [6, 73,74,75,76,77,78]. Microhardness and fracture toughness serve as characteristics of material for estimating its resistance to the nucleation and growth of microcracks. For this purpose, the indentation test as one of the simplest known mechanical method is used [68, 79, 80]. Based on the indentation technique, a variety of loading schemes and formulas for calculating fracture toughness of materials were proposed [70,71,72, 79, 81, 82]. Thus, to develop a new ceramic or composite material with required physical and mechanical properties, it should be studied in terms of strength and crack growth resistance and their relations to the chemical and phase compositions.

The work is aimed at evaluating the effect of ultrafine alloying elements on phase composition, high-temperature strength, fracture toughness, and fracture micromechanisms of Ti–Si–X and Ti–Cr–X composites.

2 Materials and Methods

In this work, titanium-based composites of various systems have been studied. The composites were prepared from raw materials by arc melting in an argon atmosphere on a water-cooled copper hearth [59, 64]. The purity of the elements was as follows: Ti > 99.6 at%; Cr > 99.99 at%; and Si, C, Al, Sn, Zr > 99.99 at%. After melting, ingots were annealed at 1400 °C for 5 min. In some cases (composites 2 and 3, Table 1), ingots were rolled at a temperature of 1050 °C with applying the thermo-mechanical deformation of about 40%.

Table 1 Types of the investigated composited and their marking
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Beam specimens 5.0 × 7.5 × 50 mm in size were machined from ingots or blanks, grinded, and polished (Table 1).

Strength tests of beam specimens were performed under three-point bending in a temperature range of 20–1000 °C. The fracture stresses (σf) were calculated using the “stress–flexure” diagrams at P = Pmax by the equation [25, 32, 36, 76]

$$ \sigma_{f} = \frac{{1.5 \cdot P_{\max } \cdot L}}{{b \cdot t^{2} }} $$
(1)

where Pmax is the maximum load (N); L is the span between two supporting rollers (mm); b and t are the specimen width and thickness (mm).

To characterize crack growth resistance of materials [83,84,85,86], along with microhardnes [87, 88], a fracture toughness characteristic, namely, the critical stress intensity factor (SIF) KIc is often used. One of the simplest methods of estimation of the fracture toughness is an indentation method implementing a variety of formulas for calculating the SIF [89,90,91,92,93,94,95,96,97,98]. For a lot of materials, the KIc values calculated by some of these formulas are consistent with those obtained by conventional methods of fracture mechanics [79, 98, 99]. Among the last ones, a single-edge notch beam (SENB) test [100,101,102] is widely used to estimate fracture toughness of ceramic and composite materials. This method was thoroughly described in [99].

In our work, fracture toughness tests of specimen series were performed under three-point bending in a temperature range of 20–900 °C using the mentioned SENB test method. For estimating the critical SIF of materials corresponding formulas [100,101,102] were used.

At least three specimens were used for each test temperature of corresponding test methods.

A scanning electron microscope (SEM) Carl Zeiss EVO-40XVP equipped with an INCA Energy 350 system was used for the study of microstructure and fracture surface morphology of specimens and an energy-dispersive X-ray (EDX) microanalysis of chemical composition of the materials both in secondary electron (SE) and back-scattered electron (BSE) imaging modes.

X-ray powder diffraction data was obtained by using a X-ray diffractometer (Aeris, Malvern Panalytical) with Cu Kα radiation operated at a voltage of 40 kV and a current of 15 mA. The angular range was 20°–90° and a step was 0.0217°. The X-ray diffraction (XRD) phase analysis was perfomed using Highscore software and referenced with the International Center for Diffraction Data (ICDD). All procedures including indexing, structure solutions, and refinement of profile and structural parameters were performed with the WinCSD [103] program package.

3 Results and Discussion

The microstructure and mechanical behavior of the titanium-based composites of Ti–Si–X and Ti–Cr–X systems in a wide temperature range have been studied.

