Tatsuya Tokunaga

Thermodynamic Study of Phase Equilibria in the Ni-Fe-B System

A thermodynamic analysis of the phase equilibria in the Pb-Sn-Sb ternary system was conducted. A regular solution approximation based on a two-sublattice model was adopted to describe the Gibbs energy of formation of the individual phases in the binary and ternary systems. In the case of some component binary systems, the effect of pressure also was considered. Experimental data obtained by differential thermal analysis (DTA) and electron probe microanalysis (EPMA) in the present study, along with literature data on phase boundaries and thermochemical properties, form the basis for the evaluated thermodynamic parameters used in the calculation. Calculated and experimental phase boundaries agree fairly well.

Thermodynamic analysis of the phase equilibria in the Nb-Ni-Zr system

A thermodynamic study of phase equilibria in the Nb-Ni-Zr system has been carried out experimentally and using the CALPHADmethod. To enable the thermodynamic description of the constituent binary systems, the results from a previous evaluation were adopted for the Nb-Ni, Ni-Zr and Nb-Zr systems. However, some modifications of the thermodynamic parameters of the Ni-Zr system were made based on recent experimental data on the binary and ternary phase equilibria. The phase boundaries involving the liquid phase in the Nb-Ni-Zr ternary system at the constant 60mol%Ni and 20mol%Zr were determined experimentally using differential scanning calorimetry (DSC). The thermodynamic parameters of the Nb-Ni-Zr ternary system were evaluated by combining the experimental results from DSC with reported phase boundaries of the isothermal sections at 773 and 1073K. The calculated results reproduced the DSC results as well as the experimental isothermal sections. Furthermore, the amorphous-forming ability of Nb-Ni-Zr ternary alloys was evaluated by incorporating the thermodynamic properties from the phase diagram calculations into the Davies-Uhlmann kinetic formulations. The calculated critical cooling rates in the observed metallic glass forming compositional range were found to be lower than those in the observed amorphous forming range by one or more orders of magnitude. [doi:10.2320/matertrans.MB200713]

Thermodynamic evaluation of the phase equilibria and glass-forming ability of the Ti-Be system

The glass-forming ability of Ti−Be alloys is of great interest. Experimental and theoretical evaluations of the glass-forming ability of this binary alloy show that the formation of a metastable TiBe phase with a CsCl-type B2 structure controls the glass-forming ability in this system. However, there is no information on the thermochemical properties of metastable TiBe for the quantitative evaluation of the glass-forming ability using Davies-Uhlmann kinetic formulations. We have carried out a thermodynamic analysis using experimental phase diagram data and the energy of formation of the stoichiometric compounds from ab initio calculations. Furthermore, the Gibbs energy of formation for the body-centered cubic (bcc) phase was evaluated over the entire composition range by applying the cluster expansion method (CEM) to the total energy of some bcc-based ordered structures obtained from ab initio calculations. For the bcc phase, the two-sublattice formalism, (Ti, Be)0.5(Ti,Be)0.5, was adopted to describe the A2/B2 transformation. A good agreement between the calculated values and experimental phase equilibria was obtained. Evaluation of the glass-forming ability was also attempted utilizing the thermodynamic quantities obtained from the phase diagram assessment. The calculated glass-forming ability agrees well with the experimental results.

Phase transformation behaviour of creep-strength enhanced 9%Cr steels

Abstract The phase transformation behaviour of Grade 92 steel has been investigated using differential thermal analysis (DTA) and hardness measurements. In our DTA experiments, disk-shaped samples were normalized at 1070°C and then tempered at temperatures between +10 and −40°C from the ferrite-to- austenite transformation temperature of 878°C (Ac1) at a rate of 30°C/min. From the DTA heating curve during normalization, the magnetic transition and Ac1 transformation were found to occur at 744 and 778°C, respectively. Two overlapping exothermic peaks corresponding to the magnetic transition and the formation of ferrite from austenite were observed in the temperature range 700 – 800°C in the DTA cooling curves after tempering at temperatures between −10 and +10°C from Ac1. The hardness values measured after the DTA experiments decreased with increasing tempering temperature up to around Ac1−15°C and then increased slightly with increasing temperature.