Electron Theory Calculation of Thermodynamic Properties of Steels and Its Application to Theoretical Phase Diagram of the Fe–Mo–B Ternary System

Masanori Enoki, Kota Takahashi, Soei Mitomi, Hiroshi Ohtani
2020 ISIJ International  
In this study, the ground structures of the Fe-Mo-B ternary systems were estimated by first-principles calculations based on genetic algorithm, and the free energies of their structures are evaluated by electronic calculations and statistical thermodynamic techniques. In addition, the phase diagram at finite temperature was theoretically constructed using the calculated free energies, and the result was compared with the experimental knowledge. The space groups and compositions of many ground
more » ... ructures obtained by the calculations correspond well with the experimental findings, but the agreement is not perfect. However, by including metastable structures by only a few kJ/mol than the ground state, it becomes clear that the appearance of almost all structures can be predicted based on this technique. The new calculation technique of such theoretical phase diagrams suggested in the present study is expected to open up the possibility of estimation of unknown phase diagram, reexamination of experimental phase diagrams and discovery of new phases. On the other hand, examining the calculation conditions for improving the accuracy of energy calculation, consideration of the anharmonicity of atomic vibration, magnetic entropy effect, handling of solid solution, etc. are mentioned as problems requiring further consideration. KEY WORDS: first-principles calculation; thermodynamics; phase diagram; Fe-Mo-B. analyzed using appropriate thermodynamic models, and the thermodynamic properties of the target system are parameterized. Since the physical properties of the multicomponent system can be reasonably predicted by the thermodynamic extrapolation method, it has the advantage of accurately predicting the actual phase equilibria. Then this method has been greatly developed in the field of material development by utilizing the advantages. However, it is very difficult to apply this technique to the system which has not been clarified in experiments, metastable regions, and nonequilibrium phases. Therefore, referring to the Fe-Mo-B ternary system in this study, the thermodynamic properties of this alloy system are evaluated by the electronic theory calculations based on the first-principles. In addition, using the results, this paper proposes a new technique to theoretically construct alloy phase diagrams at finite temperature, and aims at arranging the present problems.
doi:10.2355/isijinternational.isijint-2020-189 fatcat:seews424rvhenecyssd2bofcaa