INTRODUCTION

Ternary carbides Cr2AlC and Ti2AlC are the representatives of the family of MAX phases with the formula Mn + 1AXn, where M is a transition d metal, A is an element from Groups IIIA–VIA (Al, Si, Ge, etc.), and X is carbon or nitrogen (n = 1–5). MAX phases are characterized by hexagonal close packing P6/mmc and have a layered crystal structure, in which the [Mn + 1Xn] carbide or nitride blocks are spaced by the monolayers of elements from Groups IIIA–VIA. The interest in such compounds is determined by a special combination of physicochemical properties associated with the layering at the level of the crystal structure. The materials based on MAX phases possess a high potential for use in industrial sectors because they have a unique combination of the characteristic features of both metals and ceramics [1, 2] and are promising for application under the conditions of high temperatures and oxidizing environments. Similar to metals, they are characterized by high electrical and thermal conductivity, are readily processed, and are not sensitive to thermal shocks, while as ceramics they have a low density and a high Young modulus, heat resistance, and high-temperature strength [3–6]. Currently, over 70 compounds have been obtained which belong to the family of MAX phases, among which Cr2AlC is the most widely studied after Ti2AlC, Ti3AlC2, and Ti3SiC2 [7–9].

The main preparation methods of Cr2AlC are the synthesis from elements by hot isostatic pressing (HIP), spark plasma sintering (SPS), and vacuum pulse discharge sintering; here, Cr5Al8, Cr2Al, and Cr7C3, as well as unreacted Cr and C, are present in the composition of the material in addition to the main phase [10–12]. A series of solid solutions of MAX phases with the 211 composition—(Cr1 – xTix)2AlC with x = 0.05–0.2—was obtained from a mixture of the powders of CrCx, TiCx, and Al by hot isostatic pressing [13]. It turned out to be impossible to synthesize (Cr1 – xTix)2AlC at x > 0.2; secondary carbide phases (Cr7C3, Cr3C2, TiC) appeared in the composition of the material when increasing the concentration of CrCx in the initial mixture. MAX phases with the 312 and 413 compositions—(Cr2/3Ti1/3)3AlC2 and (Cr5/8Ti3/8)4AlC3—were obtained by hot isostatic pressing of the powders of the elements at t = 1500°C for 1 h under a pressure of 30 MPa in an argon flow [14]. The (Cr2/3Ti1/3)3AlC2 formulation showed a high stability. At the same time, it is challenging to synthesize a material that contains the (Cr1 – xTix)n + 1AlCn phase only, and the product contains the impurities of carbide (TiCx, (TiCr)Cx, CrCx) and intermetallic phases of the Ti–Al–Cr system.

According to the thermodynamic calculations [15], the regions of existence of the solid solutions Ti2AlC–Cr2AlC are extremely limited. In [16], attempts were undertaken to synthesize MAX phases with the 211 and 312 compositions using reactive sintering of the powders of Cr, TiH2, Al, and graphite. It was noted that both the replacement of titanium by chromium in Ti3AlC2 and mutual solubility of Ti2AlC and Cr2AlC are limited by several atomic percents. The materials contained a significant amount of the TiC and Al–Cr secondary phases.

Therefore, upon obtaining MAX phases in a Cr–Ti–Al–C system by HIP, SPS, and reactive sintering, the end products always contain the impurities of carbide and intermetallic phases.

A promising preparation method of MAX phases is self-propagating high-temperature synthesis (SHS). This technology requires almost no energy expenditures and possesses high productivity and environmental friendliness [17–19]. Ti2AlC, Ti3AlC2, and Ti3SiC2 MAX phases were obtained by SHS from the elements [20–22]. In these works, the authors used the initial mixtures consisting of the powders of titanium, aluminum, carbon, and silicon. One of the technological trends of SHS is SHS metallurgy, which makes it possible to obtain cast materials due to the full melting of the components in the combustion wave. Its characteristic feature consists in the use of the mixtures consisting of metal oxides, a reducing metal (Al or Mg), and carbon, as well as high-energy additives, e.g., CaO2 + Al. The process is based on the occurrence of the following exothermic reactions:

