Experimental study and thermodynamic evaluation of Mg–La–Zn system

Abstract

The Mg–La–Zn alloys exhibit good mechanical properties both at room and elevated temperatures due to the formation of intermetallic compounds. However, the ternary compounds and their phase equilibrium relationships in the Mg-rich region still exist uncertainty, which impedes the design of new Mg–La–Zn alloys. Thus, seven ternary alloys were prepared to find out the phase relations in the Mg-rich corner concerning the intermetallics LaMg12, τ1, LaMg3 and τ3. The analysis of Transmission Electron Microscopy (TEM) and Synchrotron Radiation Powder X-ray Diffraction (SR-PXRD) results shows that τ1-Mg62.0La8.1Zn29.9 is a ternary compound which crystal structure is resolved as space group of Cmc21 with a = 9.86895 Å, b = 11.37308 Å and c = 9.67478 Å. The solubility range of Zn in τ1 is determined to be from 9.5 to 59.9 at.%, while the solubility of Zn in LaMg12 is considered to be less than 4.96 at.% with Zn mainly substituting Mg. τ3-La(Mg, Zn)3 is a ternary phase differing from LaMg3. The thermodynamic description of the Mg–La–Zn system is developed through CALPHAD method based on the current experimental results and those in literature. The calculated isothermal sections of the Mg–La–Zn system at 300, 350 and 400 °C agree well with the experimental data. The self-consistent thermodynamic database is helpful to interpret the solidification path and useful to predict the phase constitution of Mg–La–Zn alloys.