Mechanism of synthesis of ultra-high temperature Ta₄ZrC₅ carbide by thermal explosion with preliminary mechanical alloying of metals

Introduction

Binary carbides of the Ta–Zr–C system are considered promising candidates for the development of ultra-high temperature ceramics (UHTCs) [1–8]. Despite extensive research on this system, the available literature data remain contradictory, with the central question concerning the possibility of forming a single-phase carbide Ta_1–xZr_xC. Calculation of the phase diagram of the binary Ta–Zr system has shown that below 800 °C these two metals are practically immiscible [6]. In this temperature range, a mixture of two solid solutions is formed: one based on α-zirconium with an HCP structure and the other based on tantalum with a BCC structure. The solubility of tantalum in zirconium does not exceed 2 at. %, while that of zirconium in tantalum is less than 9 at. %. A continuous series of solid solutions with a BCC structure is formed only near 1700 °C [6].

Thermodynamic investigation of the Ta–Zr–C system using the CALPHAD method led to the paradoxical conclusion that no ternary phases exist within the temperature range of 200–3600 °C [7]. This conclusion contradicts the results obtained using the CASTEP (Cambridge Serial Total Energy Package) code, which demonstrated the stability of Ta_1–xZr_xC and its structural similarity to the ultra-high temperature carbide Ta₄HfC₅ [8]. The stability of the Ta_1–xZr_xC phase was also demonstrated by first-principles calculations and confirmed experimentally in [9]. In particular, binary carbides with x = 0.9, 0.8, 0.6, and 0.3, synthesized by self-propagating high-temperature synthesis (SHS) from mechanically activated Ta–Zr–C mixtures and consolidated by hot pressing, showed no signs of decomposition into simple carbides after annealing at 800 °C for 40 h. Preliminary mechanical activation promotes the formation of a more homogeneous reactive mixture, enabling the synthesis of binary carbide under SHS conditions [10–12]. Along with hot pressing, a highly promising method for consolidating complex UHTCs is spark plasma sintering (SPS), in which synthesis and densification occur simultaneously [13–18]. Samples of Ta_1–xZr_xC UHTCs have also been produced using an original electrothermal explosion technique [19].

In studies devoted to the synthesis of Ta_1–xZr_xC by SHS, the primary focus has been placed on ignition and combustion parameters. The processes occurring during the preheating stage prior to ignition and the mechanism of molten metal interaction with carbon remain largely unexplored.

The aim of this work was to experimentally investigate the macrokinetic characteristics of mechanical alloying, thermal explosion, and heating of the resulting Ta₄Zr bimetallic particles, as well as the high-temperature interactions in the Ta–Zr–C system leading to the formation of Ta_1–xZr_xC carbide.

Materials and methods

Commercially produced powders manufactured in the Russian Federation were used in this study:

– tantalum powder, grade TaP-1 (TU 1870-258-00196109-01), 99.9 wt. % purity, particle size d = 40–63 μm;

– zirconium powder, grade PTsRK (TU 48-4-234-84), 99.6 wt. % purity, particle size d = 40–63 μm;

– carbon black, grade P804T, particle size d < 2.2 μm.

High-energy ball milling (HEBM) of tantalum and zirconium powder mixtures in a molar ratio of 4Ta:Zr was carried out in an AGO-2 planetary ball mill under an argon atmosphere. The rotational speed of the steel vials was 2220 rpm, the mass of powder mixtures was 10 g, and the mass of steel balls was 200 g. The milling time varied from 2 to 30 min. After processing and removal of the vials from the mill, their surface was heated to above 100 °C; therefore, the vials were opened only after several hours of cooling to room temperature.

As a result of HEBM, a Ta₄Zr alloy powder was obtained, a significant portion of which (up to 50 wt. %) adhered to the balls and vial walls. To grind and recover more of the alloy powder, 30 ml of hexane was added to the vials, followed by additional HEBM for 3 min. After drying, carbon black was added to the Ta₄Zr alloy powder in stoichiometric proportion to obtain Ta₄ZrC₅, and the mixture was homogenized in the AGO-2 planetary ball mill for 3–5 min.