$$\begin{gathered} {\text{M}}{{{\text{e}}}^{1}}{{{\text{O}}}_{x}} + {\text{M}}{{{\text{e}}}^{2}} = {\text{M}}{{{\text{e}}}^{1}} + {\text{M}}{{{\text{e}}}^{2}}{{{\text{O}}}_{x}}, \\ {\text{M}}{{{\text{e}}}^{1}} + {\text{Al}} = {\text{M}}{{{\text{e}}}^{1}}{\text{Al}}, \\ {\text{M}}{{{\text{e}}}^{1}} + {\text{C}} = {\text{M}}{{{\text{e}}}^{1}}{\text{C}}, \\ {\text{M}}{{{\text{e}}}^{1}}{\text{Al}} + {\text{M}}{{{\text{e}}}^{1}}{\text{C}} = {\text{Me}}_{2}^{1}{\text{AlC}}, \\ \end{gathered} $$

where Me1 is an early transition metal and Me2 is Al or Mg.

SHS metallurgy was applied for the first time for the synthesis of the Cr2AlC MAX phase in [23]. At a certain ratio of the reagents, the combustion temperature exceeds the melting point of the initial reagents and end products. As a result, the product is formed in the liquid state in the combustion wave. The separation of the heavy metal-like and light oxide phases of the formed products occurs under the action of gravity due to the different specific gravities [23–25]. Initial mixtures consisting of chromium(VI) and chromium(III) oxides with aluminum and carbon were used in these works. It was shown that the key synthesis parameter determining the composition of the end products is the lifetime of the melt, which depends on the combustion temperature of the initial mixture. In [26], mixtures based on chemically coupled chemical reactions were used for the synthesis of the Cr2AlC cast phase: weakly exothermic Cr2O3 + 3Al + C (a heat acceptor) and strongly exothermic 3CaO2 + 2Al (a heat donor). It was found that the maximum concentration of the Cr2AlC phase of 66% is achieved at the concentration of the 3CaO2 + 2Al additive in the initial charge of 30%.

The aim of this study was to determine the possibility of obtaining a titanium-doped Cr2AlC phase upon combining two processes, namely, SHS from elements and SHS metallurgy.

MATERIALS AND EXPERIMENTAL PROCEDURE

Powders of analytical grade chromium oxide Cr2O3 (Pervoural’skoe PO Khrompik) and chemically pure grade calcium oxide CaO2 (NPK Reaktiv, Novosibirsk), as well as aluminum of the ASD-1 brand (Volgogradskaya aluminievaya kompaniya), PTM titanium (AO POLEMA, Tula), and MPG graphite (OOO Grafit-resurs, Chelyabinsk oblast) with a particle size smaller than 100 μm, were used as the initial components. The ratios of the components of the initial mixtures were calculated using the following chemical reactions:

$${\text{C}}{{{\text{r}}}_{2}}{{{\text{O}}}_{3}} + 3{\text{Al}} + {\text{C}} = {\text{C}}{{{\text{r}}}_{2}}{\text{AlC}} + {\text{A}}{{{\text{l}}}_{2}}{{{\text{O}}}_{3}},$$
(1)
$$3{\text{Ca}}{{{\text{O}}}_{2}} + 2{\text{Al}} = 3{\text{CaO}} + {\text{A}}{{{\text{l}}}_{2}}{{{\text{O}}}_{3}},$$
(2)
$$2{\text{Ti}} + {\text{Al}} + {\text{C}} = {\text{T}}{{{\text{i}}}_{2}}{\text{AlC}}{\text{.}}$$
(3)

A charge corresponding to schemes (1) and (2) was used as the base charge, the composition of which 70% (Cr2O3 + 3Al + C) + 30% (3CaO2 + 2Al) was tested in [26]. This mixture is combustible in the steady-state mode. After the passage of the combustion wave, the material is in the liquid-phase state, which leads to the separation of the product to two layers, namely, a lower metal-like layer and an upper oxide layer, due to the different specific gravities.

