Mo–Nb–Si–B Alloy: Synthesis, Composition, and Structure

Abstract

Cast refractory alloys Mo–Nb–Si–B were prepared by centrifugal self-propagating high-temperature synthesis (SHS) from metallothermic mixtures containing MoO₃, Nb₂O₅, Al, Si, and B powders, and additive of Al₂O₃ as a temperature-moderating and chemically inert agent. Variation in the centrifugal acceleration and amount of the additive affected the composition and structure of cast Mo–Nb–Si–B alloys. In a wide range of values, the combustion temperature was found to exceed 3000 K, and the combustion products were obtained as two-layer ingots of target Mo–Nb–Si–B alloy (lower) and Al₂O₃ slag (upper).

1. Introduction

Mo–Si alloys have high resistance to oxidation in air at temperatures of 1000–1650 °C; however, at the intermediate temperatures of 600–800 °C, they are prone to catastrophic oxidation [1,2]. The addition of B makes it possible to form a dense borosilicate glass that protects ceramics against oxidation. Mo–Si–B alloys prepared by heating and subsequent cooling were shown in [3] to represent Mo-based solid solution with Mo₃Si and/or Mo₅SiB₂ inclusions. These alloys possess a far greater oxidation resistance than previously known molybdenum ones, but they are not as good as Mo₅Si₃–Mo₃Si–Mo₅SiB₂ alloys [4], and yet they contain a plastic α-Mo phase. As mentioned in [5], varying the volume fraction and morphology of the α-Mo phase in these Mo–Mo₃Si–Mo₅SiB₂ intermetallics allows it to achieve high values of fracture toughness and creep strength. Additional doping with Nb strengthens the molybdenum matrix but leads to no changes in the structural composition [6,7].

Mo–Si–B or Mo–Nb–Si–B alloys manufactured by the powder metallurgy method exhibit high heat resistance and high-temperature strength, thereby having the potential for turbo-engine applications. Because of the high melting points of Mo–Si-based alloys, multistage methods of powder metallurgy are favorable over melting ones. One of them includes the following stages: (1) mechanical activation in a vertical attritor, for 10 h, to completely dissolve Nb, Si, and B in the Mo matrix; (2) sintering at 1450 °C, and (3) hot isostatic pressing (HIP) at 1500 °C and under a pressure of 200 MPA [8]. An alternative route is the cost-effective, productive, and environment-friendly centrifugal self-propagating high-temperature synthesis (SHS) process, which provides the synthesis of Mo–Nb–Si–B alloy from a mixture consisting of a thermite composition MoO₃/Nb₂O₅/Al/Si/B and an elemental composition Mo/Nb/Si/B, such as a temperature-moderating additive [9]. In this work, we added Al₂O₃ instead of the costly and scarce Mo and Nb.

Thus, this work aimed at the preparation of Mo–Mo₃Si–Mo₅SiB₂ alloy by centrifugal SHS of MoO₃–Nb₂O₅–Al–Si–B powder mixtures containing Al₂O₃ as a temperature-moderating agent with special emphasis on optimizing process conditions.

2. Thermodynamic Calculation

Table 1. Composition of the green mixture and nominal weight percentage of elements in a combustion product (wt %).

	Mo	Si	Nb	B	MoO3	Nb2O5	Al
Green mixture	–	1.5	–	0.5	68.9	2.4	26.7
Combustion product	92.5	3.0	3.4	1.0	–	–	–

shows the composition of the green mixture and nominal weight percentage of elements (Mo, Si, B, and Nb) in a combustion product.

Thermodynamic calculation (Figure 1a, where P is mass fraction of phases in combustion products), carried out using the software package TERMO 2.0, ISMAN, Chernogolovka, Russia [10], showed that the combustion of MoO₃/Nb₂O₅/Al/Si/B mixture occurs at high temperatures in the range of 2225–3500 K, at which condensed combustion products—alloy (Mo–Nb–Si–B) and slag (Al₂O₃)—are in a liquid phase state.

