Experimental study and thermodynamic modeling of the phase equilibria in the Mg-rich corner of Mg-Zn-Mn system

Mg-Zn-Mn alloys have the advantages of low cost, excellent mechanical properties and high corrosion resistance. To clarify the phase equilibria of Mg-Zn-Mn alloy in the Mg-rich corners, the present work experimentally investigated the phase equilibria in the Mg-rich corner at 300-400℃ with equilibrated alloy method using electron probe micro analyzer (EPMA), X-ray diffractometer (XRD), transmission electron microscopy (TEM) and differential scanning calorimeter (DSC). Mn atoms were found to dissolve into MgZn2 to form a ternary solid-solution type compound, in which Mn content can be up to 15.1% at 400°C. Three-phase equilibrium of α-Mg + MgZn2 + α-Mn and Liquid + α-Mg + MgZn2 were confirmed at 400°C. Subsequently, thermodynamic modeling of the Mg-Zn-Mn system was carried out using the CALPHAD method based on the experimental data of this work and literature data. The calculated invariant reaction Liquid + α-Mn → α-Mg + MgZn2 at 430℃ shows good agreement with the DSC results. In addition, the results of solidification path calculations explain the microstructure in the as-cast and annealed alloys well. The agreement between the calculated results and experimental data proves the self-consistency of the thermodynamic database, which can provide guidance for the compositional design of Mg-Zn-Mn alloys.