The processes occurring during heating of Ta₄Zr alloy and during thermal explosion in the Ta₄Zr + 5C and 4Ta + Zr + 5C mixtures were studied using the setup described in [20]. Cylindrical samples, 3 mm in diameter and up to 1.5 mm in height, were pressed from the powder mixtures and placed in a boron nitride crucible on a WRe 5/20 thermocouple rolled to a thickness of 30 μm. The crucible was positioned on a graphite strip heated by electric current. The samples were heated under constant electrical power supplied to the graphite strip, in argon at atmospheric pressure. The heating rate and maximum temperature were controlled by adjusting the electrical power. After ignition of the pellets, heating of the strip was stopped. The accuracy of temperature measurement, determined using the melting points of Zn, Al, and Cu as reference standards, was ±10 °C.

Melting and reaction of Ta₄Zr alloy powders with carbon were investigated in a VUP-5 vacuum system at a pressure of 1.3·10^–3 Pa. The powders were placed on substrates (25×5×1 mm) of fine-pored graphite (GMP) or glassy carbon (SU-2500), which were heated by direct current with a power of 1–4 kW [21]. The substrate had a fillet-like shape, allowing the narrow section to reach temperatures above 3000 °C. Before the reaction, the substrate was degassed by heating in vacuum to ~3000 °C. After cooling, bimetallic powders were deposited on its surface.

Results and discussion

Mechanical alloying

During high-energy ball milling (HEBM) of the 4Ta + Zr powder mixture, the combined effects of friction, rolling, and impacts result in mechanical alloying (MA) of tantalum and zirconium particles. Adhesion of the powder mixture to the milling balls allows observation of alloy formation in their cross-sections (Fig. 1, a). Initially, bimetallic layers form on the balls, which, after repeated cycles of deformation, detachment, and fracture, produce particles with a structure similar to that shown in Fig. 1, b–d. With increasing milling time, both the coating and the alloy particles formed upon its fracture become more homogeneous (Fig. 2). Elemental distribution maps across particle cross-sections reveal that after 10 min of HEBM, large zirconium particles and steel fragments abraded from the milling balls are not fully broken down. Iron contamination originating from the abrasion of the steel balls and vials is an undesirable characteristic of HEBM, as it contaminates the product. The processes of particle fragmentation and iron contamination become especially pronounced during HEBM in hexane, due to the wedge action of the liquid (Rebinder effect).

At mechanical alloying times up to 5 min, complete homogenization of the alloy was not achieved; therefore, the duration was increased to 10–30 min. However, even in this case, the elemental distribution remained insufficiently uniform (see Fig. 2). Further prolongation of milling time led to significant contamination of the Ta₄Zr alloy with iron, and thus the MA time was limited to 30 min.

Results of X-ray diffraction analysis
of the bimetallic powders

Fig. 3 shows the diffraction patterns of powders at different stages of mechanical alloying. The zirconium powder used contains some zirconium hydride, originating from the hydride–calcium reduction process by which it was produced. During MA, zirconium hydride decomposes with the release of hydrogen, and its diffraction peaks, along with those of zirconium, disappear after only 3 min of MA. This disappearance may be associated both with the refinement of zirconium crystallites and with its dissolution in tantalum.

As shown in Fig. 2, complete dissolution of zirconium does not occur even after 10 min of milling, and moreover, the mutual solubility of Zr and Ta is quite limited [6]. Therefore, the most likely reason for the disappearance of the peaks is intensive plastic deformation of zirconium, which results in crystallite refinement (reduction of coherent scattering domain size) and peak broadening. After 30 min of MA, the diffraction pattern exhibits three strongly broadened peaks characteristic of the BCC structure. The asymmetric shape of the most intense peak suggests the presence of two phases with closely spaced interplanar distances. Such a diffraction pattern may indicate the coexistence of two BCC solid solutions based on Ta.

