SHS of cast materials in the Mo-Al-C system

Introduction

There are several binary compounds in Mo–Al system – MoAl₃, Mo₃Al₈, MoAl, and Mo₃Al [1]. These compounds, given their high melting points and mechanical properties, are promising materials for use under high temperature and intense wear conditions [2–4]. In Al–С system, the aluminum carbide phase Al₄С₃ is stable. Al₄С₃ particles finely dispersed in the aluminum matrix reduce the creep tendency of the material, especially in combination with silicon carbide particles. Aluminum carbide can be used as an abrasive material in high-speed cutting tools [5; 6]. In the Mo–C system, molybdenum and carbon form carbides Mo₂С and МоС_1 – х . Among these, molybdenum carbide Mo₂C has the most widespread use in practice. It is used as a catalyst in hydrodesulfurization reactions, in dry reforming of methane, for the decomposition of hydrazine, and in regulators of small rocket engines [7–9]. Various methods are used for the synthesis of molybdenum carbide: carbothermal reduction of molybdenum (VI) oxide with graphite in an inert medium, electrochemical synthesis, melting with graphite, reduction of oxide using a methane–hydrogen mixture or other carbon sources [10–12].

Among the ternary molybdenum aluminum carbon compounds, the Mo₃Al₂C compound, a superconductor with a transition temperature of ~9 K, is of greatest interest. In the studies [13–16], this compound was obtained by the arc and high-frequency melting at high pressures (up to 10 GPa). Its transport, magnetic, and thermodynamic properties were investigated. The bulk modulus of elasticity is estimated at 221 GPa. Known methods for obtaining Mo₃Al₂C are inefficient and energy-intensive. A one-stage method – self-propagating high-temperature synthesis (SHS) – is a promising method for obtaining such compounds. It requires practically no electricity, exhibits high capacity and is environmentally friendly [17; 18].

The SHS method allowed to obtain a large number of binary and ternary element compounds [19–22]. One of the technological trends of application of self-propagating high-temperature synthesis is SHS-metallurgy, which allows to obtain “cast” materials by the complete melting of components in a combustion wave. A specific feature of the process is the use of mixtures consisting of metal oxides, a reducing metal (Al), and carbon. At a certain ratio of reagents, the combustion temperature exceeds the melting point of the initial reagents and final products. As a result, the product during synthesis is formed in the liquid state. Under the action of gravity, the heavy metal-like and light oxide phases of the formed products are separated. Cast materials based on binary and ternary compounds (MAX phases) in the systems: Cr–Al–C [23–25], Nb–Al–C [26; 27], and V–Al–C [28] obtained by SHS-metallurgy are of great interest. The mentioned studies demonstrate that the main synthesis parameter that determines the composition of the final products is the existence time of the melt, which depends on the combustion temperature of the initial mixture. To date, the authors of this paper have not identified any studies on the production of cast materials in the Mo–Al–C system by the SHS method.

The purpose of this paper was to study the possibility of obtaining cast products within the Mo–Al–C system by combining the methods of SHS from elements and SHS-metallurgy.

Материалы и методика исследований

Powders of MoO₃ (“Ch”, purity 99.9 %), Al (ASD-1, purity 99.2 %, particle size d < 30 µm), Mo (PM-M, 99.9 %, d < 10 µm), and graphite (PG, 99.2 %, d < 400 µm) were used as initial reagents. In the experiments, stoichiometric mixtures were used as base mixtures; their composition was calculated from a combination of two chemical reactions:

The mass ratio of the mixtures \(\alpha = \frac{{{M_{{\rm{II}}}}}}{{{M_{\rm{I}}}}}\) varied in the range from 0 to 1, where М_I and М_II are the masses of the mixtures calculated from reactions (I) and (II), respectively.

The experiments revealed that the combustion of the mixture, the composition of which was calculated according to reaction (I), is characterized by a high rate and a strong ejection of reagents and synthesis products from the reaction vessel. The mixture, the composition of which was calculated according to reaction (II), does not burn and, when added to the first mixture, behaves as a “reactive” inert component. The ratios of the initial compounds in reactions (I) and (II) remained constant in all experiments.

