Structural, vibrational, and thermodynamic properties of Al-Sc alloys and intermetallic compounds - art. no. 094104

We present results of a theoretical study of the temperature-dependent stru ctural and thermodynamic properties of solid-phase Al-Sc alloys and compoun ds based upon first-principles calculations of electronic free energies and ionic vibrational spectra. This work extends a previous first-principles s tudy of the fcc portion of the Al-Sc phase diagram which demonstrated a lar ge effect of vibrational free energy upon calculated Sc solid-solubility li mits [V. Ozolins and M. Asta, Phys. Rev. Lett. 86, 448 (2001)]. Here the co ntributions of nonconfigurational (electronic and vibrational) entropies to the free energies of solid-phase Al-Sc alloys and compounds are analyzed i n further detail, and the accuracy of the approximations employed in these calculations is assessed. For each of the reported inter-metallic compounds in this system, calculated formation enthalpies agree to within 10% (0.05 eV/atom) of published calorimetry measurements. Large negative entropies of formation, equal to -0.77k(B) /atom. -0.58k(B)/atom, and -0.24k(B)/atom ar e calculated for cubic Al3Sc, cubic AlSc, and orthorhombic AlSc compounds, respectively, resulting primarily from the stiffening of nearest-neighbor A l-Sc bonds in the intermetallic phases relative to elemental Al and Sc. The net effects of nonconfigurational free energy contributions to the fcc por tion of the Al-Sc phase diagram are 100 and 450 K decreases in the calculat ed Al solvus phase boundary temperatures associated with electronic and vib rational entropy, respectively, at the maximum measured Sc solid-solubility limit.