Exothermic and Slag Formation Behavior of Aluminothermic Reduction of Mo and V Oxides

Abstract

Vanadium (V), molybdenum (Mo), and aluminum (Al) are important alloying elements in titanium alloys, typically introduced through master alloys such as V-Al and Mo-Al. Current preparation of these master alloys predominantly relies on the spontaneous reduction of V₂O₅ or MoO₃ by aluminum. However, separate production and addition of master alloys increase the cost of the titanium alloy. Insufficient understanding of the exothermic behavior and slag-forming process during the aluminothermic reaction often leads to low alloy yield and elevated impurity levels due to splashing and poor alloy–slag separation. This study focused on the controllable aluminothermic reaction of V₂O₅ and MoO₃ to produce high-quality and high-yield V/Al/Mo alloy. Thermodynamic calculations indicate that the reduction of MoO₃ to Mo by aluminum is more favorable than the reduction of V₂O₅ to V. Al% in the V-Al-Mo alloy is crucial for controlling reaction temperature. When the Al/O ratio in the raw materials exceeds 1.0, increasing aluminum reduces both the reaction exothermicity and theoretical reaction temperature. A combination of thermodynamic calculations and high-temperature experiments demonstrates that the heat generation and slag composition can be effectively controlled by Al/O ratio in raw materials. When the Al/O ratio in raw materials is 1.6–2.0, the yields of Mo and V exceed 99% and 95%, respectively. This study provides an effective approach to producing V/Al/Mo alloy under controllable conditions, which shows great potential for other aluminothermic reactions. Extensive solid solutions of V/Al/Mo also provide invaluable data for the optimization of the alloy database.

1. Introduction

Titanium alloys, renowned for their high specific strength, exceptional resistance to extreme temperatures, superior corrosion resistance, excellent weldability, and biocompatibility, have emerged as core materials in aerospace, marine engineering, energy chemical industries, and biomedical applications [1,2,3,4]. Their strategic significance is particularly pronounced in aerospace, where the escalating demand for lightweight materials capable of withstanding extreme operational conditions has solidified their indispensable role [5,6,7,8]. The performance of titanium alloys is intrinsically linked to the selection and precise proportioning of alloying elements. Aluminum (Al), functioning as an α-phase stabilizer, enhances both ambient and high-temperature mechanical properties via solid solution strengthening, while molybdenum (Mo) and vanadium (V), acting as isomorphous β-phase stabilizers, optimize heat treatment responsiveness and improve strength toughness synergy [9,10,11,12,13]. Currently, Al, Mo, and V are predominantly introduced into titanium melt as binary master alloys (e.g., Al-Mo and Al-V). However, the production of the master alloys separately increases their cost. The sequential addition of multiple components complicates the smelting process and exacerbates compositional segregation [14]. Development of V/Al/Mo multicomponent master alloys presents a viable solution to these challenges.

Preparation of V/Al/Mo alloys primarily relies on the simultaneous aluminothermic reduction of MoO₃ and V₂O₅, a process whose exothermic characteristics critically influence alloy yield and purity. Zhang et al. [15] attempted to synthesize V/Al/Mo alloys via ex-furnace aluminothermic methods, but inadequate control over reaction intensity resulted in heterogeneous microstructures and elevated impurity levels. Alternative approaches, such as pre-pelletizing MoO₃ and V₂O₅ powders to mitigate splattering [16], face challenges including silicon contamination from binders and inconsistent pellet size distribution, thereby compromising process stability and product quality.

Suboptimal reaction conditions may induce splashing, raw material loss, low alloy yield, and impurity accumulation, collectively degrading alloy quality and escalating production costs [17,18,19]. Research has demonstrated that tailored strategies, such as raw material design (e.g., employing low-valent vanadium oxides [20,21,22]), slag composition modulation (e.g., CaO addition [23,24]), cooling material incorporation [25,26], and electric furnace control [27,28], can balance exothermic behavior and enhance slag-metal separation. For instance, Sun [20] used a V₂O₅ and V₂O₃ mixture to control the heat for vanadium–aluminum alloy production. Similarly, Yee et al. [27] revealed that moderate CaO additions (6–8 wt.%) promoted the formation of low-melting-point slag phases (e.g., CaO·Al₂O₃), whereas excessive CaO (>12 wt.%) generated refractory phases (e.g., CaO·6Al₂O₃), impairing slag fluidity. Despite these advancements, critical gaps persist in the aluminothermic reduction of V-Mo oxides: (1) the thermodynamic competition mechanisms governing multi-oxide co-reduction remain unclear; (2) strategies to suppress reaction violence and splashing are underdeveloped; (3) achieving compositional homogeneity and impurity control in the alloys remains challenging.