Before mixing, the powders were dried for 3 h at a temperature of 60 °C. The charge was prepared by manually stirring it in a porcelain mortar. The bulk density reaction mixture was placed in a 20 mm in diameter and 50 mm high quartz mold (Fig. 1, a). The weight of the initial mixtures in all experiments was 20 g. The syntheses were carried out in a 3 L reactor (Fig. 1, b) in an argon atmosphere at an initial pressure of 5 MPa according to the procedure described in work [23]. The reaction was initiated with a 0.5 mm diameter molybdenum wire coil. The combustion process was recorded using a video recorder. The combustion rate was calculated from the process video by measuring the time of passage of the combustion wave along the height of the sample. The parameters of synthesis were determined from the following values:

\({\eta _1} = \frac{{{M_{{\rm{ing}}}}}}{{{M_{\rm{0}}}}} \) – the ratio of the product yield in the ingot and the weight of the mixture;

\({\eta _2} = \frac{{{M_{{\rm{ing}}}}}}{{M_{{\rm{ing}}}^{{\rm{calc}}}}}\) – the completeness of the reaction, the ration of the product yield in the ingot relative and its calculated value;

\({\eta _3} = \frac{{{M_0} - {M_{{\rm{ing}}}}}}{{{M_0}}}\) – mass loss during combustion due to the ejection of components from the reaction vessel,

where M_ing – the weight of the target product (ingot), \({M_{{\rm{ing}}}^{{\rm{calc}}}}\)  – the calculated weight of the ingot, M₀ – the mass of the initial mixture, M_k – the total mass of products after burning.

The end products of the synthesis were studied using X-ray diffraction (XRD) and microstructural analysis. The methods for studying the obtained products are described in more detail in the authors' previous works [25].

Results and Discussion

The samples after synthesis consist of two ingots that are easily separated mechanically. The material in the upper part of the sample, according to the XRD results, is mainly an α-Al₂O₃ phase. At the bottom, an ingot (target product) with a characteristic metallic sheen is formed (Fig. 1, c). The formation of an ingot indicates that in the combustion wave a liquid phase was formed, i.e. the synthesis temperature is higher than the melting temperature of the initial components and the resulting products. Different specific masses of the final products lead to their separation due to gravity – the heavy “metal” phase settles at the bottom of the crucible, whereas the light oxide phase forms at the top.

The values of the combustion rate of the initial mixtures and the pressure increase in the reactor depending on the value of α are shown in Fig. 2, a. As α increases, these parameters decrease. When a “cold” mixture, the composition of which is determined from reaction (II), is added to a highly exothermic mixture, the composition of which is determined from reaction (I), part of the heat released as a result of a reaction (I) is spent on its melting, which leads to a decrease in the combustion rate and pressure increase in the reactor. Figure 2, b shows the dependences of the reaction completeness (η₂), product yield (η₁), and spread of combustion products (η₃) on α. The parameters η₁ and η₂ increase in the range of α from 0 to 0.4, then there is a gradual decrease. The values of η₃ decrease monotonically over the entire range of α.

The maximum product yield (η₁ = 67 %, η₂ = 93 %) is observed at α = 0.4 (Fig. 2, b). Yield is affected by two competing factors: reagent spread and combustion temperature. An increase in the amount of the “cold” mixture, on the one hand, reduces the ejection of reagents from the vessel and, accordingly, increases η₁, and, on the other hand, it reduces the combustion temperature, which leads to a decrease in η₂ due to a reduction in the time spent by the product in the liquid state, where oxide and “metal” phases spatially separate. X-ray phase analysis of the products (Figs. 3–5) obtained by combustion of mixtures with different α showed that as a result of synthesis, a multiphase material is formed, which includes Mo₃Al₂C, Mo₂C, Mo₃Al, and Mo₃Al₈. The quantitative ratio of the phases depends on the composition of the initial charge (see Table).

The combustion of mixtures of compositions 1 and 2 is non-stationary with a non-linear front and a considerable ejection of material from the crucible. X-ray phase analysis of the obtained products (see Fig. 3) revealed that as a result of synthesis, materials with a high content of molybdenum aluminides (Mo₃Al and Mo₃Al₈), more than 65 %, are formed, while the total content of carbide phases (Mo₃Al₂С and Mo₂C) does not exceed 35 %. The low content of the latter is apparently associated with a deficit of carbon due to its ejection from the crucible in the form of particles or gaseous oxides. In the combustion wave, the mixture of initial reagents undergoes a number of physical and chemical transformations. In the heating zone, aluminum and molybdenum oxide melt, forming a liquid-phase medium with distributed carbon particles. In the zone of chemical transformation, aluminum and carbon interact with molybdenum oxide:

As a result, part of the carbon escapes from the reaction zone in the form of a gas, causing its deficiency in the system. The higher the combustion temperature of the mixture, the greater the likelihood of carbon participation in the redox reaction (IV). The material obtained as a result of the combustion of mixture 1 contains phases of Mo₃Al–Mo₃Al₈ intermetallic compounds forming an eutectoid, as well as Mo₃Al₂C and Mo₂C (see Fig. 3).