Heating of bimetallic powders and ignition
of their mixtures with carbon black

Fig. 4 shows the thermograms of primary and repeated heating of samples prepared from the initial 4Ta + Zr mixtures (curves 1a and 1b), Ta₄Zr bimetallic particles obtained by MA for 10 and 30 min (curves 2a, 2b and 3a, 3b, respectively), and the Ta₄Zr + 5C mixture (curves 4a, 4b). The inflections in the thermograms in the temperature range of 700–900 °C (curve 3) are associated with the α → β phase transition in zirconium. Tantalum lowers the transition temperature from 863 to 800–785 °C [22; 23]. According to the phase diagram of the Zr–Fe system, dissolution of more than 4 at. % Fe in tantalum also reduces the α → β transition temperature in zirconium to 785 °C [22]. Increasing the MA time to 30 min leads to a clearer manifestation of this phase transition during powder heating. Above the phase transition temperature, an exothermic effect is observed. Heat release is proportional to the area under the Δt curve, which represents the temperature difference between the primary and repeated heating of the mixtures (curves 1c–4c). The exothermic effect increases with milling time and is most likely caused by the interaction of Ta₄Zr alloy with the abraded iron. During ignition and combustion, however, this reaction contributes only a small fraction of the heat release. For example, the enthalpy of formation of Zr₃Fe (ΔH(298.15) = −13.51 kJ/mol) [6] is an order of magnitude smaller than that for the carbides ZrC (ΔH(298.15) = −207.1 kJ/mol) and TaC (ΔH(298.15) = −141.8 kJ/mol) [24]. This is confirmed by the heating curves of the Ta₄Zr + 5C mixture (see Fig. 4, curves 4a, 4b, 4c).

Fig. 5 presents the results of studying the effect of heating and thermal explosion on the phase composition of powders. Curves 1 and 2 correspond to a BCC solid solution, while curve 3 represents the Ta₄ZrC₅ compound with an FCC structure. During heating of Ta₄Zr + 5C mixtures to the ignition temperature, ordering of the alloy structure occurs. This is evident from the reduced peak broadening and peak shifts in the diffraction pattern of Ta₄Zr bimetallic particles after short-term heating in argon (curve 2 in Fig. 5). Their reaction with carbon leads to the formation of an alloy close in composition to Ta₄ZrC₅, although in some experiments traces of Ta and its carbide Ta₂C were observed at the background level in the diffraction patterns. This can be attributed to incomplete homogenization of the Ta₄Zr alloy at HEBM times shorter than 60 min.

Fig. 6 shows the thermograms of Ta₄Zr bimetallic particles (τ_МС = 5 min) mixed with carbon black and heated at different heating rates (v). Below 1100 °C, heating does not lead to ignition (Fig. 6, curve 8). The critical ignition temperature (t_c) corresponds to the intersection of tangents drawn to the heating section of the sample and to the steep temperature rise. The value of t_c depends on the duration of MA and the mixing time of bimetallic powders with carbon black, and may vary within 900–1250 °C. With increasing heating rate, the ignition temperature t_c also increases (Fig. 7).

Experimental results show that the ignition temperature of the mixture is significantly lower than the melting points of zirconium (1852 °C) and Ta₄Zr alloy (1855–2600 °C, depending on the degree of homogeneity) [14]. Thermodynamic calculations for the ternary system [7] also indicate that a liquid phase appears only above 1800 °C. The heating of Ta₄Zr + 5C powder mixtures observed in the 900–1100 °C range, followed by their ignition, demonstrates that the reaction initiates in the solid state. The calculated adiabatic combustion temperatures using the Thermo software [25] for the reaction 4Ta + Zr + 5C = 4TaC + ZrC are 2640 and 3180 °C at initial temperatures of 25 and 900 °C, respectively. Thus, during thermal explosion and in the combustion wave, the melting point of the bimetallic particles is reached, and the reaction proceeds via mass transport through the liquid phase.

