Experimental and thermodynamic investigation of RE-Mg-Zn (RE=Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) systems

摘要

In a thermodynamic "optimization/modeling", adjustable model parameters are refined based on all available thermodynamic and phase-equilibrium data in order to obtain one set of model equations as functions of temperature and composition. From the model equations, all the thermodynamic properties and phase diagrams can be back-calculated by Gibbs energy minimization using software such as FactSage. Generally, in the optimization of a ternary system one begins by optimizing the three binary sub-systems. The binary model parameters are then used to estimate the properties of the ternary phases, and these estimates are then refined by introducing ternary model parameters where required to reproduce available ternary data.;All binary Mg-Zn and Mg-RE (where RE = rare earth) systems have already been optimized and the model parameters of these systems are readily available in the FactSage software. In the present project, all binary RE-Zn and most of the ternary RE-Mg-Zn systems are optimized.;Firstly, all available phase diagram and thermodynamic data for the RE-Zn (Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) systems have been collected and critically assessed. The rare earth elements have very similar properties. The phase diagrams of all RE-Zn systems are very similar. Trends in the properties of the rare earth (RE)-Zn systems as one traverses the rare earth series have been exploited for purposes of estimating missing data and for checking existing data for consistency. The Miedema Model is also used in the present project to estimate the enthalpy of mixing of the liquid phases. Based on all available data, critical thermodynamic evaluation and optimization of these systems have been carried out and model parameters for the thermodynamic properties of all phases have been obtained.;Secondly, in-situ neutron diffraction (ND) experiments have been performed on selected samples in the Ce-Mg-Zn and Nd-Mg-Zn systems to identify phases and transition temperatures. Due to the high penetrating power of neutrons, large samples (10-20 grams) are used in the present work, leading to better control of composition and increased resistance to oxidation. More accurate information about phase relationships and transformation behavior can also be expected from the present ND experiments because they are performed in-situ at high temperatures. All the ND experimental data are used to validate and refine the thermodynamic modelling.;Finally, all phase diagram data for the RE-Mg-Zn systems have been critically assessed and all ternary RE-Mg-Zn (excluding Sc-Mg-Zn, Pm-Mg-Zn, Eu-Mg-Zn and Yb-Mg-Zn) systems have been optimized, based on the binary Mg-Zn, Mg-RE and RE-Zn systems. Observed trends and regularities are again used in the optimization of the ternary systems. As expected, all RE-Mg-Zn systems are similarly closely related. The Ce-Mg-Zn and Nd-Mg-Zn systems are critically optimized taking into account the new ND data. Thermodynamic optimization of all other RE-Mg-Zn systems is greatly aided by simultaneous optimization of the Ce-Mg-Zn and Nd-Mg-Zn systems.;It should be noted that the Modified Quasichemical Model (MQM) is used in the present project to describe the liquid phase. Since short range ordering (SRO) is taken into account by this model, better description of the liquid phase is expected.;The present project is aimed at building the most complete and accurate thermodynamic database for the RE-Mg-Zn systems from which all investigators of Mg alloys can benefit.