3.1 XRD Analysis of the Studied Composites

Ti–Si–Al–Sn–C composite (1). The XRD patterns of the composites under study (Fig. 1) show in detail the peculiarities of their phase balances. The XRD pattern of composite 1 (Ti–Si–Al–Sn–C system) contains only peaks of the α-Ti and TiC0.67 phases (Fig. 1a). The α-Ti phase percentage was found to be about 90 wt%, whereas the TiC0.67 phase percentage was about 10 wt%. The morphology of these phases was investigated in details using the microstructure images made at various magnifications. At a low magnification, one can see quite homogeneous microstructure of the Ti–Si–Al–Sn–C composite (Fig. 2a). At a higher magnification, we can observe distinct microstructural components of this composite (Fig. 2b). According to a general EDX analysis (spectrum 1 in Fig. 2b and Table 2), this material contains 87.41 wt% Ti, 6.09 wt% Al, 3.15 wt% Si, and 3.35 wt% Sn. The results of EDX analysis showed some difference in chemical composition of the Ti–Si–Al–Sn–C composite material as compared to the results of XRD analysis. According to EDX analysis (Fig. 2b and Table 2), this material is a metal-matrix composite of Ti–Si–Al–Sn–C system with high titanium content. It possibly comprises the titanium matrix phase, Ti5Si3 phase, Ti3SiC2 MAX phase, and titanium carbide phase.

Fig. 1
figure 1

XRD patterns of the investigated composites a 1, b 2, c 3, and d 4 (Table 1) showing peaks for the α-Ti (light circles), Ti(Cr) (dark circles), TiC0.67 (light triangles), and Ti3SiC2 phases (dark triangles) and corresponding Miller indices (in parentheses)

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Fig. 2
figure 2

SEM a, b microstructures (SE images) with marked zones of b general (spectrum 1) and local (spectra 2, 3, 4, and 5) EDX analyses, and c–f fractography (SE images) of specimens of composite 1 after fracture toughness tests at c, d 20 °C and e, f 650 °C (Table 1)

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Table 2 The data of the EDX spectra 1–5 marked in Fig. 2 for a specimen of composite 1 (Table 1)
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The titanium phase (α-Ti of about 87 wt% Ti, spectrum 2 in Fig. 2b and Table 2) with some amounts of aluminum (6.26 wt%), silicon (2.39 wt%), and tin (3.88 wt%) is a matrix phase. The total amount of the α-Ti phase estimated optically using the microstructure image (Fig. 2b) is about 66–70 vol%.

The Ti5Si3 phase with small amounts of aluminum (4.35 wt%) and tin (2.2 wt%) looks like thin elongated curved areas of light-gray color about 25 µm in length (spectrum 3 in Fig. 2b). These areas are located at the boundaries of titanium lamella packets. The total area occupied by them (Fig. 2b) is about 11–12 vol%. The Ti5Si3 phase was not detected by XRD analysis because of its small content.

The Ti3SiC2 MAX phase with small amounts of aluminum (3.25 wt%) and tin (2.05 wt%) looks like thick elongated areas of dark-gray color about 15 µm in size (spectrum 4 in Fig. 2b). These areas are located at the boundaries of titanium lamella packets similar to the Ti5Si3 phase. The total amount of the MAX phase (Fig. 2b) is about 14–15 vol%. Both the Ti5Si3 phase and the Ti3SiC2 MAX phase were not detected by XRD analysis because of their small content.

The titanium carbide phase (TiC0.67, spectrum 5 in Fig. 2b) is in the form of distinct round-shaped particles of dark-gray color about 3 µm in size. The particles are distributed uniformly both in the matrix the α-Ti phase and at the boundaries of titanium lamella packets. The total area occupied by them (Fig. 2b) is about 5–7 vol%. Besides, small amounts of the Ti5Si3 phase and the Ti3SiC2 MAX phase are probably neighboring these carbide phase particles since some amount of silicon (9.68 wt%), along with small amounts of aluminum (2.74 wt%), iron (0.21 wt%), cadmium (1.31 wt%), and tin (1.64 wt%), were also detected in these areas.

