Thermal explosions in (Ti, Zr, Hf, Nb, Ta) carbon mixtures

Introduction

Since 2004, high-entropy alloys (HEA) and high-entropy ceramics based on oxides, borides, carbides, nitrides, and hydrides have been subject of intensive research [1]. Among these materials, high-entropic carbides (HEC) have received particular attention due to their unique features [1–3]. Specifically, Ti, Zr, Hf, Nb and Ta-based HECs form stable monophase compound [4–7] with exceptional mechanical properties [8–11], low thermal conductivity [12], good oxidation resistance [13–15] and biocompatibility [16; 17].

To date, more than 20 manufacturing processes for high-entropy alloys have been developed [1]. Among the most common processes are mechanical activation (MA), spark plasma sintering (SPS), reduction from oxides, and hot pressing. Additionally, self-propagating high-temperature synthesis (SHS) can be utilized to produced HEAs by exploiting the exothermic reactions between group IV–V transition metals and carbon, boron, nitrogen or silicon [18; 19]. The SHS product is typically processed using spark plasma sintering to obtain a single-phase HEC. Although SHS is a rapid and advantageous process, the thermal explosion and combustion of HEC multicomponent mixtures have not been extensively studied.

The aim of this study is to examine the kinetics and product formation during a thermal explosion in (Ti, Zr, Hf, Nb, Ta) + C₅ mixtures.

Materials and methods

We utilized commercially available domestic powders:

– hafnium (Hf), GFM-1 grade (TU 48-4-176-85 Specs), 99.1 % purity, 180 μm average particle size;

– tantalum (Ta), TaP-1 grade (TU 1870-258-00196109-01 Specs), 99.9 % purity, d = 40÷63 µm;

– titanium (Ti), PTM-1 grade (TU 14-22-57-92 Specs), 99.2 % purity, d = 5÷15 µm;

– niobium (Nb), NbP-1a grade (GOST 26252-84), 99.7 % purity, d < 63 µm;

– zirconium (Zr), PCrK-1 grade, (TU 48-4-234-84 Specs), 99.6% purity, d = 40÷63 µm;

– graphite powder, (GOST 23463-79), 99.9999 % purity, ASC 8–4, d ≤ 140 µm.

Activation and ignition occurred in argon of 99.998 % purity. The powder mixture was prepared in two stages. In the first stage, an equimolar mixture of Ta + Ti + Nb + Zr + Hf was activated in an Activator 2S planetary mill (Activator, Novosibirsk, Russia). The mixture was placed in pre-vacuumed steel drums and subsequently filled with argon at 6 atm. The ball-to-powder weight ratio was 1:18, with a ball weight of 360 g (5–7 mm diameter) and powder weight of 20 g. The drum rpm was 347 and the milling time was 120 min, resulting in the production of a metallic composite.

To prevent oxidation and self-ignition, the composite powder was unloaded in an argon-filled glove box. Graphite powder was added to the mixture inside the glove box to create (Ti, Zr, Hf, Nb, Ta)C₅. The resulting mixture was similarly activated for 60 min and then passivated. The drum was open for 1–2 s to allow air in, then closed and held for 10–12 h. To obtain a homogeneous mixture with the (Ti, Zr, Hf, Nb, Ta)C₅ high-entropy alloy made by SHS, a portion of the composite/graphite powder was stirred in a porcelain mortar for 30 min. The final product concentrations in these mixtures were 25 and 50 %. Samples, measuring 3 mm in diameter and up to 1.0–1.5 mm in height, were pressed from the mixtures.

The procedure for the ignition temperature test is depicted in Figure 1 [20]. The cylindrical samples were positiond on a flat thermocouple that was 30 µm thick, and then placed into either a boron nitride or graphite crucible. The crucible was subsequently placed on an electric graphite strip heater and heated to the ignition or melting temperature. The accuracy of temperature measurement was validated using the Zn, Al and Cu melting points as a reference. The margin of error at t ≤ 1100 °C was no more than ±10 °C. The thermocouple readings were recorded at 1 kHz.

For XRD of the initial samples and products after ignition, we employed a DROn 3M diffractometer with CuK_α-radiation (Burevestnik, St. Petersburg, Russia). Furthermore, we used an LEO 1450 VP microscope (Carl Zeiss, Germany) for canning electron microscopy (SEM).

Results and discussion

Figure 2 depicts the thin section microstructure of the initial Ti, Zr, Hf, Nb, and Ta composite particles. It was not possible to produce homogeneous composite particles from the metal powder mixture, as the particles contained both layered inclusions (1) and individual large tantalum particles (2).

Due to insufficient probe positioning accuracy, it was not possible to analyze smaller particles and layers. However, the SEM analysis indicated that they included all of the original elements. XRD analysis (Figure 3) indicated that the metal peaks had shifted to the left, indicating the formation of solid solutions. The tantalum and niobium peaks were nearly identical. The small peak located at approximately 40° was close to the 100 % titanium peak. Ti was found to form solid solutions with tantalum, niobium, hafnium and zirconium.

The formation of solid solutions was confirmed by the plateau observed in the thermal curves at various heating rates (V) during the initial stage (t < 450 °C), as shown in Figure 4. A thermal curve of the heating of the titanium sample was also included for comparison.

