Determination of Gibbs Energy of Mixing of Tungsten-Boron Binary System by Electromotive Force Measurement Using Solid E
- PDF / 1,002,899 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 7 Downloads / 171 Views
melting points, high electric conductivity, superior chemical inertness, and high hardness, especially tungsten tetraboride (WB4) is expected as a superhard material.[1–4] The multi-component materials including the tungsten-boron binary system are used for electrode materials and heat-, wear-, and corrosion-resistant alloys. To consider the high-temperature oxidation characteristics of such multi-component materials, it is important to clarify the relationship between the equilibrium phases on the oxidation path of the tungsten-boron binary alloy and their equilibrium oxygen partial pressures. For the calculation of equilibrium oxygen partial pressure, the
HIROAKI YAMAMOTO, MASAO MORISHITA, YUTA MIYAKE, and SHUSUKE HIRAMATSU are with the Department of Chemical Engineering and Materials Science, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 6712280, Japan. Contact e-mail: [email protected] Manuscript submitted August 21, 2016. Article published online February 21, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
Gibbs energies of mixing, DmixG, for the tungsten-boron binary system are indispensable. There are four intermediate compounds in the tungsten-boron binary system at the temperature of 1273 K (1000 C), namely W2B, aWB, W2B5, and WB4.[1] Strictly speaking, they are not stoichiometric compounds and their solubility widths change with temperature. The low-boron composition of W2B is approximately 32.0 mol pct B at 2873 K (2600 C), and the high-boron composition of W2B is 34.0 mol pct B at 2853 K (2580 C).[5] For the composition range of aWB, the low-boron composition is approximately 48.5 mol pct B at 2383 K (2110 C), and the high-boron composition is approximately 52.0 mol pct B at 2443 K (2170 C).[5] Since the solubility widths for W2B and aWB gradually narrow as the temperature decreases and both of them seem to be less than 1 mol pct B at lower temperature than 2073 K (1800 C),[1] W2B and aWB were treated as stoichiometric compounds in the present study. Rudy and Windisch described that the solubility width of boron in W2B5 extends from 66.5 to 70.0 mol pct B at 2173 K (1900 C).[5] Lundstro¨m derived the low-boron composition of W2B5 to be approximately 66.7 mol pct B,[6] and Romans et al. analyzed the highboron composition of W2B5 using microprobe analysis to VOLUME 48B, JUNE 2017—1703
be 69.4 mol pct B.[7] For the solubility width of WB4, the low-boron composition is approximately 81.0 mol pct B at the temperature for the peritectic reaction of W2B5 and liquid phase at 2293 K (2020 C), and the high-boron composition is approximately 82.0 mol pct B at the temperature for the eutectic reaction of W2B5 and boron solid solution at 2243 K (1970 C).[5] Massalski reviewed that W2B5 and WB4 were nonstoichiometric compounds having composition ranges of 0.670 £ XB £ 0.690 and 0.805 £ XB £ 0.822, where XB is mole fraction of boron, on the basis of information of the tungsten-boron binary phase diagram in References 5 through 7. Therefore, the formulae for W2B5 and
Data Loading...
