Electrochemical Fabrication of Niobium Silicon Alloys from Oxide Powder Mixtures

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