ISRN Metallurgy

Volume 2013 (2013), Article ID 429818, 5 pages

http://dx.doi.org/10.1155/2013/429818

Research Article

Electrochemical Fabrication of Niobium Silicon Alloys from Oxide Powder Mixtures

Fanke Meng and Huimin Lu

School of Materials Science and Engineering, Beihang University, No.37 Xueyuan Road, Haidian District, Beijing 100191, China

Received 14 January 2013; Accepted 31 January 2013

Academic Editors: Y. S. Chang, P. Lukac, A. Pinkerton, and Y.-d. Wang

Copyright © 2013 Fanke Meng and Huimin Lu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

NbSi alloys were prepared by direct electrochemically reducing four mixed Nb₂O₅ and SiO₂ powders (Nb-10Si, Nb-20Si, Nb-30Si, and Nb-37.5Si) in molten CaCl₂ electrolyte at 900°C. The samples were characterized with scanning electron microscope (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). No oxidized phases were remained by XRD tests. Under SEM, Nb phase was scattered in Nb₅Si₃ phase for the samples of Nb-10Si, Nb-20Si, and Nb-30Si. For the sample of Nb-37.5Si, only Nb₅Si₃ was obtained after electrolysis.

1. Introduction

Nickel alloys are the mostly used superalloys in aviation industry. Due to the demand of exploring new generation aircraft turbine engines, scientists and engineers are devoted to investigate less expensive superalloys with higher melting points to substitute the Ni-based alloys [1, 2]. Following this idea, NbSi alloys (0.5–37.5 at % Si) are attracting more and more attention because of the high melting points of the two composed phases (melting points: Nb, 2469°C; Nb₅Si₃, 2520°C) in the alloy system [1]. Moreover, NbSi alloys still have three other excellent properties: close coefficient of heat expansion of the two compositions (Nb, 7.3 × 10 k⁻¹; Nb₅Si₃, 6.1 × 10 k⁻¹), relative lower density (7.16 g cm⁻³), and reasonable strength at high temperature. Thus, NbSi alloys are considered to be a potential candidate of the next generation superalloys. Until now, the mainly used techniques for fabricating NbSi alloys are orientation solidification [3], smelting with argon arc [4], physical vapor deposition (PVD), as well as some other methods of powder metallurgy. Generally, all of these techniques have the following procedures: first, extracting pure Nb and Si from minerals; second, melting and mixing molten Nb and Si by using arc or electron beam melting method; third, casting the mixed molten alloys to ingots. However, the notorious weakness of the process is resulted from high energy consumption and complex production procedures. As a typical example, the only commercial process for the production of metallic Nb is reducing K₂NbF₇ or Nb₂O₅ with sodium reduction or carbothermal reduction, respectively [5]. For production of silicon, the conventional technique is reducing SiO₂ with carbothermal reduction technique. The typical reduction temperature is as high as 1700°C, which therefore needs intensive energy and releases carbon oxides as wasted gases in the process [6]. Also, another critical disadvantage should be paid more attention: it is difficult to homogenize Nb and Si in any melting process due to the very high melting point of metallic Nb.

During the recent decade, in order to produce high-melting-point metals and superalloys, many new skills were studied by scientists and engineers. Among them, direct electrochemical reduction of metal oxides or mixed oxides in molten electrolytes was an effective method for production of metals and alloys. The typical examples were FFC Cambridge Process and the derived methods. In this process, metal oxides were pressed to solid pellets as a cathode and the graphite were acted as an anode, which were soaked into the molten CaCl₂ at about high temperatures (700–900°C) for electrolysis simultaneously. Consequently, a potential was loaded between the two electrodes. Generally, this technique was used in production of many metals and nonmetals for example, Cr, Si, Nb, Ni, and Zr [7–12]. The whole reaction process can be described with the following equations:

Cathode

Anode

In these equations, the A is a symbol of metal or nonmetal elements. If the cathode was mixed oxides, the reduction production might be oxygen-free alloys. Therefore, this electrochemical method can be effectively used for refractory metals, high-melting-point nonmetals, and superalloys.

Herein, we used this electrochemical reduction method for the production of NbSi alloys in the molten CaCl₂ at high temperature (900°C). The processors were mixed Nb₂O₅ and SiO₂ powders of four various atomic ratios of Nb and Si. After electrolysis, the NbSi alloys were successfully obtained. After scanning electron microscope (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX) analysis, the NbSi alloys were composed of different phases and almost free of oxygen. Therefore, this electrochemical reduction method had paved a way for the NbSi alloys production, which is also able to be applied to fabricate other refractory alloys.