To address these issues, this study proposes a simultaneous aluminothermic reduction process utilizing V₂O₅, MoO₃, CaO, and Al as raw materials. Through integrated thermodynamic simulations and experimental validation, we elucidate the preferential reduction sequence of MoO₃ versus V₂O₅, establish quantitative correlations between the aluminum-to-oxygen ratio (Al/O) and reaction temperature/phase composition, and optimize process parameters to achieve high-yield (Mo > 99%, V > 95%), low-impurity, and compositionally controlled V/Al/Mo alloys. This work provides both theoretical insights and practical methodologies for advancing the cost-effective production of high-performance titanium alloys.

2. Research Methodology

2.1. Raw Materials and Experimental Plan

The raw materials used in this study include V₂O₅, MoO₃, aluminum granules, and calcium oxide (CaO). Detailed specifications of these materials are summarized in

Table 1. Raw materials used in the present work.

Material	Form	Purity (%)	Manufacturer
V2O5	Powder	≥99.7	Chengde Vanadium Titanium Co., Ltd. (Chengdu, China)
Al	Granules	≥99.9	Beijing Kerry New Materials Co., Ltd. (Beijing, China)
CaO	Powder	≥99.5	Xilong Scientific Co., Ltd. (Shantou, China)
MoO3	Powder	≥99.5	Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China)

.

During the production of V/Al/Mo alloy, excess metallic aluminum reacts with V₂O₅ and MoO₃ through an aluminothermic reduction process. The resultant metallic vanadium (V) and molybdenum (Mo) subsequently combine with excess aluminum to form the V/Al/Mo alloy. The ratio of V to Mo in the alloy was kept at 1:1, which is a typical one in Ti alloys. The concentrations of (V + Mo) are primarily controlled by the excess aluminum used in the reaction system. The principal chemical reactions and physical process reactions are:

(i)

Chemical reactions: Reduction of V₂O₅ and MoO₃ by Al (Equation (1)), supported by stoichiometry and exothermicity;

(ii)

Physical process: Alloy formation of V, Mo, and excess Al (Equation (2)). This distinction ensures accurate representation of both the redox chemistry and solid-state alloying. The V:Mo = 1:1 ratio and the role of excess Al in controlling (V + Mo) content are retained.

$$12\mathrm{Al}+3{\mathrm{V}}_2{\mathrm{O}}_5+\mathrm{Mo}{\mathrm{O}}_3=6\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+6\mathrm{V}+\mathrm{Mo}$$

(1)

$$\mathrm{Al}+\mathrm{V}+\mathrm{Mo}\to \mathrm{V}/\mathrm{Al}/\mathrm{Mo} \mathrm{alloy}$$

(2)

The aluminum in the reactants serves dual roles as both reducing agent and alloying element. The total aluminum requirement equals the sum of: (1) aluminum consumed in Reaction (1) for oxide reduction and (2) aluminum incorporated into the alloy in Reaction (2). Al/O is used to express the amount of aluminum required to complete Reaction (1) as a reducing agent. The compositions and weights of slags are planned to be the same in all experiments. Detailed experimental conditions are provided in

Table 2. Experimental conditions.

Exp No.	Al (g)	V2O5 (g)	MoO3 (g)	CaO (g)	Al:V:Mo in Alloy	Al in Alloy (wt%)	Al/O (wt)
#1	2.8	3.0	2.5	4.5	2:9:9	10	1.2
#2	3.3	3.0	2.5	4.5	1:2:2	20	1.4
#3	3.9	3.0	2.5	4.5	6:7:7	30	1.6
#4	4.7	3.0	2.5	4.5	4:3:3	40	2.0
#5	5.8	3.0	2.5	4.5	2:1:1	50	2.4
#6	7.4	3.0	2.5	4.5	3:1:1	60	3.1

.