Ti–Si–Al–Zr–C composite (2). The XRD pattern of composite 2 (Ti–Si–Al–Zr–C system) contains peaks of the α-Ti, TiC0.67, and Ti3SiC2 MAX phases (Fig. 1b). Its phase composition was found to be as follows: α-Ti phase (about 70 wt%), TiC0.67 phase (about 12 wt%), and Ti3SiC2 MAX phase (about 18 wt%). The microstructure image of the Ti–Si–Al–Zr–C composite made at a low magnification showed its homogeneous microstructure (Fig. 3a). The microstructure image of a higher magnification presents uniformly distributed areas of arbitrary shapes differing in colors (Fig. 3b). A general EDX analysis (spectrum 1 in Fig. 3b and Table 3) showed 76.79 wt% Ti, 6.77 wt% C, 3.88 wt% Al, 5.75 wt% Si, and 6.81 wt% Zr in this material. Thus, this material is a metal-matrix composite of Ti–Si–Al–Zr–C system possibly comprising the titanium matrix phase, (Ti, Zr)5Si3 phase, Ti3SiC2 MAX phase, and titanium carbide phase.

Fig. 3
figure 3

SEM a, b microstructures (SE images) with marked zones of b general (spectrum 1) and local (spectra 2, 3, and 4) EDX analyses, and c–f fractography (SE images) of specimens of composite 2 after fracture toughness tests at c, d 20 °C and e, f 700 °C (Table 1)

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Table 3 The data of the EDX spectra 1–4 marked in Fig. 3 for a specimen of composite 2 (Table 1)
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The α-Ti phase (89.65 wt% Ti, spectrum 2 in Fig. 3b and Table 3) with some amounts of aluminum (5.14 wt%), silicon (1.1 wt%), and zirconium (4.11 wt%) is a matrix phase. The total amount of the α-Ti phase (Fig. 3b) is about 52–55 vol%.

The (Ti, Zr)5Si3 phase looks like round-shaped particles of dark-gray color about 5 µm in size (spectrum 3 in Fig. 3b). These areas are adjacent to the titanium lamella packets. The total area occupied by them (Fig. 3b) is about 4–6 vol%. Besides, small amounts of the Ti3SiC2 MAX phase and the TiC0.67 phase are probably neighboring these particles since some amount of carbon (6.97 wt%), along with a small amount of aluminum (1.46 wt%), was also detected in these areas. However, the (Ti, Zr)5Si3 phase was not revealed by XRD analysis because of its small percentage.

The titanium carbide phase TiC0.67 with small amount of the Ti3SiC2 MAX phase and also small amounts of aluminum (5.09 wt%) and zirconium (2.69 wt%) looks like textured bulk MAX phase regions about 35–60 µm in size consisting of thin lamellae (spectrum 4 in Fig. 3b). These regions are uniformly distributed in the titanium matrix. The total amount of these regions (Fig. 3b) is about 35–45 vol%.

Ti–Si–Al–Zr–C composite (3). The XRD pattern of composite 3 (Ti–Si–Al–Zr–C system) is similar to that of composite 2 and contains peaks of the α-Ti, TiC0.67, and Ti3SiC2 MAX phases (Fig. 1c). Its phase composition is as follows: α-Ti phase (about 75 wt%), TiC0.67 phase (about 17 wt%), and Ti3SiC2 MAX phase (about 8 wt%). The microstructure image of the Ti–Si–Al–Zr–C composite made at a low magnification showed a quite homogeneous microstructure with some resemblance to the microstructure of composite 2 (Fig. 4a). The microstructure image of a higher magnification presents randomly distributed areas of arbitrary shapes (Fig. 4b). As a result of a general EDX analysis (spectrum 1 in Fig. 4b and Table 4), 83.1 wt% Ti, 5.07 wt% Al, 5.94 wt% Si, and 5.89 wt% Zr were found in this material. Unexpectedly, no signs of carbon were detected. Like composite 2, this material is a metal-matrix composite of Ti–Si–Al–Zr–C system possibly comprising the α-Ti matrix phase, (Ti, Zr)5Si3 phase, Ti3SiC2 MAX phase (only according to XRD analysis), and titanium carbide phase.

Fig. 4
figure 4

SEM a, b microstructures (SE images) with marked zones of b general (spectrum 1) and local (spectra 2, 3, and 4) EDX analyses, and c–f fractography (SE images) of specimens of composite 3 after fracture toughness tests at c, d 20 °C and e, f 700 °C (Table 1)

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Table 4 The data of the EDX spectra 1–4 marked in Fig. 4 for a specimen of composite 3 (Table 1)
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The α-Ti matrix phase (89 wt% Ti, spectrum 2 in Fig. 4b and Table 4) with some amounts of aluminum (5.99 wt%), silicon (1.07 wt%), and zirconium (3.94 wt%) is presented in an amount of about 56–60 vol%.

