Reassessment of Al-Ce and Al-Nd binary systems supported by critical experiments and first-principles energy calculations

The present study reinvestigates the Al-Ce and Al-Nd phase diagrams and reoptimizes their thermodynamics using the CALPHAD method. First-principles energy calculations play an important role in terms of sublattice formalism and phase-stability prediction, demonstrating that they should be effetively integrated into experimental investigations and thermodynamic assessments. Specifically, current experimental results and theoretical calculations show that Al2Nd (or Al2Ce) should be treated as a stoichiometric compound phase rather than as the solution phase that was proposed in previous studies. Further, a new comport nd, AlCe2, is found stable at high temperatures (648 degrees C to 775 degrees C) in the Al-Ce system. It forms through a peritectic reaction of liquid and AlCe phases at 775 degrees C, and decomposes into AlCe and beta AlCe3 at 648 degrees C and below. Since the AlCe2 phase is not retained at room temperature by quenching experiments, it is suggested that AlCe2 may be isostructural with the previously known compound AlNd2 (oP12). Based on current differential thermal analysis (DTA) measurements and theoretical calculations, it is also proposed that there is an alpha/beta Al3Ce polymorphous transition occurring at 973 degrees C in the Al-Ce system and an alpha/beta Al3Nd polymorphous transition occurring at 888 degrees C in the Al-Nd system. The beta Al3RE phase may be isostructural with beta Al3Y (hP12). Finally, the previously described beta Al11RE3 phase (rare earth elements (RE) = La, Ce, Nd, or Pr) is proposed to have a stoichiometry of Al4RE (iI10), based on direct evidence from differential scanning calorimetry (DSC) measurements.