2.2. Experimental Procedure

The powders of V₂O₅, MoO₃, and CaO were dried at 120 °C for 2 h to remove moisture and then mixed in an agate mortar according to the predetermined ratio for 30 min. The mixture was mixed with aluminum particles (~1 mm) evenly and pelletized in a stainless steel mold (inner diameter: 2 cm). The pellets were subsequently loaded into a high-purity alumina crucible (purity ≥ 99%, dimensions: 25 mm inner diameter × 55 mm height), which was nested inside a graphite crucible. The crucible assembly was supported by a thermocouple sheath and positioned within the isothermal zone of a high-temperature tube furnace. A type-B thermocouple inside an alumina protection sheath was embedded in an upside-down graphite crucible to continuously monitor the temperature during the reaction. A schematic diagram of the experimental setup is shown in Figure 1.

The entire experiment was conducted under an argon protective atmosphere to prevent oxidation of metallic aluminum and the resulting V/Al/Mo alloy at high temperatures. The sample was precisely positioned in the furnace’s hot zone, with real-time temperature monitoring via a high-precision Type B thermocouple for precise thermal control. A programmed heating profile was implemented to optimize alloy quality and slag separation: the temperature was raised at 10 °C/min to 1550 °C, followed by a 15-min hold to achieve thermal equilibrium, promote slag–alloy segregation, and enhance elemental diffusion for the uniform microstructure and composition of the alloy. The experiments were conducted according to the ratios/proportions outlined in Table 2. After the aluminothermic reaction, all samples were cooled to room temperature in the furnace. The crucible was then carefully fractured to retrieve the alloy and slag, which underwent subsequent microstructural analysis and compositional testing to evaluate phase distribution and elemental homogeneity.

2.3. Sample Characterization

Both slag and alloy samples were mounted in epoxy resin and subjected to sequential grinding and polishing. Microstructural features and polishing quality were examined using a LEICA DMC5400 optical microscope (Leica, Wetzlar, Germany, Europe) Phase compositions and microstructure were analyzed with a JXA-ISP100 electron probe microanalyzer (JEOL, Tokyo, Japan). The standards used for EPMA included alumina (Al₂O₃) for Al, magnesia (MgO) for Mg, wollastonite (CaSiO₃) for Ca and Si, vanadium for V, and molybdenum for Mo. Note that EPMA can only measure elemental composition. All vanadium oxides in different electronic states are expressed as “V₂O₅” for presentation purposes. Bulk chemical compositions of the alloy and slag were determined using an Agilent ICPOES730 (Agilent, Santa Clara, CA, USA) inductively coupled plasma optical emission spectrometer (ICP-OES).

2.4. Thermodynamic Predictions

The FactSage 8.3 software [29,30] provides preliminary information for slag design and metallurgical process optimization, reducing the number of high-temperature experiments. In this study, thermodynamic calculations were performed using FactSage 8.3. The Reaction module was employed to calculate the Gibbs free energy of the aluminothermic reactions, while the Equilib module simulated thermodynamic equilibrium conditions. Selected databases included FactPS, FToxid, and SGnobl, with configured solution phases, FToxid-SLAGA, FToxid-SPINC, FToxid-MeO_A, FToxid-CORU, FToxid-VO_ and SpMCBN-LIQU.

3. Thermodynamic Analysis of Aluminothermic Reactions

3.1. Gibbs Free Energy and Enthalpy Changes in Aluminothermic Reactions

In aluminothermic reduction processes, aluminum acts as a reducing agent to progressively reduce vanadium oxides to V. The reactions using different vanadium oxides can be described as follows:

$$\mathrm{Al}+0.3 {\mathrm{V}}_2{\mathrm{O}}_5=0.5 \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+0.6 \mathrm{V}$$

(3)

$$\mathrm{Al}+0.375 {\mathrm{V}}_2{\mathrm{O}}_4=0.5 \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+0.75 \mathrm{V}$$

(4)

$$\mathrm{Al}+0.5 {\mathrm{V}}_2{\mathrm{O}}_3=0.5 \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3 +\mathrm{V}$$

