Thermodynamic study of the zirconium-aluminum system

Abstract

The heats of formation of the zirconium-aluminum intermetallic compounds ZrAl 3, ZrAl 2, Zr 2Al 3, ZrAl, Zr 5Al 4, Zr 3Al 2, Zr 5Al 3, and the solid solution of aluminum in bcc zirconium have been studied using a Knudsen cell mass spectrometric technique. The high-temperature compound Zr 5Al 4 was identified in the residue of some of these experiments and this led to further heat treatment/X-ray diffraction experiments which indicated that Zr 4Al 3, previously reported to form from the melt, decomposes in the solid state at temperature in excess of 1050°C. By measuring aluminum vapor pressures over the two-phase ranges of the system from 0 < X Al < 0.75 the enthalpy changes for the decomposition reactions were determined by second- and third-law methods, and these were used along with the measured vapor pressure of aluminum over the solid solution of aluminum in bcc zirconium to derive the enthalpies of formation of the intermetallic phases (in kcal/mole): ZrAl 3, −38.96; ZrAl 2, −32.86; Zr 2Al 3, −56.12; ZrAl, −21.36; Zr 5Al 4, −93.76; Zr 3Al 2, −48.78; Zr 5Al 3, −74.57.