Thermodynamic Modeling Of Be-Sr, Be-Mg And C-Co-W-Nb Systems

The CALPHAD (CALculation of PHAse Diagram) method has been widely utilized in the research of phase diagram, and it provides important significance on the guidance of the research, development and design of materials. This work studied the phase diagram and thermodynamic properties of the Be-Sr, Be-Mg systems of Al alloys and the C-Co-W-Nb system of cemented carbides by means of CALPHAD approach. The main results obtained are as follows:(1) After critically evaluating the experimental data in the literatures on the Be-Sr and Be-Mg systems, the thermodynamic models that describing the phases the two systems were built. With the entalpy of formation for Be13Sr compound computed by first-principle calculations, the thermodynamic parameters of these two systems were optimized based on the available experimental data. A set of self-consistant parameters for each system was finally obtained. The phase diagrams calculted by the obtained parameters agree well with experimental data.(2) Based on the evalutaion of the Co-W-Nb ternary system and its sub-binary systems, appropriate thermodymic models were estabilished. Then, the Co-W-Nb system was optimized using reported experimental data. A set of parameters were obtained, and the isothermal sections calculated at1000,1100and1200℃agree with experimental data.(3) In order to fit well with the experimental data in the C-Co-W-Nb system, a re-assessment of the C-Co-Nb system was conducted. The optimized parameters were used to calculate the isoactivity section of Co-NbCx at different carbon activities, and the results were compared with the experimental data. Besides, three isothermal sections were predicted.(4) A thermodynamic database for the C-Co-W-Nb system was built through evaluation and optimization. To test the validation of the database, several calculations were performed using the database and then compared with the experimental data. Then the database was used to calculate phase equilibria closing to the sintering region and study the varying solubility of alloy elements in the binder and cubic phases along with the temperature.