Thermodynamic consideration predicts the formation of up to 10 wt % (P) of gaseous Al₂O, BO, B₂O₂, SiO₂, NbO₂, and so on (Figure 1a). The addition of aluminum oxide (α) to green composition decreases the combustion temperature T and mass fraction P of gas phase, as well as contributing to the appearance of B and Si in the combustion products (Figure 1b, where k is the percentage of constituents in the metal phase).

3. Experimental

Green mixtures containing powders of MoO₃, Nb₂O₅, Al, Si, and B in the amount of 40 g were ignited in a quartz tube (25 mm in diameter, 70 mm high) using a centrifugal machine described in [11] at the centrifugal acceleration a = 1–400 g.

In our experiments, the burning velocity U, the material loss (η₁) caused by sputtering, and the yield of target metallic phase into ingot η₂ were calculated using the following relationships:

U = h/t

(1)

η₁ = [(m₁ − m₂)/m₁] × 100%

(2)

η₂ = (m_exp/m_cal) × 100%

(3)

where h is the mixture height, t is the burning time, m₁ is the mass of green mixture, m₂ is the mass of combustion product, and m_exp and m_cal are the experimental and calculated mass of metallic ingot, respectively. m_cal was worked out on the basis of chemical equation with the complete alumothermal reduction of initial oxides and alloying elements (Si and B) in the corresponding weighed portions.

The combustion products were characterized by scanning electron microscopy SEM (Carl Zeiss Ultra Plus microscope, Carl Zeiss, Jena, Germany) and X-ray diffraction analysis XRD (DRON-3M diffractometer, Cu-K_α radiation, Burevestnik, St. Petersburg, Russia). Concentrations of Mo, Nb, Si, and Al in the final product were determined by spectrophotometry. The determination of boron was carried out by potentiometric titration of mannitol-boric acid, while that of oxygen was performed by reductive melting in an inert carrier gas flow. The Vickers hardness of the synthesized samples was measured using a 100 g load and a 15 s loading time (Instron 402MVD tester, Wilson Instruments, Norwood, MA, USA).

4. Results

The experiments showed that the combustion of high-exothermic mixture is accompanied by the splashing of burning products. This was suppressed by the introduction of Al₂O₃ (α) into the mixture and forces of artificial gravity. The mixtures were found to burn within the range α = 0–50%. As α increased within the indicated range, material loss η₁ significantly decreased, as seen in Figure 2. The cast product was formed for α = 0–40%. In this case, the combustion products were prepared as two-layer ingots: Mo–Nb–Si–B target alloy (lower) and Al₂O₃ slag (upper).

As shown in Figure 3, the action of gravity forces makes it possible to increase the product yield η₂ from 80 to 90 wt %. An additional point to emphasize is that, as a/g grows, the amount of pore space in the resultant cast Mo–Nb–Si–B material, according to microstructural analysis, decreases. For a > 100 g, a pore-free structure is formed.

Figure 4 and Figure 5 illustrate the influence of α and a/g on the percentage of constituents (k) of cast Mo–Nb–Si–B material, respectively. For α = 10–40% and a = 40–400 g, EDS analysis for Nb gives 2–2.5 wt %, which is lower than the calculated value. The measured contents of Si and B show the values close to nominal ones (3–3.5 and 1 wt %, respectively). With increasing α and a/g, Al concentration is seen to decrease from 6 to 2 wt %. The content of the main element (Mo) is within the range of 85–90 wt %.

The XRD pattern of Mo–Nb–Si–B ingot collects the peaks belonging to the following phases: (1) α-Mo, (2) Mo₃Si, and (3) Mo₅SiB₂ (Figure 6a).

