Reassessment of the Al-Mn system and a thermodynamic description of the Al-Mg-Mn system

A thermodynamic optimization for the Al-Mn system is performed by considering reliable literature data and newly measured phase equilibria on the Al-rich side. Using X-ray diffraction, differential thermal analysis, and scanning electron microscopy with energy dispersive X-ray spectroscopy methods, the melting behavior of λ-Al₄Mn was correctly elucidated, and two invariant reactions associated with λ-Al₄Mn (L + μ-Al₄Mn ↔ λ-Al₄Mn at 721 ± 2°C and L + λ-Al₄Mn ↔ Al₆Mn at 704 ± 2°C) are observed. The model Al₁₂Mn₄(Al, Mn) ₁₀ previously used for Al₈Mn₅ was modified to be Al₁₂Mn₅(Al, Mn)₉ based on crystal structure data. In addition, the high-temperature form of Al₁₁ (Mn₄ is included in the assessment. Employing fewer adjustable parameters than previous assessments, the present description of the Al-Mn system yields a better overall agreement with the experimental phase diagram and thermodynamic data. The obtained thermodynamic description for the Al-Mn system is then combined with those in the Al-Mg and Mg-Mn systems to form a basis for a ternary assessment. The thermodynamic parameters for ternary liquid and ternary compound Mn₂Mg₃Al₁₈ (τ) are evaluated on the basis of critically assessed experimental data. The enthalpy of formation for τ resulting from CALPHAD (CALculation of PHAse Diagrams) approach agrees reasonably with that via first-principles methodology. Comparisons between the calculated and measured phase equilibria in the Al-Mg-Mn system show that the accurate experimental information is satisfactorily accounted for by the present description. A reaction scheme for the whole ternary system is presented for practical applications.