The (Ti, Zr)5Si3 phase with a small amount of aluminum (2.07 wt%) looks like particles of arbitrary shapes about 5–25 µm in size united in colonies or distributed randomly (spectrum 3 in Fig. 4b). They occupy the total area of about 26–30 vol% (Fig. 4b). For an unknown reason, this phase was not detected by XRD analysis.

The TiC0.67 phase with a small amount of the Ti3SiC2 MAX phase and some amounts of aluminum (5.09 wt%) and zirconium (2.69 wt%) looks like particles of arbitrary shapes about 1–5 µm in size distributed randomly in titanium matrix (spectrum 4 in Fig. 4b). The total amount of these particles (Fig. 4b) is about 14–18 vol%.

Ti–Cr–Al–C composite (4). The XRD pattern of composite 4 (Ti–Cr–Al–C system) contains peaks of the Ti(Cr) and TiC0.67 phases (Fig. 1b). Its phase composition was found to be as follows: Ti(Cr) phase (about 78 wt%) and TiC0.67 phase (about 22 wt%). At a low magnification, distinctly grained microstructure of the Ti–Cr–Al–C composite can be observed (Fig. 5a). The microstructure image of a higher magnification presents grains of a matrix phase with uniformly distributed tiny particles inside and the fringe-like grain boundary regions. The fringes consist of needle-shaped particles differing in colors (Fig. 5b). A general EDX analysis (spectrum 1 in Fig. 5b and Table 5) showed 54.4 wt% Ti, 37.34 wt% Cr, 5.12 wt% C, and 3.14 wt% Al in this material. The material presenting a metal-matrix composite of Ti–Cr–Al–C system possibly comprises the Ti(Cr) matrix phase, Al2O3 phase, and titanium/chromium carbide phase.

Fig. 5
figure 5

SEM a, b microstructures (SE images) with marked zones of b general (spectrum 1) and local (spectra 2, 3, 4, and 5) EDX analyses, and c–h fractography (SE images) of specimens of composite 4 after fracture toughness tests at c, d 20 °C, e, f 700 °C, and g, h 800 °C (Table 1)

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Table 5 The data of the EDX spectra 1–5 marked in Fig. 5 for a specimen of composite 4 (Table 1)
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The Ti(Cr) matrix phase comprises titanium (57.3 wt%, spectrum 2 in Fig. 5b and Table 5) and chromium (39.2 wt%) with some amount of aluminum (3.5 wt%). The total amount of the Ti(Cr) phase (Fig. 5b) is about 66–72 vol%.

The titanium/chromium carbide phase and the Al2O3 phase (spectrum 3 in Fig. 5b) containing in total 49.17 wt% Ti, 25.14 wt% Cr, 14.3 wt% C, 8.08 wt% O, and 3.31 wt% Al were detected at the grain boundaries in the areas of black color. The total amount of these areas (Fig. 5b) was about 4–6 vol%.

A local EDX analysis of a round-shaped particle of dark-gray color about 1 µm in size (spectrum 4 in Fig. 5b and Table 5) showed 50.98 wt% Ti, 34.05 wt% Cr, 11.99 wt% C, and 2.98 wt% Al. This particle probably was the titanium carbide phase TiC0.67 identified by XRD analysis, whereas some amounts of chromium and aluminum were detected by EDX analysis in the surrounding Ti(Cr) matrix phase. The total amount of these carbide particles was about 10–12 vol%.

The thin needle-shaped particles of light-gray color about 15 µm in length (spectrum 5 in Fig. 5b) forming the fringe-like grain boundary regions and containg 50.29 wt% Ti, 36.82 wt% Cr, 6.29 wt% C, 3.86 wt% O, and 2.74 wt% Al present a mixture of the TiC0.67 and Al2O3 phases that surrounds the Ti(Cr) phase grains. Their total amount is about 14–16 vol%. However, the Al2O3 phase was not detected by XRD analysis because of its small percentage.