No other reacted phases were found. It is pertinent to note that XRD analysis shows no peaks of Nb insofar as it was completely dissolved in the Mo matrix phase during synthesis. A Mo/Si ratio in Mo₃Si is 91.1/8.9 (wt %), while a Mo/Si/B ratio in Mo₅SiB₂ is 90.6/5.3/4/1. The slag is seen in Figure 6b to contain Mo in addition to conventional phase (Al₂O₃). Optic metallography confirmed the presence of individual spherical Mo particles in the oxide layer. Within the ranges α = 0–10% and α = 40–50%, there is a high Mo content that favors the formation of a metal–ceramic structure in the slag layer. Thus, α = 10–40% was chosen as optimal.

The SEM images presented in Figure 7 show a cast structure consisting of Mo solid solution (marked in Figure 7b by 1) and two intermetallic phases, Mo₃Si and Mo₅SiB₂ (2 and 3, respectively). A quantitative analysis by X-ray diffraction revealed that the main phase is Mo₃Si; its volume fraction approximates 40%. α-Mo phase has a volume fraction of around 30%, and, as seen in Figure 7a, forms to be discontinuous. According to [5], this fact can positively affect the creep strength of Mo–Mo₃Si–Mo₅SiB₂ intermetallics.

In order to evaluate the mechanical properties, we measured the Vickers hardness of ingots derived from a = 100 g and α = 20 and 30 wt %. For α = 20 wt %, the average hardness value was 1350 HV. It is 80 HV lower than the hardness of ingot prepared at α = 30 wt %.

5. Discussion

The process of obtaining Mo-based cast alloys by centrifugal SHS includes 3 stages: (1) the combustion of MoO₃/Nb₂O₅/Al/Si/B highly exothermic mixture and the formation of two-phase—Mo–Nb–Si–B and Al₂O₃—melt; (2) the gravitational separation of melts insoluble in each other under artificial gravity; and (3) the cooling, crystallization, and formation of the composition and structure of Mo–Nb–Si–B and Al₂O₃.

The combustion of MoO₃/Nb₂O₅/Al/Si/B mixture is accompanied by the splashing of burning material. The latter is caused by the formation and release of gas under the action of Archimedean force. Thermodynamic calculation showed (Figure 1a) that up to 10 wt % of the gas phase (G.P.: Al₂O, BO, B₂O₂, SiO₂, NbO₂, and so on) can be formed. The introduction of Al₂O₃ into green mixture reduces gas formation, thereby markedly suppressing the splashing of mixture. However, as Al₂O₃ content increases, the combustion temperature decreases (see Figure 1a) and, as a result, the starting mixture loses its ability to burn.

The completion of the combustion results in a continuous Al₂O₃ melt containing Mo–Nb–Si–B drops. Under the action of gravity forces, heavy drops move to the bottom of the quartz mold and form a metal layer. The completeness of gravitational separation is determined by the ratio of velocities of the drops and the cooling of melt. Drop velocity is determined by the value of the centrifugal acceleration.

Under optimal conditions (α = 20–30% and a > 100 g), it is possible to suppress the splashing and to progress to 90 wt % of the yield of the target product (see Figure 3).

At the final stage of centrifugal SHS, the metal layer containing Mo–Nb–Si–B-based solid solution and two phases of Mo₃Si and Mo₅SiB₂ are formed. SHS-produced Mo–Mo₃Si–Mo₅SiB₂ alloy is characterized by high hardness values, which far exceeds (by approximately 3 times) those attainable in Mo–Nb–Si–B alloy fabricated by a powder metallurgical method (425 HV) [7].

6. Conclusions

Mo-based composition materials reinforced with Nb, Si, and B, possessing good high-temperature and heat-resistance properties, can be prepared by centrifugal SHS from the highly exothermic composition MoO₃/Nb₂O₅/Al/Si/B containing temperature-moderating additive (Al₂O₃) under the conditions of artificial gravity. Such materials seem promising as a candidate for the next generation of high-temperature structural materials and as a high-hardness coating material [12].