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

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, tau(1), LaMg3 and tau(3). The analysis of Transmission Electron Microscopy (TEM) and Synchrotron Radiation Powder X-ray Diffraction (SR-PXRD) results shows that tau(1)-Mg62.0La8.1Zn29.9 is a ternary compound which crystal structure is resolved as space group of Cmc21 with a = 9.86895 angstrom, b = 11.37308 angstrom and c = 9.67478 angstrom. The solubility range of Zn in tau(1) 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. tau(3)-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 degrees 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. (C) 2019 Elsevier B.V. All rights reserved.