CC BY
38ISHS 2019 Moscow, Russia
MOLECULAR DYNAMICS SIMULATION OF REFRACTORY COMPOUNDS AND METHODS OF THEIR SYNTHESIS
S. A. Rogachev
Merzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia e-mail: RogachevSA@gmail.com
DOI: 10.24411/9999-0014A-2019-10138
With increasing computing power, the use of molecular dynamics methods to predict the properties of materials is becoming more common. This approach is also applicable to the investigation of the properties of refractory materials. For the analysis of the properties of any materials, the first-principle (ab-initio) modeling method is the most applicable. However, this method has a big weakness, which is a high computational complexity (systems containing more than a thousand atoms are difficult to calculate). Classical molecular dynamics simulation to determine the melting point uses a large number of atoms, therefore, for an ab-initio modeling, it is necessary to develop a new approach. The developed method is a modification of the two-phase method. The idea of a two-phase method consists in the equilibrium coexistence of a solid and a liquid phase of a substance. To do this, we create a sample consisting of half of the melt, and half of the crystal structure. Then we set a certain initial temperature and observe the evolution of the system under isoenthalpic ensemble. Depending on what the initial temperature was chosen, the system can go either to a completely liquid state, or to a completely crystalline state, or reach the phase equilibrium of the liquid and the crystal structure. In this process, the temperature will vary from the initial to some certain value. This certain value will be the melting point. The number of atoms, available in ab-initio modeling, is a small statistical selection, but its increase will lead to computational difficulties. Therefore, it was proposed to increase not the number of atoms in the system, but the number of model experiments with slightly different initial temperatures. In this case, since the system is extremely small, the result of evolution in a two-phase system is either a completely liquid phase or a completely solid phase. In this case, the initial temperature is taken as the melting point, at which out of 12 numerical experiments, 6 ends in the solid phase, and 6 in the liquid phase. The approximate error in calculating the temperature in this way is about 50 K. By this method, using the VASP software package and using the PAW potentials, the melting points were calculated for pure metals such as Fe (1850 K), Pd (1850 K), Ti (1950 K), Pt (2050 K), Zr (2150 K), Mo (2900 K), Ta (3300 K), W (3700 K), and also for titanium and tantalum carbides. The results obtained are in good agreement with the experimental data. In the future we plan to apply this approach for a wide range of materials. Due to the smallness of the systems it is difficult to predict methods of synthesis, using the ab-initio simulation. Therefore, it is necessary to develop EAM and MEAM potentials for carrying out molecular-dynamic calculations in a large system. The MEAM interaction potential for the Ta-C system was developed. For interaction between atoms of the same type, MEAM potentials were taken from the literature. The main difficulty was the development of the interaction potential for atoms of different types. To do this, using the VASP software package, calculations such as the cohesive energy of tantalum carbide in a cubic face-centered lattice, the heat of formation, the equilibrium distance between atoms in the crystal lattice, and the elastic modulus were carried out from first principles. The tantalum carbide melting point calculated using this MEAM potential differs from the experimental value, but it makes it possible to investigate various methods of synthesis. In the future we plan to develop MEAM potentials for Hf, Ta, C, N systems.
This work was supported by the Russian Foundation for Basic Research (no. 18-33-00641).
S. A. Rogachev
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