Papers by Author: T. Tokunaga

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Authors: T. Tokunaga, H. Kaku, H. Miyazaki, F. Masuyama
Abstract: The phase transformation behavior of 9Cr–3W–3Co–VNb steels with boron contents of 47 and 130 ppm has been investigated using differential thermal analysis (DTA). In our DTA experiments, disk-shaped samples were normalized and then tempered at temperatures between 10 and −50 °C from the ferrite-to-austenite transformation temperature (Ac1) at a rate of 30 °C/min. The measured Ac1 temperatures for 47 ppm B steel and 130 ppm B steel were 878 and 884 °C, respectively. The general features of the phase transformation behavior and the changes in hardness with tempering temperature for these boron containing steels were almost identical, irrespective of the boron content. In the DTA cooling curves during tempering at temperatures between −30 and 10 °C from the Ac1 temperatures, an exothermic peak due to the formation of fresh martensite was observed and the peak area increased with increasing tempering temperature. The Vickers hardness values measured after the DTA experiments decreased gradually with increasing tempering temperature, reaching a minimum value of about 180 HV around Ac1−40 °C, and then increased markedly, reaching a value of about 350 HV at Ac1+10 °C.
Authors: T. Tokunaga, N. Hanaya, Hiroshi Ohtani, Mitsuhiro Hasebe
Abstract: A thermodynamic analysis of the Fe-Mn-P ternary system has been carried out using the CALPHAD method. Among the three binary systems relevant to this ternary phase diagram, the thermodynamic parameters of the Mn-P binary system were evaluated in this study. The enthalpy of formation of the binary phosphides obtained from our first-principles calculations was utilized in the present analysis to compensate for the lack of available experimental data. The thermodynamic descriptions of the Fe-Mn and Fe-P binary systems were taken from previous studies. The phase equilibria in the Fe-Mn-P ternary system were analysed based on the experimental data on the phase boundaries. The calculated phase diagrams are in agreement with the experimental results.
Authors: T. Tokunaga, Hiroshi Ohtani, Mitsuhiro Hasebe
Abstract: Thermodynamic assessment of the Al-Cr system has been carried out by incorporating first-principles calculations into the CALPHAD approach. A regular solution approximation was adopted to describe the Gibbs energy of the solution phases. The several phases appearing in the composition range between about 30 and 42 at.%Cr were treated as a single homogeneous γ-phase, based on recent experimental results, and the Gibbs energy of the γ-phase was represented using the four-sublattice model with the formula (Al,Cr)8(Al,Cr)8(Cr)12(Al)24. The calculated results enable the reproduction of experimental results on both the phase equilibria and thermochemical properties. In addition, a B2 ordered bcc phase, which was suggested to form as an equilibrium phase in a previous X-ray diffraction study, is not likely to form in either the stable state or metastable state based on our first-principles calculations.
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