Papers by Author: Yo Tomota

Abstract: Martensite morphology such as connectivity or dispersivity in ferrite (F)/martensite (M) dual-phase (DP) steels was investigated from topological viewpoint to reveal the effect of the martensite morphology on the mechanical properties. Topological analysis permits evaluation of the microstructural connectivity and dispersivity by measuring the number of handles, independent bodies and genus, etc. The topological analysis was performed on three-dimensional (3D) reconstructed images of the microstructure with different connectivity, dispersivity, volume fraction and hardness of martensite in DP steels that were prepared by changing the intercritical annealing temperature. The connectivity and the volume fraction of martensite increased while the dispersivity and hardness of it decreased with increasing annealing temperature. The effect of connectivity and dispersivity as well as volume fraction and hardness, in particular, on work hardening behavior was individually evaluated at a given strain.

Abstract: Tensile deformation behaviors of three austenitic stainless steels, JIS-SUS310S, 304 and 301L, were studied by static tensile tests and in situ neutron diffraction. In the mechanical properties obtained by the static tensile tests, the 304 and 301L steels showed better balance of tensile strength and uniform elongation than the 310S one because of TRIP effect. The angular dispersion neutron diffractions with a wavelength of 0.16 or 0.182 nm were performed during stepwise tensile testing by using a neutron diffractometer for residual stress analysis (RESA) at the Japan Atomic Energy Agency. The lattice plane strain, stress-induced martensite volume fraction, dislocation density and so on were estimated by the profile analysis as a function of applied stress. The change in lattice plane spacing for austenite indicated four deformation stages. In the comparison of lattice plane strain among the tested steels, a phase stress caused by the stress-induced martensite seems to overlap the intergranular stress of austenite phase. Judging from the results of profile analysis, the strain partitioning of austenite phase in metastable austenitic steels became larger with increasing of the volume fraction of stress-induced martensite during tensile deformation.