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

Abstract

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 effectively 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 compound, AlCe₂, is found stable at high temperatures (648 °C to 775 °C) in the Al-Ce system. It forms through a peritectic reaction of liquid and AlCe phases at 775 °C, and decomposes into AlCe and βAlCe₃ at 648 °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 α/βAl₃Ce polymorphous transition occurring at 973 °C in the Al-Ce system and an α/βAl₃Nd polymorphous transition occurring at 888 °C in the Al-Nd system. The βAl₃RE phase may be isostructural with βAl₃Y (hP12). Finally, the previously described βAl₁₁RE₃ phase (rare earth elements (RE)=La, Ce, Nd, or Pr) is proposed to have a stoichiometry of Al₄RE (tI10), based on direct evidence from differential scanning calorimetry (DSC) measurements.