(5)

$$\mathrm{Al}+1.5 \mathrm{VO}=0.5 \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+1.5 \mathrm{V}$$

(6)

Similarly, the overall aluminothermic reduction reaction for molybdenum oxide can be expressed as:

$$\mathrm{Al}+0.5 \mathrm{Mo}{\mathrm{O}}_3=0.5 \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+0.5 \mathrm{Mo}$$

(7)

For the Al-V₂O₅ system, researchers such as Jing et al. [31], Duan et al. [32], and Yin et al. [33] investigated the thermal behavior of aluminothermic reactions using differential scanning calorimetry (DSC) at varying heating rates. Their results revealed an endothermic peak near 660 °C, corresponding to the melting of aluminum, and a minor exothermic peak within 670–700 °C, attributed to the initiation of the aluminothermic reaction, indicating that the reaction commences after aluminum melting. The standard molar Gibbs free energy change for a reaction can be expressed in two conventions: (1) based on the formation of 1 mole of compound; (2) based on the reaction of 1 mole of elemental reactant [34]. In this study, the Reaction module of FactSage was employed to calculate the Gibbs free energy changes (ΔG) and enthalpy changes (ΔH) for Reactions (3)–(7) at 660 °C (aluminum melting point), using the 1-mole elemental reactant (Al) convention. The computational results are illustrated in Figure 2a.

The x-axis in Figure 2a represents the reduction of vanadium oxides from different oxidation states to metallic vanadium, while the y-axis indicates the corresponding Gibbs free energy and enthalpy changes. The reduction of MoO₃ to metallic Mo is also included for comparison. The results in Figure 2a reveal the following:

(1)

All reactions exhibit negative values for both Gibbs free energy (ΔG) and enthalpy changes (ΔH), confirming that these reactions are thermodynamically spontaneous and exothermic at 660 °C.

(2)

Comparing the free energy and enthalpy changes for the reduction of vanadium oxides from different oxidation states to metallic vanadium, higher oxidation states (e.g., V⁵⁺ in V₂O₅) demonstrate greater thermodynamic favorability and release more heat. Notably, the reduction of MoO₃ to metallic Mo is thermodynamically more favorable and releases significantly more heat compared to the reduction of V₂O₅ to metallic V.

Figure 2b shows the Gibbs free energy changes as a function of temperature for reactions (3) to (7). Because all aluminothermic reactions are exothermic, ΔG of the reactions (3) to (7) increases with increasing temperature. Negative ΔG indicates that all reactions shown in Figure 2b are thermodynamically spontaneous, and the order of the reaction tendency does not change up to 2000 °C.

3.2. Thermodynamic Analysis of the Reaction System

As shown in Figure 2, the aluminothermic reduction reaction releases substantial heat, which elevates the system temperature. Assuming no heat loss, the maximum reaction temperature (T_max) can be calculated based on the actual mass of raw materials. The calculation method is as follows: under the experimental conditions listed in Table 2, using V₂O₅, Al, MoO₃, and CaO as starting materials, and assuming complete reduction, the compositions and masses of all slags are fixed as the same, while the mass ratio of V to Mo in the alloy is maintained at 1:1. The Equilib module in FactSage was employed to simulate the reactions shown in Table 2. The FactPS, FToxid, and SGnobl databases were selected for the calculations. The heat released per kg of reactants (unit heat) is presented in Figure 3a as a function of Al/O in raw materials. Assuming the ΔH = 0 and the reaction system is closed, the maximum reaction temperature T_ma, as a function of Al/O in raw materials, is shown in Figure 3b.

It can be seen from Figure 3 that the aluminum addition significantly influences the unit heat and T_m. When Al/O < 1, unit heat and T_m rise rapidly with increasing Al/O ratios because all aluminum reacts with the oxides as a reducing agent, releasing heat from the reaction and raising the system temperature. At Al/O = 1, the system reaches its theoretical maximum temperature (2442 °C), where the oxides of vanadium and molybdenum are fully reduced. However, when Al/O > 1, excess aluminum only reacts with the V and Mo to form a V/Al/Mo alloy, increasing the alloy mass and absorbing heat for melting, thereby lowering T_m. When the aluminum content in the alloy increases from 10% (#1) to 60% (6), the unit heat reduces from 2980 to 2500 kJ/kg, and T_m decreases from 2427 to 2002 °C.