In general, the subsequence of phases formation in the studied composites may be as follows: in the beginning of the solidification process, titanium carbides and titanium/zirconium silicides and MAX-phase were formed; then, the recrystallization of titanium/chromium grains occurred.

3.2 Mechanical Behavior of the Studied Composites and Microstructure Related Fracture Mechanisms

The studied composites exhibited distinct temperature dependences of both strength and fracture toughness (Fig. 6). In particular, the composites 1 (Ti–Si–Al–Sn–C system), 2 (Ti–Si–Al–Zr–C system), and 3 (Ti–Si–Al–Zr–C system) showed high and invariant values of fracture toughness (Fig. 6b) in a temperature range of 20–500 °C. In this range, fracture toughness of these composites is about 20 MPa·m1/2. In contrast, the monotonously changing temperature dependences of strength (increasing for composite 1 and decreasing for composites 2 and 3, Fig. 6a) were revealed.

Fig. 6
figure 6

Temperature dependences of a strength and b fracture toughness of the studied composites (Table 1)

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A specimen of composite 1 undergone to the fracture toughness test at 20 °C exhibited a distinct fracture surface (Fig. 2c, d) corresponding to a mixed fracture along the boundaries of titanium lamella packets and transgranular fracture across titanium grains in the case when a cleavage plane coincides with the direction of crack propagation. This fracture micromechanism is related to the comparatively high fracture toughness (Fig. 6b). Strength of this composite increased from about 760 MPa at 20 °C to 1000 MPa at 500 °C.

Fracture surface of a specimen of composite 2 tested at 20 °C (Fig. 3c, d) corresponds to a fracture along the boundaries of titanium lamella packets. No transgranular fracture across titanium grains occurred. Therefore, a coarse relief of fracture surface was formed that was a reason of high fracture toughness of the composite (Fig. 6b).

Similarly to this material, fracture surface of a specimen of composite 3 tested at 20 °C (Fig. 4c, d) exhibited a coarse relief corresponding to high fracture toughness of the composite (Fig. 6b).

Strength of composites 2 and 3 decreased from 1400 and 1250 MPa at 20 °C to 1230 MPa and 1240 MPa at 500 °C, respectively. Nevertheless, such a level of strength is high enough and meets the requirements to materials of this system.

A testing temperature in a range of 600–850 °C is critical for these three composites since for each composite a maximum of fracture toughness appeared on the corresponding dependence is shifted toward a certain temperature. Its location is related to the microstructural peculiarities of a composite, its chemical and phase compositions, as well as dominant fracture micromechanism.

For composite 1, a temperature above 600 °C is critical since it corresponds to the maximum of fracture toughness (Fig. 6b). A specimen undergone to the fracture toughness test at 650 °C exhibited blunted edges of titanium lamella packets (Fig. 2e), microregions of transverse fracture of thin Ti3SiC2 MAX phase lamellae (Fig. 2f), and signs of quazi brittle failure of the Ti5Si3 phase grains on fracture surface (Fig. 2f). The fracture toughness is as high as in the case of testing at 20 °C (Fig. 6b). Obviously, a transition from quazi brittle (at 600 °C) to high-temperature ductile fracture (at 650 °C) occurred that was followed by some lowering of fracture toughness (from 24 MPa·m1/2 at 600 °C to 19 MPa·m1/2 at 650 °C, Fig. 6b). Probably, tin also contributes to the transition process. Strength of this composite decreased from about 1000 MPa at 600 °C to 650 MPa at 700 °C (Fig. 6a).

A temperature corresponding to the maximum of fracture toughness for composite 2 is about 700 °C (Fig. 6b). Such a shift by 100 °C compared to composite 1 is important in terms of high-temperature mechanical stability of the studied composites. Fracture surface of a specimen of composite 2 tested at 700 °C (Fig. 3e, f) exhibited a coarse relief with signs of plastic elongation of titanium grains corresponding to the highest fracture toughness of the composite among the tested ones (Fig. 6b). No transgranular fracture across titanium grains occurred and no signs of debonding between Ti5Si3 phase grains or Ti3SiC2 MAX phase lamellae and titanium matrix were detected (Fig. 3f).

