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

Abstract

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°C 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 15.1at% 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°C 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.

1. Introduction

Owing to low density, high specific strength, and recyclability, magnesium (Mg) alloys are increasingly used in aerospace, automotive, and aircraft industries [1–5]. However, poor plastic deformability and corrosion resistance constrain the industrial application of pure Mg. Alloying is an effective way to improve the properties of Mg alloys. For example, adding Zn [6], Al [7], Li [8], Mn [9], Ca [10], Si [11], Sr [12], Y [13], Gd [14], Ce [15], and Nd [16] can significantly enhance the deformation properties, corrosion resistance, and other properties. Among numerous Mg-based alloys, Mg–Zn–Mn alloys have attracted considerable attention for their low cost and excellent properties [17–21]. It is worth noting that the intermetallic compounds and phase equilibria of the Mg–Zn–Mn system is still uncertain, which impedes the design of novel Mg–Zn–Mn alloys and the establishment of the multi-component thermodynamic database.

In order to better assist the design of Mg–Zn–Mn alloys, it is necessary to establish an accurate thermodynamic database. Databases of sub-systems in Mg–Zn–Mn system, including Mg–Mn [22], Mg–Zn [23], and Zn–Mn [24], were constructed by researchers with sufficient experimental data and have good self-consistency. However, experimental data in the literature about the phase equilibria in Mg–Zn–Mn ternary system are very limited. Gladyshevskii and Cherkashin [25] measured the solubility of Mn in MgZn2, which was determined to be 10.8wt% at 400°C. The solubility of Zn and Mn in α-Mg solid solution at 200–325°C was reported by Bumazhnov [26] using XRD analysis and microstructural observation. It should be noted that the phase equilibria were not mentioned in these works [25–26]. Until 2016, Huang et al. [27] experimentally investigated the phase equilibria of the Mg–Zn–Mn system in the whole range at 320°C with diffusion couples and equilibrated alloys. It was found that the MgZn2 phase with a wide composition range (Mg27.2–43.8Zn46.4–65.3Mn0.3–14.0) can coexist in equilibrium with α-Mg. However, the wide range of the MgZn2 phase had even covered the Mg-rich compounds Mg2Zn3 (or denoted as Mg4Zn7) and MgZn (or denoted as Mg12Zn13). Moreover, some of the annealed samples with chemical compositions in the three-phase region α-Mg + MgZn2 + α-Mn exhibit different phase species. Therefore, it is essential to verify the crystal structure and compositions of the compounds in equilibrium with α-Mg and the phase equilibria in the Mg-rich corner.