Experimental Investigation And Thermodynamic Assessment Of The Phase Diagrams For The MN-ND, MG-MN-ND And MG-MN-GD Systems

The work is combining experimental method and thermodynamic assessment to investigate phase diagram of the Mn-Nd、Mg-Mn-Nd and Mg-Mn-Gd systems. We selected the key alloys depending on extrapolating isothermal section of the Mg-Mn-Nd and Mg-Mn-Gd ternary systems by CALPHAD method. The alloys of Mn-Nd binary system were prepared with Electric Arc Furnace, the Mg-alloys were puted into sealed Ta tube, reached symmetrical alloys by liquid-solid diffussion in muffle furnace. The equilibrium or as-cast alloys were measured with X-ray Diffraction(XRD), Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM / EDS) and Differential Scanning Calorimetry (DSC). Comprehensive analysising all the experimental results, we identified the phase diagram of Mn-Nd binary system and the isothermal sections at 400℃ of Mg-Mn-Nd and Mg-Mn-Gd ternary systems. 　Basing on presented experimental datas and reliable literature datas, we optimized the Mn-Nd binary system, and Mg-Mn-Nd, Mg-Mn-Gd ternary systems with CAPHAD method. Our major works are summary as follows:1. Investigated the Mn-Nd binary phase diagram. The Mn17Nd2 was not observed in this work; The eutectoid decomposition temperatures of Mn23Nd6 and Mn2Nd were deduced as>500℃ and >400℃, respectively, according the EDS results.2. The isothermal section of Mg-Mn-Nd ternary system at 400℃ has been experimental investigation. No ternary compound was found in this system. This isothermal section consists of six single-phase regions, nine two-phase regions and four three-phase regions. There are three binary compounds Mg41Nd5, Mg3Nd and MgNd at this temperature and no obvious solubility of the third element were found among the three binary compounds.Three alloys were prepared and annealed at 700℃ to investigate the Mg2Nd phase, it was detected that it contained quite large solubility of Mn (at least 11.4 at.% Mn) at high temperature and decompose at about 630℃ in the ternary system.3. The isothermal section of Mg-Mn-Gd ternary system at 400℃ has been experimentally investigated. No ternary compound was found in this system. Two binary compounds Mg2Gd and MgGd were contained solubility of Mn, the maximum solubility were up about 10 and 15 at.% Mn, respectively. This isothermal section consists of ten single-phase regions, seventeen two-phase regions and eight three-phase regions.4. Based on present experimental data and reliable literature data, the thermodynamic assessment has been performed for Mn-Nd and Mg-Mn-Nd and Mg-Mn-Gd systems, thermodynamic parameters in the present descriptions can well described all the experimental data of this work.Generally speaking, our work provide a precious phase diagram information and thermodynamic description for the application and developing new Mg alloys...