2. Experimental Details

2.1. Materials Preparation

In this research work, the electrolyte was CaCl₂. In this work, CaCl₂ (≥99.0%), Nb₂O₅ (>99.9%), and SiO₂ (>99.8%) powders were purchased from Chengde Optical and Electrical Materials Ltd., Hebei Province, China. The pellets preparation was following the below steps. First, CaCl₂ powders were dried at 80°C overnight and then pressed to small cylindrical blocks with a pressure of 30 MPa. Second, the cylindrical blocks were dried again at 80°C for several hours and then pulverized to small CaCl₂ pellets with an average sized less than 1 mm. For the preparation of the Nb₂O₅/SiO₂ pellets, the process was identical as that used for CaCl₂ pellets preparation. The Nb₂O₅ and SiO₂ powders were mixed with the atomic ratios of Nb-10Si (Nb, 90 at %; Si, 10 at %), Nb-20Si (Nb, 80 at %; Si, 20 at %), Nb-30Si (Nb, 70 at %; Si, 30 at %), and Nb-37.5Si (Nb, 62.5 at %; Si, 37.5 at %) between Nb and Si. The total mixed powder was 200 g. The graphite crucibles (inner diameter: ~9.0 cm, height: ~18.0 cm, wall thickness: ~1.0 cm) were cleaned with ethanol (>95%, Beijing Keshi Co. Ltd. Beijing, China) for at least 5 times until the inner walls of the graphite crucible were almost free of graphite powders. At last, the mixed Nb₂O₅/SiO₂ pellets were placed in the cleaned crucible and then the CaCl₂ pellets (~2000 g) were placed on the top of the Nb₂O₅/SiO₂ pellets.

2.2. Potentiostatic Electrolysis

The process of electrolysis was conducted in a programmable temperature-controlled furnace produced by China Iron & Steel Research Institute Group. The design of the instrument was shown in the schematic of Figure 1. The graphite crucible filled with Nb₂O₅/SiO₂ and CaCl₂ was placed in a graphite stage in the furnace. As an anode, a graphite rod (diameter: 6.0 cm) was just hanged above the crucible vertically by a mechanic instrument, which can push the graphite rod move up and down to ensure that the rod can contact with the molten electrolyte during the whole electrolysis process. The direct current for the electrolysis was supplied by a KGHF-1000A/6V rectifier, made by Beijing Chunshu Electric rectifier Ltd., Beijing, China. The temperature-controlled furnace was vacuumed and then filled with argon (≥99.9%, O₂ ≤ 10 ppm, H₂O ≤ 15 ppm, N₂ ≤ 5 ppm) as soon as possible. It should be noted that the argon flow was maintained with a rate of 1.0–1.5 L min⁻¹ and the outlet was adjusted with a valve to ensure that the gas pressure inside the furnace was about 0.01 MPa larger than the ambient pressure, which was helpful for avoiding air to ingress into the furnace in the electrolysis process. The heating process was of three steps: first, the temperature was heated to 780°C with a ramping rate of 10°C min⁻¹; second, kept at 780°C for 30 min, the CaCl₂ electrolyte was completely melted in the crucible; third, the temperature continued to rise up to 900°C with a heating rate of 10°C min⁻¹. Consequently, the potential was loaded and then stabilized at 3.0 V for electrolysis between the two electrodes. The electrolysis would be lasted for about 4-5 hours, and the graphite crucible with the products was removed out the furnace as the temperature in the furnace was decreased to room temperature after electrolysis. The product was obtained by smashing the crucible.

Figure 1: Schematic of eletrolysis system.

2.3. Characterizations of Morphology and Composition

Micrograph and composition analyses were conducted with a SEM system equipped with a CCD camera, from TESCAN. EDX equipment was purchased from OXFORD and analysis software was INCA. The phase composition of the NbSi alloys was identified by a Rigaku D/Max 2200 PC X-ray diffractometer.

3. Results and Discussion

After electrolysis at 900°C in molten CaCl₂, the samples were obtained. Figure 2 showed the pictures of the obtained product after electrolysis. As shown in Figure 2(a), the CaCl₂ electrolyte and the sample were molten together after electrolysis. By smashing the crucible, a block of ingot was obtained in Figure 2(b). Clearly, the ingot had two layers of different materials. A metal-like layer with a color of grayish was rich of alloys and other layer was mainly composed of electrolyte.

(a)

(b)

Figure 2: (a) top view of the crucible after electrolysis; (b) the sample with electrolyte after electrolysis.

