An isothermal section of the Mg-Nd-Gd ternary system at 723 K was established by diffusion triple technique and electron probe microanalysis （EPMA）. Mg3Gd and Mg3Nd form a continuous solid solution （Gd,Nd）3Mg, and a continuous solid solution （Gd,Nd）Mg is also formed between MgGd and MgNd. Mg7Gd, Mg5Gd, Mg2Gd, Mg41Nd5, （Gd,Nd）3Mg and （Gd,Nd）Mg are found in the ternary system. In these intermetallic phases, Mg7Gd has been reported to be a metastable phase in previous literatures. The solubilities of Mg, Gd and Nd in all the phases were detected. Furthermore, four three-phase equilibria, α（Mg）＋Mg7Gd＋Mg41Nd5, Mg7Gd＋Mg5Gd＋Mg41Nd5, Mg5Gd＋Mg41Nd5＋（Gd,Nd）3Mg and （Gd,Nd）3Mg＋（Gd,Nd）Mg＋Mg2Gd, were identified in the isothermal section.
The phase equilibria in Mg-rich corner of Mg-Ca-Gd and Mg-Ca-Nd ternary systems at 400℃ were determined through the equilibrated alloy method by using XRD, SEM, EPMA and DSC. Partial isothermal sections in Mg-rich corner of Mg-Ca-Gd and Mg-Ca-Nd ternary systems at 400 ℃ were constructed from 13 alloys. A three-phase region of a-Mg, Mg41RE5 and Mg2Ca was determined in both ternary systems. It is formed by a similar ternary eutectic reaction L→a-Mg＋Mg2Ca＋Mg41RE5 at 499.6 ℃ and 505.6 ℃, respectively. It is found that the maximum solubility of Ca in Mg5Gd is 3.68% （molar fraction） and 3% of Gd can be dissolved in Mg2Ca in the Mg-Ca-Gd system at 400 ℃. While in the Mg-Ca-Nd system, the maximum solubility of Ca in Mg41Nd5 is 3.57% and 1.24% of Nd can be dissolved in Mg2Ca at 400 ℃. Other three-phase equilibria existing in Mg-rich corner of Mg-Ca-Gd system are a-Mg＋MgsGd＋T and MgsGd＋Mg2Ca＋T and the three-phase equilibrium in Mg-rich corner of Mg-Ca-Nd system is Mg3Nd＋Mg2Ca＋ Mg41Nd5.
Like Al3Sc,the Al30Dy7Zr3compound has an L12structure,implying that it could probably be used to improve the strength of aluminum alloys at elevated temperatures.Thus,it is extremely important to understand the phase relations of the Al–Dy–Zr system in the Al-rich corner.Firstly,the phase diagram of the Al–Dy binary system was re-assessed with the C15_Laves phase treated as a stoichiometric compound.Then,using the special quasirandom structure containing 16 atoms(SQS_16)to simulate Dy1-xZrx(x=0.25,0.5,and 0.75)hcp_A3 solid solutions,their enthalpies of mixing at 0 K were calculated by the first-principles method.Based on these results and the evaluated phase diagram in literature,the Dy–Zr binary system was optimized thermodynamically.Integrating the thermodynamic descriptions of the three constituent binary systems,the phase diagram of the Al–Dy–Zr ternary system in the Al-rich corner was optimized finally by coupling the first-principles calculation with the CALculation of PHAse Diagram method.A good agreementhas been reached between the calculated phase relations and the experimental results,which indicates that the current thermodynamic description is reasonable.
The Fe-Ti binary system was re-assessed using the CALPHAD method in order to improve the capability of being extrapolated to a ternary or higher-order system. Compared with previous assessments, the main focus was put on the thermodynamic description of the two intermetallic compounds Fe2Ti and FeTi. The C14_Laves phase Fe2Ti was described by the two-sublattice model, which is widely used at present. By checking the homogeneity range on the boundary of the ternary systems involving the binary, the phase boundary of this compound was further confirmed. The FeTi phase with a BCC_B2 crystal structure was treated as the ordered phase of the BCC_A2 phase and a unified Gibbs energy function was used to describe both the ordered and disordered phases. Reproduction of the specific heat capacities of these compounds was another aspect paid particular attention to. Comprehensive comparisons of the calculated and experimental results regarding the phase diagram and thermodynamic properties show a good agreement between them and prove the validity of the present thermodynamic description.