Figure 2 and Figure 3 collectively demonstrate that the reaction temperature can be controlled through exothermic or endothermic processes. In practice, using lower-valence oxides (e.g., VO or V₂O₃ instead of V₂O₅) reduces the heat generated, while increasing the mass of the alloy or slag enhances heat absorption, effectively lowering the system temperature. This thermodynamic framework provides critical insights for optimizing industrial aluminothermic processes through stoichiometric adjustments and material selection.

4. Experimental Results and Discussion

Figure 4 presents macrographs of post-reaction samples from #1 and #3, illustrating the effects of reaction conditions on alloy–slag separation. When the aluminum content in the alloy is low (e.g., #1 and #2, corresponding to Al/O ratios of 1.2 and 1.4, respectively), the reaction releases significant heat, generating a theoretical maximum temperature over 2400 °C. As shown in Figure 4, under these conditions, the alumina crucible fractures during the reaction, causing melt leakage and incomplete aluminothermic reduction. The alloy yield is minimal, and the alumina crucible sticks with the graphite crucible. In contrast, when the aluminum content in the alloy is high (e.g., #3–#6), the reduced exothermicity lowers the theoretical maximum temperature. This prevents crucible failure, allowing the aluminothermic reaction to proceed to completion. Post-reaction samples exhibit clear separation between the alloy and slag, with intact crucible structures. This phenomenon confirms the critical role of Al/O ratio in balancing exothermicity and thermal management. Excessive heat generation at low Al/O ratios exceeds the crucible’s thermal tolerance, while controlled heat release at higher Al/O ratios ensures process stability and product integrity.

As shown in Figure 1, a B-type thermocouple was placed underneath the reaction system to monitor the temperature changes during the reaction. The thermocouple could not measure the real temperature of the sample because it was separated by the alumina sheath, graphite crucible, and alumina crucible. However, the rapidly increased temperature due to the reaction heat can still be observed, as shown in Figure 5. The temperature started to increase between 1477 and 1488 °C, indicating a large amount of heat was generated in this temperature range. The temperature was increased by 39 °C for #3 and 23 °C for #6, respectively, which is consistent with the calculated heat generation shown in Figure 3.

After separating the alloy from the slag, the compositions were analyzed by ICP and plotted together with the designed alloy compositions (from Table 2) on the V/Al/Mo ternary phase diagram [35,36], as shown in Figure 6. Triangular symbols represent the designed alloy compositions, while circular dots denote the actual alloy compositions. It can be seen from the figure that #1 and #2 alloys are in the (Mo, V) primary phase field. The compositions of the rest of the alloys are in the Al₈V₅ primary phase field. The results reveal that the compositions of #3-#6 alloys are close to the designed values, while #1 and #2 alloys contain lower V than the designed ones. Combining the results shown in Figure 3 and Figure 4, it seems that for low-Al alloys (#1 and #2), excessive exothermicity (theoretical maximum temperature > 2400 °C) caused crucible rupture, prematurely terminating the reaction and leaving excess unreacted Al and vanadium oxides, resulting in higher Al and lower V in the alloys than the designed values. The preferential reduction of MoO₃ over V₂O₅, consistent with the lower Gibbs free energy for MoO₃ reduction (Figure 2), explains the lower V/Mo ratios. These results confirm that controlling reaction exothermicity through Al/O ratio optimization is critical for achieving high alloy yield and compositional accuracy, consistent with the literature [17].

Figure 7, Figure 8 and Figure 9 show the typical microstructures of the slag and alloy, while

Table 3. Bulk (ICP) and phase compositions (EPMA) in the slow-cooled slag after aluminothermic reaction.