It should be noted that Figure 2 only showed typical pictures after electrolysis of the processor powder for the alloy of Nb-37.5Si. The other three samples were also of a similar picture, which were not shown here. The morphologies of the alloys were shown in the SEM images in Figure 3. The white small particles with an average size about 10–20 nm had Nb concentration of 98.2 at % in Table 1. On the contrary, the concentrations of the Si and the O were only 1.6 at % and 0.2 at %, respectively. Therefore, it was clearly to show that the white small particles were mainly composed of Nb. In the grey areas, the concentrations of NB and Si were 89.4 and 10.1 at % respectively, which was almost equated to the ratio of Nb-10Si alloy. It should be noted that only a very small part of O was still remained in both the white small particles and the grey areas. Although the O was not completely electrochemical reduced, it can be removed by other metallurgical methods in the consequent process. Typically, the mixed Nb₂O₅/SiO₂ powder was almost completely electrodeoxidized in molten CaCl₂ at 900°C. For the processor powder mixture of Nb-20Si alloy, the product after electrolysis the SEM image was shown in Figure 3(b), which was similar to the image in Figure 3(a). The EDX results in the Table 1 also showed that the white particles were Nb and the grey areas were NbSi alloy (Nb-20Si). The similar SEM image and EDX results were shown for the sample Nb-30Si alloy. Compare the three alloys (Nb-10Si, Nb-20Si, and Nb-30Si), two conclusions were obtained. First, the grey areas in all the three images had Nb and Si concentrations close to 62.5 and 37.5 at %, respectively, which was the atomic ratio in the Nb₅Si₃ alloy. Second, the white small particles were almost mainly composed of metallic Nb. Third, although O was remained in a little part in all the three samples, most of the O were still removed electrochemically. For the powder mixture of Nb-37.5Si alloy, the product after electrolysis, the SEM image, and EDX result were shown in Figure 3(d) and Table 1, respectively. Clearly, the product in the SEM image showed a homogenous phase with a Nb/Si ratio of 62.2/37.0, which was also close to the atomic ratio of Nb5Si3.

Table 1: EDX results of the four samples in Figure 3.

Positions
Nb (at %)
Si (at %)
O (at %)

a-white
98.2
1.6
0.2

a-grey
62.1
37.6
0.3

b-white
99.1
0.8
0.1

b-grey
62.6
37.4
—

c-white
98.7
1.1
0.2

c-grey
62.7
37.0
0.3

d
62.2
37.5
0.4

a-white, b-white, and c-white showed the concentrations of Nb, Si and O in the positions of white small particles in Figures 3(a), 3(b), and 3(c), respectively. Similarly, a-grey, b-grey, and c-grey showed the concentrations of Nb, Si and O in the positions of grey areas in Figures 3(a), 3(b), and 3(c), respectively. The concentrations of Nb, Si, and O in the sample Nb-37.5Si alloy in Figure 3(d) were shown in position d.

(a)

(b)

(c)

(d)

Figure 3: SEM images of four samples with different Nb/Si ratio: (a) Nb-10Si; (b) Nb-20Si; (c) Nb-30Si; (d) Nb-37.5Si.

To address the phase composition of the products in the four figures, the XRD patterns were obtained in Figure 4. All of the samples have a phase of Nb₅Si₃ (JCPDS: 30-0874) as indicated with solid circle. Only the alloys of the Nb-10Si, Nb-20Si, and Nb-30Si had a phase of Nb (JCPDS: 35-0789) shown in Figures 4(a), 4(b), and 4(c). Based on the data of the XRD, SEM, and EDX, three critical conclusions can be drawn. First, all the samples were composed of two phases: Nb and Nb₅Si₃, which were the two phases in NbSi alloys. Second, in Figures 3(a), 3(b) and 3(c), the small white particles were Nb phase, and grey phase was Nb₅Si₃. Third, the Nb-37.5Si sample was completely composed of Nb₅Si₃ phase after electrolysis.

(a)

(b)

(c)

(d)

Figure 4: XRD patterns of four samples with different Nb/Si ratio: (a) Nb-10Si; (b) Nb-20Si; (c) Nb-30Si; (d) Nb-37.5Si.

4. Conclusion

We used four mixed powders of different Nb-Si compositions (Nb-10Si, Nb-20Si, Nb-30Si, and Nb-37.5Si) for NbSi alloys preparation. NbSi alloys can be prepared by direct electrochemical reduction method in molten CaCl₂ at 900°C, which is an easy and one-plot procedure. After electrolysis, Nb phase showed white scattered particles in the grey Nb₅Si₃ phase under SEM. XRD, and EDX results have proved that the NbSi alloys were successfully obtained, which had paved a creative way for production of alloys with direct electrochemical reduction method.

Acknowledgments

The authors are grateful for the Science and Technology Ministry of China for the financial support to the national high technology project.

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