To obtain a titanium-doped Cr2AlC phase, a mixture composed by Eq. (3) was added to the base composition. The weight ratios between mixtures (Cr2O3 + 3Al + C) (1) and (2Ti + Al + C) (3) were varied, while the concentration of high-energy component (3CaO2 + 2Al) (2) was constant of 30% of the weight of the charge (Table 1). The charge was prepared in a planetary mixer. The ready-to-use mixture with a weight of 20 g was placed into a quartz container with a diameter of 20 mm and a height of 50 mm. The synthesis processes were performed in a 3-L SHS reactor (Fig. 1) at an initial pressure of argon of 5 MPa.

Table 1. Composition of the initial mixtures and synthesis parameters
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Fig. 1.
figure 1

SHS reactor diagram: (1) a body, (2) a base, (3) inspection windows, (4) a quartz mold with the mixture, and (5) an initiating spiral.

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The reaction was initiated using a tungsten spiral. The combustion rate was determined by the formula

$${{U}_{{\text{c}}}} = {h \mathord{\left/ {\vphantom {h \tau }} \right. \kern-0em} \tau },$$

where h is the height of the powder filling; τ is the burnup time of the sample, which was determined using a stopwatch and a video camera by the average value from three experiments.

The following quantities were used to evaluate the synthesis parameters:

(i) the ingot yield of the product

$${{\eta }^{1}} = {{{{M}_{{{\text{ing}}}}}} \mathord{\left/ {\vphantom {{{{M}_{{{\text{ing}}}}}} {{{M}_{{{\text{mix}}}}} \times 100\% }}} \right. \kern-0em} {{{M}_{{{\text{mix}}}}} \times 100\% }},$$

(ii) the weight loss due to the dispersion of the components upon combustion

$${{\eta }^{2}} = {{\left( {{{M}_{{{\text{mix}}}}} - {{M}_{{{\text{end}}}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{M}_{{{\text{mix}}}}} - {{M}_{{{\text{end}}}}}} \right)} {{{M}_{{{\text{mix}}}}}}}} \right. \kern-0em} {{{M}_{{{\text{mix}}}}}}} \times 100\% ,$$

where Ming is the weight of the ingot, Mmix is the weight of the initial mixture, and Mend is the total weight of the product after combustion.

The X-ray diffraction analysis (XRD) was performed on a DRON-3 diffractometer with a graphite monochromator on a secondary beam (a CuKα radiation). The X-ray diffraction patterns were recorded in the mode of step-by-step scanning in a range of angles 2θ = 12°–100° in increments of 0.02° and with the exposure of 4 s in a point. The quantitative analysis was performed by the Rietveld method in the PDWin software package (NPP Burevestnik, Russia). The structural data of the identified phases presented in the Crystallography Open Database [27] were used as the initial model for the refinement. The profile parameters of the reflections, background, unit cell parameters, and percentage of the phases were refined.

The weighted divergence factor calculated during the refinement, which takes into account the background, was within a range of Rwp = 8–12% for all the samples. A method of internal standard, in the capacity of which silicon (NIST SRM 640b) was used, was utilized for the precision determination of the unit cell parameters. The study of the microstructure and elemental analysis of the samples were performed on an Ultra plus ultra-high-resolution field-emission scanning electron microscope based on Ultra 55 (Carl Zeiss, Germany).

RESULTS AND DISCUSSION

The XRD data for the ingot (Fig. 2) formed upon the combustion of the base charge showed that the product included the Cr2AlC, Cr7C3, and Cr5Al8 phases (Table 2). Upon adding mixture 3 to the charge, synthesis parameters such as combustion rate Uc and weight loss η2 insignificantly change (Table 1). However, the ingot yield of the desired product η1 increases by 30% for the formulations with compositions 2 and 3 in comparison with the base mixture.

