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

Abstract

Abstract 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 Al11Mn4 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.