PUBLICATION: M.C. Gao, N. Unlu, G.J. Shiflet, M. Mihalkovic and M. Widom, "Re-assessment of Al-Ce and Al-Nd binary systems supported by critical experiments and first-principles energy calculations", Met. Mat. Trans. A 36 (2005) 3269-79 (reprint)
ABSTRACT: The present study reinvestigates the Al-Ce and Al-Nd phase diagrams and reoptimizes the thermodynamics via 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 investigation and thermodynamic assessment. Specifically, our experimental results and theoretical calculations show that Al2Nd (or Al2Ce) should be treated as a stoichiometric compound phase rather than the solution phase that was proposed in previous studies. Further, we find that a new compound AlCe2 is stable at high temperatures (748-775C) in the Al-Ce system. It forms by peritectic reaction of liquid and AlCe phases at 775C, and decomposes into AlCe and beta-AlCe3 at 648C and below. Since the AlCe2 phase is not retained at room temperature by quenching experiments, we suggest that AlCe2 may be isostructural with the previously known compound AlNd2 (oP12). Based on our DTA measurement and theoretical calculations, we also propose that there is an alpha/beta-Al3Ce polymorphous transition occurring at 973C in Al-Ce system and an alpha/beta-Al3Nd polymorphous transition occurring at 888C in Al-Nd stem. The beta-Al3RE phase may be isostructural with beta-Al3Y (hP12). Finally, we propose that the so-called beta-Al11RE3 phase (RE=La,Ce,Nd,Pr) should have a stoichiomestry of Al4RE (tI10), based on direct evidence from our DSC measurement.