Phase Diagram Measurement, Thermodynamic Modeling And Application Of Magnesium-rare Earth Systems

Magnesium alloys are one of the lightest metallic structural materials and have been widely used in aviation, automotive and 3C electronic products fields. However, due to the poor mechanical property and creep-resistance at elevated temperature, the application of magnesium alloys is subject to many restrictions. It was revealed that the addition of the rare earth elements can significantly improve the performances of the magnesium alloys, owing to the precipitation strengthening effect of the stable magnesium-rare earth metal compound. Therefore, knowledge of the phase equilibria in the Mg-RE1-RE2 systems and information on the interaction between Mg and RE become very important, which could provide the basis of designing new magnesium alloys to the researchers.This work is aimed at determining the isothermal sections at certain temperature of rare earth magnesium alloys (Mg-La-Nd, Mg-Gd-La and Mg-Gd-Mn-Sc) by means of the diffusion couple technique and alloy method. Then based on the present results and previously reported results in the literature, the thermodynamic databases of the related systems are established by using the CALPHAD (CALculation of PHAse Diagram) approach. Finally, the applications of the database in the practical alloys were analyzed. The main results of the present work are summarized as the following:(1) By means of the diffusion couple technique and equilibrated alloy method, the phase equilibria of Mg-La-Nd system at 500? was determined. It was found that Mg41La5 is stablized with the addition of Nd. Mg2La exist stable up to much lower temperature than that previously repored in the literature, the solubilitiy of Nd in Mg12La, Mg17La2 and Mg2La, and of La in Mg41Nd5 were measured. Continuous solutions Mg(La, Nd) and Mg3(La, Nd) form between MgLa and MgNd, and between Mg3La and Mg3Nd, respectively.(2) The isothermal section of Mg-Gd-La system at 500? was determined. The stable compound Mg41La5 has been measured due to addition of Gd element. the solubilitiy of Gd in Mg12La, Mg17La2 and Mg41La5, and of La in Mg5Gd and Mg2Gd were measured. Continuous solutions Mg(Gd, La) and Mg3(Gd, La) form between MgLa and MgGd, and between Mg3La and Mg3Gd, respectively.(3) The isothermal section of Mg-Gd-Mn system at 500? was determined. Combined with the experimental data of this work and previous work, the Mg-Gd-Mn system was assessed thermodynamically and a set of self-consistent thermodynamical parameters were obtained. Using this thermodynamical database, the case research has been done.(4) The thermodynamic database of Mg-Gd-Mn-Sc system has been build. According to phase diagram and phase collection rule, the solidification processes with different concentrations and the change laws along with variation of Gd and Mn have been analyzed. The applicable alloy compositions were recommended and the appropriate solution and aging treatment temperature were proposed.