Fig. 2.
figure 2

X-ray diffraction pattern of the material obtained upon the combustion of the base mixture with composition 1.

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Table 2. Phase composition of the synthesis products
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The phase composition of the ingots formed after the combustion of mixtures 2 and 3 is characterized by the presence of more than five compounds (Fig. 3, Table 2). A comparison with the ICDD PDF2 powder diffraction database showed that the angular position of the reflections of one of the phases corresponded to Ti2AlC. The positions of the diffraction lines of the second MAX phase are close to Cr2AlC; however, they are displaced towards smaller angles; i.e., its unit cell parameters are greater when compared to Cr2AlC.

Fig. 3.
figure 3

X-ray diffraction patterns of the materials obtained upon the combustion of the mixtures with compositions (a) 2 and (b) 3.

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The fragment of the X-ray diffraction patterns in a region of angles 2θ = 52.5°–58.0°, which shows the angular displacement of the 106 reflection of the MAX phases, is presented in Fig. 4. It can be assumed that this displacement is induced by the isomorphous replacement of some Cr atoms by Ti atoms in the cell. It is known that doping of the Cr2AlC phase with titanium leads to an increase in its unit cell parameters [13] as a result of the difference in the atomic radii of titanium (2.0 Å) and chromium (1.85 Å).

Fig. 4.
figure 4

Fragments of the X-ray diffraction patterns with the 106 reflection of the (Cr1 – xTix)2AlC MAX phases with compositions 1–3.

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The fraction of titanium in the (Cr1 – xTix)2AlC phase obtained upon the combustion of compositions 2 and 3 was evaluated based on the linear approximation of the dependence of the unit cell volume of Cr2AlC [28], Ti2AlC [29], and (Cr1 – xTix)2AlC [13] on the composition (Fig. 5) and data of this work (Table 3). It turned out that the replacement of 18–28 at % Cr by titanium occurs depending on the initial composition of the mixture.

Fig. 5.
figure 5

Unit cell volume of Cr2AlC, Ti2AlC, and (Cr1 ‒ xTix)2AlC depending on the atomic fraction of Ti based on the data of [13, 28, 29] and results of this work.

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Table 3. Unit cell parameters of the МAX phases in a Cr2AlC–Ti2AlC system
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The microstructure of the ingot fracture (composition 3) and elemental analysis of the structural components confirm the presence of an MAX phase (Fig. 6). A layered structure characteristic for the MAX phases is observed on the fracture, in the bulk of which roundish carbide and intermetallic grains are located. The composition of the layered phase calculated by points 4–7 determines x = 0.29 and formula (Cr0.71Ti0.29)2AlC0.97, which is similar to the value of x = 0.28 obtained based on the data on the unit cell metric (see Table 3).

Fig. 6.
figure 6

Microstructure of the ingot fracture and composition of the structural components of the material obtained upon the combustion of mixture 3.

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The results of these studies show that, upon adding mixture 3 to the base charge, the combustion rate and weight loss insignificantly change; however, an increase in the ingot yield of the desired product is observed. This is associated with the fact that mixture 3 does not contain an oxide phase and consists of the elements (Ti, Al, and C) that migrate into the ingot in the composition of the compounds. Apparently, the presented scheme of reactions (1)–(3), based on which the equilibrium composition of the desired MAX phase was calculated, will not reflect all the interactions that actually occur in the multiphase system during SHS. Indeed, XRD of the synthesized material showed that its phase composition substantially differed from the calculated composition. This gives the evidence of the fact that the processes occurring in the liquid phase being formed in the combustion wave and upon its fast crystallization lead to the formation of a nonequilibrium composition of the product.

