Papers by Author: Naoki Yoshinaga

Abstract: To establish the method for determining the amount of carbon in the ferrite phase in ferrite + martensite dual-phase low-alloy steels, mechanical loss measurements have been performed on a series of Fe–C alloys with varying constitution. The observed mechanical loss spectra of two-phase alloys turned out to be simple superposition of those of single phase alloys, of ferrite and of martensite. The concentrations of carbon in solution evaluated from the magnitude of the Snoek relaxation in the two-phase alloys agree well with those expected from the Fe–C phase diagram. It is thus possible to selectively analyse the carbon dissolved in the ferrite phase in the complex structure, at least in simple binary alloys.

Abstract: No clear-cut information is available with regard to the effect of foreign atoms on the solubility limit of C in b.c.c. iron despite many previous studies. Against this backdrop, the influence of substitutional atoms (Mn, Cr, P, Si, Al) on the solubility limit of C in b.c.c. iron in equilibrium with cementite was investigated in low-carbon steels at a temperature of 700°C. In detail, the C solubility limit was determined from internal friction measurements combined with infrared analysis of C using a high-frequency combustion technique. It has been clarified that Mn, Cr, and Al hardly change the C solubility limit, whereas P and Si increase it. The thermodynamical calculation indicates that, under para equilibrium Si increases the C solubility limit and Mn hardly changes it, while under ortho equilibrium Mn and Si decrease it. However, the present experimental condition was verified to be close to ortho form. The discrepancy between the experiments and the calculations seems to come from the fact that: 1) single solute C atoms and the C atoms combined as Substitute-C complex are not distinguished experimentally, and 2) in the regular solution model, the non-uniform distribution of C atoms around alloying atoms is not introduced into the entropy term, which is something that should be studied further in the future.

Abstract: Electro-deposited pure iron has a quite sharp and isotropic <111>//ND fiber texture and a needle-shaped grain elongated in ND. This pure iron shows an r-value exceeding 7, which is difficult to explain from the texture alone. In this study the deformation behavior of electro-deposited pure iron was investigated to reveal the mechanism behind the extremely high r-value. The post-deformation surface slip lines indicated that the particular <110> plane slips, which are perpendicular to ND, exclusively act in the specimen. The tensile deformation caused by this slip system does not require any decrease in thickness, hence the extraordinary high r-value is mainly attributable to this limitation of the active slip system. Presumably, the needle-shaped microstructure affected the limitation of the slip system.

Abstract: Lath-shaped upper bainite structures play a very important role in many high-strength steels (HSSs) and ultra high-strength steels (UHSSs). Although bainite transformation is strongly affected by the initial structure, the effect of the second phase in a multi-phase structure is yet to be clearly understood. It is significant for the advancement of UHSS to study this effect. The aim of this study is to clarify the effect of martensite, which forms before bainite, in Fe-0.2C-8Ni alloy. The bainite transformation from an austenite and martensite dual-phase structure is faster than that from single-phase austenite and the nucleation of bainitic ferrite laths are accelerated around martensite. This effect of martensite on bainite kinetics is equivalent to that of polygonal ferrite when their volume fractions are almost the same. This suggests that the boundary between martensite and austenite is a prior nucleation site of bainitic ferrite. Martensite also affects the crystallographic features of bainite. The orientations of bainitic ferrite laths tend to belong to the same block with martensite adjacent. This tendency intensifies with an increase of the transformation temperature of bainite, resulting in the formation of huge blocks consisting of bainitic ferrite and martensite laths at high temperatures (693K and 723K). In contrast, at a low temperature (643K), bainitic ferrite laths belong to same packet as martensite and have several orientations. This change of crystallographic features with transformation temperature can explain with the driving force of the nucleation of bainitic ferrite.