The introduction of a “cold” mixture into the charge leads to an increase of the content of carbide phases in the product while the proportion of molybdenum aluminides decreases (see table). The maximum content (87 %) of the ternary compound Mo₃Al₂C in the material was obtained at α = 0.4 (see Fig. 4).

The phase composition of the combustion products of mixture 3 is practically equilibrium and is in the three-phase region Mo₃Al₂C–Mo₂C–Mo₃Al. An increase of the “cold” mixture content in the charge over the variation range α = 0.8÷1.0 leads to an increase in the content of Mo₂C in the product while reducing the content of Mo₃Al₂C and molybdenum aluminides (see Table and Fig. 5).

The studies show that when a “cold” mixture is added to the charge, the combustion rate, pressure increase, and mass loss (η₃) decrease over the entire range of α (from 0 to 1.0). At the same time, in the range of α = 0÷0.4, there is an increase in the target product yield into an ingot, whereas at α > 0.4, there is a decrease. The target product yield into the ingot is affected by two competing factors: the content in the charge of the mixture (II), consisting of the target elements (Mo, Al, C) passing into the ingot as compounds, and the combustion temperature. With an increase in the amount of the “cold” mixture, on the one hand, the proportion of elements of the target phase in the charge increases and, accordingly, η₁ and η₂ increase, and, on the other hand, the combustion temperature decreases, which leads to a decrease in η₁ and η₂ due to a decrease in the residence time of the product in a liquid state, when there is a spatial separation of the oxide and “metal” phases. The influence of the first factor prevails at α = 0÷0.4, whereas at α > 0.4, prevails the influence of the second factor.

A single-phase product containing only Mo₃Al₂C in accordance with reactions (I) and (II) was not obtained due to several reasons. Obviously, the above scheme of reactions, used for the calculation of the equilibrium composition of the target ternary phase Mo₃Al₂C, does not reflect all the interactions actually occurring in the multiphase system during SHS. X-ray diffraction phase analysis of the synthesized material showed that its phase composition differs from the calculated one. This indicates that the processes occurring in the liquid phase formed in the combustion wave and during its rapid crystallization lead to the formation of a nonequilibrium composition of the product. In addition, the combustion process is accompanied by the ejection of components as a result of the reaction (IV). It is quite likely that the resulting material is depleted in both carbon and aluminium. This is indirectly confirmed by the phase composition of mixture 5, which is practically in the two-phase region Mo₃Al₂C–Mo₂C.

The analysis of microstructural analysis data suggests the following mechanism of phase formation of the final product. The phase composition of the ingot is formed as a result of a series of phase transformations. First, refractory Mo₂C carbide grains crystallize at a temperature of about 2500 °C. As a result, a Mo–Al melt is formed, which surrounds the Mo₂C grains. Then, during cooling in the temperature range of 2500–1720 °C, Al from the melt and Mo₂C interact, which leads to the formation of a carbide grain of the Mo₃Al₂C phase on their surface in a ring pattern (see Fig. 3). The growth of the layer is caused by the diffusion of Al from the Mo–Al melt through the Mo₃Al₂C layer into the Mo₂C grain. At temperatures below 1720 °C, the intergranular melt crystallizes with the formation of Mo₃Al and MoAl. Then, at a temperature of 1467 °C, the MoAl phase undergoes the eutectoid transformation MoAl → Mo₃Al₈ + Mo₃Al [1]. Thus, the formation of a multiphase cast material is explained by the multistage nature of its formation and the rapid cooling of the melt.

Conclusion

Cast materials containing the phases Mo₃Al₂C, Mo₂C, Mo₃Al, and Mo₃Al₈ were obtained by the method of self-propagating high-temperature synthesis, combining two modes of the process – SHS from elemental powders and SHS-metallurgy. A significant effect of the ratio of reagents in the initial mixtures on the parameters of the combustion process, microstructure, and phase composition of the products was found. Introduction into the high-exothermic mixture 3MoO₃–8Al–C inert “cold” mixture 3Mo–2Al–C leads to an increase in the content of carbide phases in the ingots. The maximum content (~87 wt. %) of the ternary phase Mo₃Al₂C was obtained at α = 0.4. The presence of carbide Mo₂C and molybdenum aluminides Mo₃Al₈, and Mo₃Al in the final products is due to a change in the stoichiometric composition of the initial charge caused by the ejection of components during the combustion and insufficient existence time of the melt, which leads to the formation of a non-equilibrium composition of the product.