For composite 3, in contrast to composite 2, a temperature corresponding to the maximum of fracture toughness is about 800 °C (Fig. 6b). However, the shift by 200 °C compared to composite 1 is rather related to a difference in phase compositions of these materials. On fracture surface of a specimen of composite 3 tested at 700 °C (Fig. 4e, f), a coarse relief of fracture along titanium lamella packets with an average size smaller than in composite 2, with signs of plastic elongation of titanium grains, was observed. Such fracture surface morphology is consistent with slightly lower fracture toughness of composite 3 than composite 2 (Fig. 6b). Similarly to composite 2, no transgranular fracture across titanium grains was found and no signs of debonding between the (Ti, Zr)5Si3 or TiC0.67 or Ti3SiC2 MAX phase components and the α-Ti matrix phase were detected (Fig. 4f).

In contrast to mechanical behavior of above-mentioned materials, composite 4 showed invariant values of both strength (Fig. 6a) and fracture toughness (Fig. 6b) in a temperature range of 20–600 °C. Strength of this composite is about 500 MPa in this temperature range with a trend to increasing, whereas fracture toughness is about 5 MPa·m1/2. Increased strength (up to 650 MPa at 700 and 800 °C, Fig. 6a) and fracture toughness of the composite (steep increase up to 19 MPa·m1/2 at 800 °C, Fig. 6b) are the evidences of a change in the fracture micromechanism. Fracture surface of a specimen of composite 4 undergone to the fracture toughness test at 20 °C showed signs of transgranular cleavage fracture with separation of fringe-like grain boundary regions and the titanium carbide phase TiC0.67 particles in places where the advancing crack crossed them (Fig. 5c, d). A different pattern of fracture surface was observed in a specimen of composite 4 after the fracture toughness test at 700 °C (Fig. 5e, f). Because of intense plasticization of the Ti(Cr) matrix phase at this temperature, multiple microregions of ductile metal surrounding each of titanium carbide particles that are embodied into a Ti(Cr) matrix grains (Fig. 5e) with visible shear bands (Fig. 5f) as signs of plastic deformation of the matrix phase during crack growth can be seen. Such fracture micromechanism is related to temperature-assisted relaxation of stress in the crack tip vicinity that corresponds to some increase in fracture toughness of the material at 700 °C (Fig. 6b). Finally, a phenomenon of substantial increase in both strength (Fig. 6a) and fracture toughness of the composite (Fig. 6b) at 800 °C may be explained in terms of phase transformations due to high-temperature diffusion of some elements, in particular, aluminum and silicon [18, 58, 62]. This, in turn, causes a change in the fracture micromechanism [59, 76]. In this composite, high-temperature fracture occurred at 800 °C (Fig. 5g, h) with a steep increase in fracture toughness, due to diffusion of some elements and pore coalescence at the boundaries of the Ti(Cr) phase grains (Fig. 5h). No TiC0.67 particles serve as stress concentrators, even in the places where the advancing crack crossed them (Fig. 5g). Thus, this temperature promotes quazi ductile character of crack growth resulted in striations (Fig. 5h) similar to fatigue crack growth in high-strength ductile materials at ambient temperature [59].

Thus, based on results of the strength test and fracture toughness tests along with analysis of microstructure peculiarities and fracture micromechanisms revealed in the whole temperature range investigated, the general tendencies in temperature dependent mechanical behavior of titanium-based composites have been substantiated. The Ti–Si–X composites can serve as high-temperature structural materials at an operating temperature up to 750 °C, whereas the Ti–Cr–X composite exhibits high-temperature stability at a higher temperature by about 50 °C.

4 Conclusions

In this work, mechanical behavior of the Ti–Si–X and Ti–Cr–X composites have been studied in a temperature range of 20–900 °C.

  1. 1.

    The microstructure peculiarities and phase composition of the studied composites were substantiated.

  2. 2.

    It was shown that strength and fracture toughness parameters are suitable for the characterization of mechanical behavior of the composites in the investigated temperature range.

  3. 3.

    The phenomenon of increased strength and fracture toughness of Ti–Cr–Al–C composite was revealed and explained in terms of the morphology of microstructural components and dominant fracture micromechanisms.