No.	Phases	wt%				mol%
		Al2O3	CaO	“V2O5”	MoO3	Al2O3	CaO	“V2O5”	MoO3
#1	CaO·Al2O3	65.2	34.7	0.1	0.0	50.8	49.2	0.0	0.0
	Bulk	64.9	33.7	1.2	0.2	51.1	48.3	0.5	0.1
#2	Inclusion in alloy	65.1	34.7	0.2	0.0	50.8	49.2	0.0	0.0
	CaO·Al2O3	65.2	34.8	0.0	0.0	50.5	49.5	0.0	0.0
	CaO·2Al2O3	78.9	21.1	0.0	0.0	67.3	32.7	0.0	0.0
	Bulk	69.4	29.8	0.7	0.1	56.1	43.6	0.3	0.0
#3	CaO·Al2O3	64.8	35.2	0.0	0.0	50.3	49.7	0.0	0.0
	Bulk	68.9	30.3	0.7	0.1	55.4	44.3	0.3	0.0
#4	CaO·Al2O3	63.3	36.7	0.0	0.0	48.5	51.5	0.0	0.0
	12CaO·7Al2O3	54.2	45.8	0.0	0.0	39.3	60.7	0.0	0.0
	Bulk	68.4	30.6	0.8	0.2	55.0	44.6	0.3	0.1
#5	Inclusion in alloy	64.7	35.3	0.0	0.0	50.3	49.7	0.0	0.0
	CaO·Al2O3	64.8	35.2	0.0	0.0	50.3	49.7	0.0	0.0
	12CaO·7Al2O3	51.7	48.3	0.0	0.0	37.0	63.0	0.0	0.0
	Bulk	67.0	32.8	0.1	0.1	52.7	47.1	0.1	0.1
#6	Inclusion in alloy	65.4	34.4	0.0	0.0	51.2	48.8	0.0	0.0
	CaO·Al2O3	64.8	35.2	0.0	0.0	50.3	49.7	0.0	0.0
	Bulk	66.1	33.8	0.1	0.0	51.9	48.1	0.0	0.0

and

Table 4. Bulk (ICP) and phase compositions (EPMA) in the slow-cooled V/Al/Mo alloys synthesized via aluminothermic reaction.

No.	Phases	wt%				mol%
		V	Al	Mo	Ca	V	Al	Mo	Ca
#1	Alloy in slag	93.9	2.6	0.0	3.5	90.9	4.8	0.0	4.3
	(Al,Mo,V)	36.3	16.0	47.5	0.2	39.5	32.9	27.4	0.2
	Bulk	35.9	17.2	46.6	0.3	38.4	34.7	26.5	0.4
#2	Alloy in slag	93.2	0.4	6.4	0.0	95.4	0.9	3.7	0.0
	(Al,Mo,V)	35.6	19.5	44.9	0.0	36.9	38.4	24.7	0.0
	Bulk	35.6	21.1	42.7	0.6	36.0	40.4	22.9	0.7
#3	Alloy in slag	37.3	62.3	0.1	0.3	24.0	75.6	0.1	0.3
	Al(Mo,V)	37.1	26.5	36.4	0.0	34.7	47.2	18.1	0.0
	Al8(V,Mo)5	27.4	38.8	33.8	0.0	23.1	61.8	15.1	0.0
	Bulk	33.9	31.6	34.3	0.2	30.2	53.4	16.2	0.2
#4	Alloy in slag	37.5	62.0	0.1	0.4	24.1	75.5	0.1	0.3
	Al(Mo,V)	39.9	27.1	33.0	0.0	36.8	47.0	16.2	0.0
	Al8(V,Mo)5	28.9	39.8	31.3	0.0	23.9	62.4	13.7	0.0
	Bulk	29.8	40.8	29.3	0.1	24.3	62.9	12.7	0.1
#5	Al8(V,Mo)5	36.6	42.5	20.9	0.0	28.7	62.7	8.6	0.0
	Al3(V,Mo)	19.3	53.0	27.7	0.0	14.4	74.7	10.9	0.0
	Bulk	24.2	52.2	23.2	0.4	17.9	72.6	9.1	0.4
#6	Al3(V,Mo)	15.9	54.1	30.0	0.0	11.9	76.2	11.9	0.0
	Al45(V,Mo)7	16.0	76.6	7.4	0.0	9.8	87.8	2.4	0.0
	Al	0.5	99.3	0.2	0.0	0.2	99.7	0.1	0.0
	Bulk	18.0	62.6	18.7	0.7	12.2	80.5	6.7	0.6

