Thermodynamic Studies on the System Nb-Al

Abstract

The thermodynamic properties of Nb-Al alloys and intermetallics were studied using solid-state electrochemical cells based on CaF 2 electrolyte depicted as ðÀÞPt; Ar=fAl þ Ca 2 AlF 7 g jjCaF 2 jj f½Nb À Al alloy þ Ca 2 AlF 7 g=Ar; Pt ðþÞ Experiments were carried out in the temperature range of 973 to 1078 K for selected compositions in the range of X Al = 0.03 to 0.90, and the measured emf values were used to derive the thermodynamic activities of Al and Nb. The partial and integral Gibbs energies of alloys and intermetallics were calculated from the activity data. The Gibbs energy of mixing (DG M ) of all compositions studied here showed negative deviation from ideal behavior. The enthalpy of formation D f H o (298.15 K) for all the phases was calculated by the second law method. Based on the values of DG M and D f H o (298.15 K), the most stable alloy composition in the Nb-Al system was found to be NbAl 3 . For instance, the activities of Al and Nb in NbAl 3 were found to be 9.106AE10 )1 and 7.768AE10 )8 , respectively, at 1078 K. The values of DG M at 1078 K and D f H o (298.15 K) for NbAl 3 were found to be -38.4 and -46.2 kJ mol )1 , respectively.

The Al–B–Nb–Ti system

“…Activity of components in the Al-Nb system at 1078 K referred to fcc Al and bcc Nb: points are experimental data from Refs. [14,46], lines resulted from the present thermodynamic description.…”

“…In the constituent binary Al-Nb system the key experimental information was reviewed by Zhu et al [12]. Important experimental data were published later than 2003 [12][13][14][17][18][19][20][21][22][23][24][25][26], or were recently shared within the IMPRESS project [27,28]. The data pool was further enlarged with our own measurements, described below.…”

“…Experimental data for thermodynamic properties of binary Al-Nb alloys were reported in literature were based on the Knudsen effusion method [46], calorimetry [47][48][49] and electromotive forces (EMF) method [14]. Figs.…”

“…10. Enthalpy of formation of alloys in the Al-Nb system: (a) standard enthalpy at 298 K referred to fcc Al and bcc Nb; (b) standard enthalpy at 1620 K referred to liquid Al and bcc Nb; points are experimental[14,46,48,49] and calculated ab initio[50] data, lines resulted from the present thermodynamic description.This procedure gives the following value for the enthalpy of formation of one mole of formula unit Ti 0.253 Al 0.557 Nb 0.190 : 298 H • = −36.8 ± 2.2 kJ mol −1 .…”

Stable Properties of NbAl3 under High Pressure: Theoretical Predictions and First-Principles Calculations

“…Among them, NbAl 3 has a tetragonal structure [5]. Based on the Gibbs energy of formation, George et al [6] found that NbAl 3 was the most stable compound in the Nb-Al system. To the best of our knowledge, many investigations have been carried out on NbAl 3 [7][8][9][10][11][12].…”