Hiroshi Ohtani

Phase equilibria in the Fe–Co binary system

Abstract α (A2)/γ (A1) phase equilibria of the Fe–Co system between 400 and 800°C were determined by means of lattice parameter measurement using thin film specimens. Bulk specimens were also analyzed to compare the extent of attainment to the equilibrium. The thin film technique was found to be greatly advantageous for obtaining the phase equilibria at lower temperatures where solid-state reactions are too slow to reach the equilibrium state in the conventional methods using bulk specimens. It was confirmed that the α+γ two-phase region extends below the temperature at which the α (A2)/α′ (B2) transus meets the α/α+γ boundary. Thermodynamic analysis was also conducted by taking the magnetic and chemical ordering contributions of the B2 structure into account, findings of which confirmed the extension of the α+γ two-phase region below the α/α′ ordering temperature.

Calculation of the Fe-C-Ti ternary phase diagram

Abstract A thermodynamic analysis of phase equilibria in the Fe-Ti, Ti-C, and Fe-C-Ti systems has been carried out. The Gibbs energy has been expressed by the two-sublattice model being separated into the paramagnetic and ferromagnetic terms. In particular, the equilibrium between austenite and NaCl type Ti-carbide has been treated as a portion of the miscibility gap in the fcc phase. The thermodynamic parameters for each phase were evaluated on the basis of experimental phase equilibrium and activity data. The calculated phase boundary of austenite in equilibrium with Ti-carbide exhibited a characteristic shape, not only in the isothermal but also in the vertical section.

A thermodynamic assessment of the Ti-N system

Abstract A thermodynamic analysis of phase equilibria in the Ti-N binary system has been carried out. The thermodynamic parameters for each phase were evaluated on the basis of the available experimental data, by using thermodynamic models for the Gibbs energy. Most of the experimental information was well reproduced by the present set of thermodynamic descriptions. Especially, the melting behavior of titanium mononitride (γ phase) was clarified in the present assessment.

A thermodynamic assessment of the V-N system

Abstract A thermodynamic analysis of phase equilibria in the V-N binary system has been carried out. The thermodynamic parameters for each phase were evaluated on the available experimental data, by using thermodynamic models for the Gibbs energy. Most of the experimental information was well reproduced by the present set of thermodynamic descriptions. However, some uncertainty still attaches to the nature of the γ(fcc) phase, and the need of further experimental investigations is emphasized.

A thermodynamic assessment of the Fe-N-Ti system

Abstract A thermodynamic analysis of phase equilibria in the Fe-N-V system has been carried out on the basis of the established three binary descriptions and selected ternary experimental data. The two-phase equilibrium between austenite and VN phase was treated as a portion of the miscibility gap in the fcc phase. A set of isothermal sections was presented in the whole range of compositions. Some thermodynamic analysis was carried out on the solubility products of VN in ferrite and austenite.

Optimization of the Fe-Rich Fe-Mn-S Ternary Phase Diagram

Phase equilibria in the Fe-rich side of the Fe-Mn-S ternary system were analyzed on the basis of the thermodynamic analysis of the Fe-FeS, Fe-Mn, Mn-MnS and Fe-FeS-MnS-Mn systems. The Gibbs energy of individual phases was approximated by the sublattice model, assuming S to be interstitial atoms. Most of the experimental information was well reproduced by the present set of thermodynamic descriptions. The present analysis is useful for understanding sulfide formation during solidification, precipitation ip solids, and the cause of the so-called hot shortness in mild steels.

Thermodynamic Analysis of the Fe-Cr-B Ternary System

A thermodynamic analysis of the Fe-Cr-B ternary system was carried out to clarify the complex phase equilibria between several types of borides and solution phases. To satisfy the lack of experimental information on thermodynamic properties for this ternary system, a first-principles method was applied to evaluate the enthalpies of formation of binary Cr-boride phases. The calculated enthalpy of formation of CrB2 agreed well with the experimental value. However, the firstprinciples calculations suggested that the CrB phase is more stable in the ground state than previously thought. Variations of the formation enthalpies of Fe2B (IA/mcm) and CrB (Cmcm) with dissolving Cr and Fe were also evaluated for superstructures constructed by modifying the stack of atoms along a given direction of the parent lattice structures. The thermodynamic parameters of the Fe-Cr-B ternary system were optimized with the CALPHAD technique using these calculated thermodynamic properties along with available experimental information on phase equilibria and some thermodynamic data. We found that the evaluated parameters reasonably reproduced the experimental data on the isothermal sections, the pseudo-binary section, and the liquidus projection in the Fe-rich region.