list the bulk compositions analyzed by ICP and phase compositions determined by EPMA, respectively. As described in Figure 2, Figure 3 and Figure 4, variations in the Al/O ratio led to differences in exothermicity, reaction completeness, and slag–alloy separation efficiency. Integrating the microstructures in Figure 7, Figure 8 and Figure 9 with the compositional data in Table 3 and Table 4, the following general observations are obtained: (1) CaO in the slag originates entirely from the raw materials, while Al₂O₃ primarily results from the oxidation of metallic aluminum, with a minor contribution from the crucible dissolution. (2) Residual V and Mo in the slag may arise from unreduced oxides or alloy entrapment. (3) V and Mo in the alloy are reduction products, Ca exists as oxide inclusions, and Al is predominantly metallic but partially bound to the inclusions. (4) The phases present in the alloy were all reported in the literature [35], including (Mo,V), Al₈V₅, AlMo, Al₃V, and Al₄₅V₇. Because of the similarity between V and Mo, extensive solid solutions can be formed where V and Mo can replace each other.

As shown in Figure 7, the uniform microstructures indicate that the slags in both #1 and #2 were liquid at high temperature, and CaOꞏAl₂O₃ and CaOꞏ2Al₂O₃ were formed under cooling. The residual metallic vanadium remains trapped within the slag, failing to fully integrate into the alloy. This phenomenon primarily arises from excessive exothermicity, which induces violent slag turbulence, preventing effective aggregation of reduced metallic vanadium and timely formation of the ternary alloy. Fine metallic vanadium particles present in the slag reduced Al and Mo levels in the alloy. This aligns with Figure 6, where the compositions of #1 and #2 alloys tend towards low V. Notably, the excessively high temperatures (2427 °C for #1 and 2375 °C for #2) and violent reaction dynamics under low aluminum conditions cause the crucible rupture before the reaction completion and exacerbate elemental segregation. It can be seen from Table 3 that the “V₂O₅” concentration in #1 and #2 slags is 1.2 and 0.7 wt%, respectively. This indicates that the reaction process was disrupted, leading to incomplete reactant conversion.

When the Al/O in raw materials is increased to 1.6 and 2.0 (#3 and #4), the compositions of the alloys obtained from the experiments are close to the designed values. Figure 8 illustrates the microstructures of the slags and alloys for #3 and #4. As aluminum content rises, the exothermicity of the reaction system gradually diminishes. Theoretical calculations reveal that the reaction temperature ranges from 2422 °C to 2299 °C under these conditions, significantly reducing reaction violence compared to #1 and #2. This controlled heat release ensures near-complete aluminothermic reduction without crucible damage, thereby facilitating efficient reaction progression. The slags in these samples predominantly consist of CaO·Al₂O₃ with minor 12CaO·7Al₂O₃ present in #4. The dominant phase in the alloys is Al₈(V,Mo)₅, which has a relatively large primary phase field as shown in Figure 6. Compared to low-aluminum alloys, higher aluminum content in the alloys significantly enhances compositional stability and microstructural uniformity, underscoring its critical role in alloy optimization. These results confirm that moderate aluminum addition effectively suppresses overheating, prevents excessive reaction temperatures, and mitigates issues related to alloy–slag aggregation and separation. Furthermore, optimized aluminum content refines the alloy microstructure and enhances its compositional homogeneity.

Figure 9 (#5 and #6) shows the typical microstructures of the slags and alloys where the Al/O in raw materials are 2.4 and 3.1, respectively. CaO·Al₂O₃ is the primary phase in both slags. 12CaO·7Al₂O₃ and trace amounts of the remaining liquid slag are present in the #5 slag. The #6 slag is also composed of a small amount of the remaining liquid slag. In the corresponding alloys, higher aluminum content drives distinct phase evolution: higher-aluminum phase Al₃(V,Mo) and Al₈(V,Mo)₅ are present in #5 alloy, Al₄₅(V,Mo)₇ and Al₃(V,Mo) are present in #6 alloy. Metallic aluminum is also observed in the #6 alloy.

In summary, variations in aluminum addition in the reaction system critically influence the exothermic behavior of V/Al/Mo synthesis. An optimum heat is necessary for the aluminothermic reduction to be completed. Excess heat generated within a short time can rapidly raise the system temperature, which can crack the crucible and stop the reaction.