The ignition temperature (t_ig) at high heating rates is 1030 °C. The temperature decreases with the heating rate down to 760 °C. The thermal curves suggest that this decrease is due to a phase transformation with a significant endothermic effect, resulting in an isothermal segment in thermal curves. The α–β-transformation temperatures for titanium, zirconium, and hafnium in the Group IV metals in the mixture are t_α–β = 882, 865 and 1743 °C, respectively. These temperatures are considerabely higher than those indicated on the thermal curves. It should be noted that the formation of solid solutions during mechanical activation can lead to a decrease in the α–β-transformation temperature in titanium. Okamoto H. and Lyakishev N.P. [21; 22] reported that t_α–β for titanium-zirconium equiatomic solid solutions can drop to 560–600 °C. Polymorphic transformations in Ti–Nb metastable solid solutions can occur at t = 425÷600 °C [23].

Polymorphic transformation generally enhances diffusion coefficients. This, in turn, accelerates the reaction between the solid solutions and carbon and t_α–β become the critical temperature of thermal explosion (t_c). The sample heating rates at the initial stage (V₁) and after ignition (V₂) were recorded as follows:

It can be seen that the heating rate of the V₂ sample above the critical temperature is two orders of magnitude greater than the V₁ average heating rate at the initial heating stage (up to 450 °C), indicating a thermal explosion.

Figure 5 displays the XRD image of the thermal explosion products of the activated (Ti, Zr, Hf, Nb, Ta) + 5C mixture. The ignition is triggered by the formation of carbides or solid solutions of carbon in the metals, but it does not lead to the formation of the final product. Due to the short holding time at high temperatures and rapid cooling, some of the metals do not have sufficient time to react. Comparing the XRD images in Figure 3 and 5, it can be observed that the peak intensity ratio changes after the thermal explosion due to the formation of hafnium and niobium carbides.

To investigate the reaction kinetics in this system, we diluted the initial mixture with the (Ta, Ti, Nb, Zr, Hf)C₅ final product obtained by SHS. Figure 6 illustrates the thermal curves for the samples containing 25 and 50 % of the final product.

Figure 7 shows the XRD images of the products obtained by adding 25 and 50 % of the final product to the (Ti, Zr, Hf, Nb, Ta) + 5C activated mixture, and the XRD image of the final product.

The XRD images of the products obtained by heating the diluted mixture are nearly identical. The main phase formed by dilution retains its cubic lattice, but the lattice parameters differ. The XRD images reveal peaks of titanium and zirconium carbide. The XRD images of the activated mixture (see Figure 3) exhibit weakly pronounced titanium and zirconium peaks. When heated above 1300 °C, the peaks of Ti and Zr carbide become visible. We can conclude that the solid solutions formed after activation and the dilution of high-entropy phase are unstable at high temperatures (t > 1300 °C), resulting in the release of titanium and zirconium carbides. The composition of the (Ti, Zr, Hf, Nb, Ta)C₅ high-entropy phase also changes.

Since the compositions of the ignition products of the diluted mixture are nearly identical, we used the max temperature vs. heating rate curves for the two dilutions to estimate the activation energy using the Kissinger equation. The reaction rate in the diluted mixtures decreased significantly, and the sample overheating was low. Furthermore, the temperature gradient across the relatively thin samples (about 1 mm thick) was insignificant. Considering these factors, we estimated the activation energy for the reaction forming the (Ti, Zr, Hf, Nb, Ta)C₅ high-entropic phase as follows:

\[\ln \left( {\frac{\beta }{{t_{\max }^2}}} \right) = \ln \left( {\frac{{AR}}{{{E_{\rm{a}}}}}} \right) - \frac{{{E_{\rm{a}}}}}{{R{T_{\max }}}},\]

where β is the heating rate, degrees/s; E_a is the activation energy, kJ/mol; R = 8.314 J/(mol·K) is the gas constant; A is the pre-exponential factor of the Arrhenius equation (particle collision frequency, s\(^-\)¹); T_max is the max temperature, K.

Figure 8 shows the \(\ln \left( {\frac{\beta }{{t_{\max }^2}}} \right)\) vs. \(T_{\max }^{ - 1}\) curve.

The activation energy Eа for the reaction of the (Ti, Zr, Hf, Nb, Ta) + 5C activated mixtures diluted with the final product in the temperature range of 1100–1580 °C, was found to be 34 kJ/mol. This value is significantly lower, by 75–80 %, than the value estimated from previous experimental data on gasless combustion of metal-carbon systems [24]. One possible explanation for this discrepancy is that the mechanical activation process resulted in an increase in the reactivity of the metals, which may be attributed to the grinding process, the closer contacts between reactants and the introduction of more crystal lattice defects.

Conclusions

1. The mechanical activation of (Ti, Zr, Hf,  Nb, Ta) + 5C mixtures for 120 min at 347 rpm produces composite particles and solid solutions of Ti, Zr, Hf, Nb and Ta, while individual tantalum particles remain in the mixture.

2. The ignition of the (Ti, Zr, Hf, Nb, Ta) + 5C activated mixture occurs in several stages, including inert heating, progressive heating to 420–450 °C and phase transformation at 750–770 °C. A thermal explosion occurs when the temperature raises abruptly.

3. Despite the high temperatures, the reaction produces complex carbides, and unreacted tantalum remains due to the short duration of the thermal explosion.

4. The activated mixtures and high entropy solid solution are unstable. When heated above 1300 °C, they release titanium and zirconium carbides. This process also causes a change in the composition of the (Ti, Zr, Hf, Nb, Ta)C₅ final product.

5. The effective activation energy estimated for the reaction in the (Ti, Zr, Hf, Nb, Ta) + 5C mixture (E_а = 34 kJ/mol) is 75–80 % lower than the values reported for the metal + carbon combustion reactions. This could be attributed to the mechanical activation of the mixture.