X-ray diffraction analysis of the thermal explosion products shows the formation of a single-phase double carbide with a face-centered cubic (FCC) structure (curve 3 in Fig. 5).

High-temperature interaction
of Ta₄Zr melt with carbon

The study of high-temperature reaction mechanisms is complicated by the rapid processes occurring in the combustion wave. To investigate the interaction of Ta₄Zr melt with carbon, model experiments were conducted. In these, Ta₄Zr and Ta particles were placed on graphite or glassy carbon substrates, heated to the melting point of tantalum, and held for 10–30 s. The melting of Ta (99.9 % purity, melting point 2996 °C) served as a reference for estimating substrate temperature. Energy-dispersive X-ray analysis showed that iron impurities present in the initial particles were absent after heating, explained by their evaporation.

The interaction of particles with the substrate depends on particle size. Small particles (d < 10 μm) did not spread under the action of surface tension forces but instead dissolved carbon and sank into the substrate (Fig. 8). This behavior is governed by the ratio between the spreading time (τ_s) and the carburization time (τ_d). For small particles, τ_s > τ_d, for large particles, τ_s < τ_d. The critical spreading time is determined by the relation τ_s ~ R²/D, where R is particle radius and D is the diffusion coefficient of carbon in the melt [26].

Melting begins at the contact point between the particle and the hot substrate. Therefore, during melting and spreading of particles larger than 10 μm, the reaction initiates in the central region of the contact spot, producing a crater-like ring structure (Fig. 9).

The resulting droplets typically move along the substrate surface, leaving a trace. A characteristic feature of these structures is a reduced zirconium concentration on the surface of the spread droplet and an increased zirconium concentration in the imprint. This structure is most likely associated with the higher diffusivity of zirconium atoms in the melt and their strong affinity for carbon. In some cases, nearly all zirconium diffuses into the substrate (Fig. 10). In such cases, regions with zirconium concentrations close to the initial composition coexist with zirconium-depleted regions, which differ significantly in structure. The crater edges also exhibit high carbon content.

Conclusions

An experimental study was conducted on the formation of the ultra-high temperature carbide Ta₄ZrC₅ by mechanical alloying in a planetary ball mill, followed by synthesis in the thermal explosion mode. Model experiments simulating the processes of ignition and combustion in the high-temperature zone were also performed. The data obtained on the formation of bimetallic particles, ignition of their mixtures with carbon black, and high-temperature interaction with carbon provide new insights into the synthesis mechanism of ultra-high temperature carbides and their ceramic materials for high-temperature applications. The following conclusions can be drawn.

1. High-energy ball milling of 4Ta + Zr powder mixtures in a planetary mill yields BCC solid solutions that serve as initial reactants for subsequent synthesis. The mechanically alloyed powder consists of bimetallic particles with a characteristic layered (“compositional”) structure.

2. Mixtures of bimetallic powders with carbon black undergo self-ignition upon heating in argon at 900–1250 °C, depending on heating conditions. Since these temperatures are below the melting point of the bimetallic particles, the self-sustaining reaction is initiated by solid-phase interaction.

3. As a result of the exothermic reaction of the bimetallic particles with carbon, the temperature rapidly increases beyond the melting point of the metals. The final product is formed through the interaction of the metallic melt with carbon.

4. The product of synthesis in the thermal explosion mode is the ultra-high temperature carbide Ta₄ZrC₅.

5. Model high-temperature experiments in the “molten Ta₄Zr bimetallic particles–carbon” system revealed two size-dependent interaction mechanisms: large particles spread across the carbon substrate surface with preferential zirconium diffusion into the substrate, whereas small particles predominantly absorbed carbon.