It has been shown in the previous sections that the reaction heat can significantly affect the reaction progress. As shown in Figure 6, the extra heat cracked the crucible and stopped the reaction (#1 and #2). During high-temperature experiments, the slag can react with the alumina crucible, resulting in a higher Al₂O₃ concentration in the slag. The higher the reaction temperature, the more alumina is dissolved from the crucible. Therefore, different slag compositions could be obtained although they were designed to have the same value. Figure 10 shows the compositions of the slag as a function of Al/O in the raw material. It can be seen from Figure 10a that Al₂O₃ concentrations in the slags decrease with increasing Al/O from 1.4 to 3.1, which is consistent with the trend of the reaction heat. At Al/O = 1.2, excessive heat cracked the crucible and stopped the reaction, resulting in less dissolution of Al₂O₃ from the crucible. The CaO concentrations in the slags show an opposite trend to Al₂O₃.

Figure 10b shows that residual V₂O₅ and MoO₃ concentrations in the slags decrease with increasing Al/O ratio. Higher V₂O₅ is present in the slag than MoO_3, which can be explained by Figure 2, where MoO₃ has a stronger reduction tendency than V₂O₅ evidenced by ∆G. Excess aluminum promoted the reduction efficiency of V₂O₅ and MoO₃. The V₂O₅ and MoO₃ in the slags represent the loss of these elements during the production, which can be present in both oxide and alloy forms.

The recovery rates of alloy elements serve as critical indicators for the cost of the alloy synthesis. Figure 11 summarizes the recovery rates of vanadium (V) and molybdenum (Mo) as a function of Al/O ratio in raw materials. The recovery rates of V and Mo are calculated based on their remaining values in the slag, as shown in Table 3 and Figure 10b. It can be seen that the Mo recovery rate increases slightly with increasing Al/O ratio, and most of the recovery rate is over 99%. The V recovery rate is more sensitive to the Al/O ratio. The recovery rate of vanadium is increased from 95% to 99% when the Al/O ratio is increased from 1.2 to 3.1.

5. Conclusions

This study investigated the exothermic behavior of aluminothermic reduction for synthesizing V/Al/Mo alloys, with a focus on the influence of the Al/O ratio in raw materials on thermodynamic characteristics. By integrating thermodynamic calculations and experimental validation, the thermal behavior and process parameters under varying Al/O ratios were systematically analyzed to identify optimized production strategies. Experimental results demonstrate that aluminum content significantly affects the reaction heat, microstructure, elemental recovery rate, and alloy composition. An appropriate Al/O ratio is critical for achieving high recovery rates and stable alloy quality. Key conclusions are summarized as follows:

(1)

Thermodynamic analysis confirms the viability of synthesizing V/Al/Mo alloys via aluminothermic reduction. Specifically, MoO₃ exhibits higher reducibility to metallic Mo compared to V₂O₅ reduction to metallic V, with higher-valence vanadium oxides releasing greater exothermic energy during reduction.

(2)

Optimizing reductant dosage (Al/O ratio) is essential for temperature regulation. When Al/O > 1, higher aluminum content reduces both reaction exothermicity and theoretical reaction temperature.

(3)

Adjusting the Al/O ratio effectively governs the composition and phase distribution of the ternary alloy. Aluminum–vanadium-based phases dominate the alloy matrix, while the slag primarily consists of a low melting point CaO·Al₂O₃ phase.

(4)

High-quality V/Al/Mo alloys can be synthesized in a single-step aluminothermic reduction process at 1550 °C with a high recovery rate. When the aluminum content in the alloy is controlled above 30%, the recovery rates of Mo and V exceed 99% and 95%, respectively.

(5)

In actual production, it is essential to obtain a master alloy to meet the requirements of the high-melting-point elements V and Mo in the titanium alloy. The proper control of the maximum temperature and compositions of alloy and slag ensures a high alloy yield, which can be achieved by an optimum Al/O ratio in the raw materials. This work provides a theoretical foundation for aluminothermic reduction-based preparation of V/Al/Mo alloys used in titanium alloys and offers practical guidelines for industrial-scale production.