CVM-based first-principles calculations for Fe-based alloys
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CVM-based first-principles calculations for Fe-based alloys
Tetsuo Mohri1 1 Division of Materials Science and Engineering, Faculty of Engineering and Research Center for Integrative Mathematics, Hokkaido University, Sapporo 060-8628, JAPAN
ABSTRACT Cluster Variation Method (CVM) has been recognized as one of the most reliable theoretical tools to incorporate wide range of atomic correlations into a free energy formula. By combining CVM with electronic structure total energy calculations, one can perform first-principles calculations of alloy phase equilibria. The author attempted such CVM-based first-principles calculations for various alloy systems including noble metal alloys, transition-noble alloys, III-V semiconductor alloys and Fe-based alloy systems. Furthermore, CVM can be extended to two kinds of kinetics calculations. One is Path Probability Method (PPM) which is the natural extension of the CVM to time domain and is quite powerful to investigate atomistic kinetic phenomena. The other one is Phase Field Method (PFM) with the CVM free energy as a homogeneous free energy density term in the PFM. The author’s group applied the latter procedure to study time evolution process of ordered domains associated with disorder-L10 transition in Fe-Pd and Fe-Pt systems. CVM has, therefore, a potential applicability for the systematic studies covering atomistic to microstructural scales. It has been, however, pointed out that the conventional CVM is not able to include local lattice relaxation effects and that the resulting order-disorder transition temperatures are overestimated. In order to circumvent such inconveniences, Continuous Displacement Cluster Variation Method (CDCVM) has been developed. Since first-principles CDCVM calculations are still beyond the scope at the present stage, preliminary results on the two dimensional square lattice and an fcc lattice with primitive Lennard-Jones type potentials are demonstrated in the last section. INTRODUCTION First-principles calculations have been attracting broad attentions in the study of phase stability, phase equilibria and phase transition of alloys [1]. Electronic structure total energy calculations provide the information of the stability of a given phase against competing phases at the ground state, while the statistical mechanics model to evaluate the entropy is necessary to study phase equilibria at finite temperatures. Cluster Variation Method (CVM) [2] has been recognized as one of the most reliable theoretical tools to calculate entropy of a given system and it has been demonstrated that the combination with electronic structure total energy calculations enables one to perform first-principles phase equilibria calculations for various alloy systems. CVM entropy formula constitute hierarchy structure in terms of the biggest cluster considered in the entropy formula which is termed basic cluster, and bigger the basic cluster is more accurate results one can obtain.
Recently Phase Field Method (PFM) [3] has been attracting broad attentions to calculate mic
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