Let us consider the characteristic features of the synthesis of solid solutions Cr2AlC–Ti2AlC upon combining the technologies of SHS from elements and SHS metallurgy. In the first case, the process is based on the metal (Ti, Mo, Ta, etc.) + nonmetal (C, B, N) reactions when the adiabatic combustion temperature does not exceed the melting point of the interaction products, i.e., the products are in the condensed state in the combustion wave. In the second case, the SHS metallurgy process is based on the oxidation–reduction reactions of the thermite type (Me1Ox + Me2). The main chemical transformations occur in the liquid phase, being formed upon the passage of the combustion wave. The key parameter affecting the formation of the MAX phases in SHS metallurgy is the lifetime of the melt, which depends on the heat release of the thermite reaction and synthesis parameters such as the weight of the mixture and heat removal [23–25].

Introducing a 2Ti + Al + C elemental mixture with Tad = 2468 K [30] into a 70% (Cr2O3 + 3Al + C) + 30% (3CaO2 + 2Al) base mixture with Tad = 2820 K [26] leads to a decrease in the combustion temperature of the mixtures with compositions 2 and 3. As a result, the lifetime and time of homogenization of the melt decrease.

One common mechanism of formation of the MAX phases during SHS is when the highest temperature carbide phases are formed at the first stage of the process and then dissolve in the surrounding intermetallic melt, followed by the crystallization of the ternary compounds Mn + 1AXn [20–22]. As a result of the decrease in the combustion temperature, carbides (TiCr)C, Cr7C3, and Cr3C2 do not fully dissolve in the surrounding melt Al–Cr–Ti, and a certain amount of Al8Cr5 and AlTi3 intermetallic phases is present in the composition of the synthesized material (see Table 2). For composition 3 with the largest amount of the 2Ti + Al + C mixture, the decrease in the adiabatic combustion temperature is maximum and, as a result, seven phases including unreacted graphite have been found in the product.

The analysis of the results of works [13, 14] on the synthesis of MAX phases in a Cr–Ti–Al–C system showed that the end products obtained by powder metallurgy also contained the impurities of carbide (TiCx, TiCrCx, and CrCx) and intermetallic (Al8Cr5 and AlTi) phases.

The authors of [15] showed, based on thermodynamic calculations, that the regions of existence of solid solutions in a Ti2AlC–Cr2AlC system are extremely limited. Close to the side of Ti2AlC, the maximum concentration of Cr in the phase of the solid solution (Ti1 – xCrx)2AlC is x = 0.07 at t = 1600°C, and it drops with decreasing temperature almost down to zero at 450°C. Close to the Cr2AlC phase, the maximum concentration of Ti in the solid solution (Cr1 – xTix)2AlC at t = 1600°C is x = 0.055. As a result, according to [15], the existence of the (Cr1 – xTix)2AlC equilibrium phase is impossible throughout the entire range of compositions x.

A two-phase region including individual Ti2AlC and Cr2AlC ternary phases is more thermodynamically favorable. At the same time, the experimental results of [13, 31] and data obtained by us evidence the possibility of formation of a titanium-doped Cr2AlC phase containing up to 30 at % Ti.

CONCLUSIONS

A study of the phase composition of the combustion products of 70% (Cr2O3 + 3Al + C)/(2Ti + Al + C) + 30% (3CaO2 + 2Al) mixtures upon combining the processes of SHS from elements and SHS metallurgy has shown the possibility of obtaining a cast material based on a titanium-doped Cr2AlC phase. The concentration of (Cr1 – xTix)2AlC in the product is 43–62%, and the fraction of doping Ti that replaces Cr is 0.18–0.28%.

The microstructure of the material is characterized by the presence of laminate layers with the inclusions of carbide grains. The maximum concentration of the titanium-doped MAX phase (62%) has been obtained upon using 15% (2Ti + Al + C) in the base mixture, which corresponds to composition 2 (see Table 1).

Upon increasing the fraction of the elemental mixture in the charge, twofold the adiabatic combustion temperature decreases, which leads to a decrease in the lifetime of the melt. As a result, for composition 3, which contains 30% (2Ti + Al + C), the combustion product includes a significant amount of undesired phases—intermetallics, carbides, and graphite.