Faculty Members

Faculty of Engineering

Department of Mechanical Engineering

Key words
Crystal plasticity analysis, Dislocation, Metal materials, Mechanical properties

Doctor of Philosophy / Associate Professor

Yohei Yasuda


Department of Mechanical Engineering, Faculty of Engineering, Hokkaido University,
Hokkaido University Graduate School of Engineering, Physics Master’s Program,
Hokkaido University Graduate School of Engineering, Department of Mechanical and Space Engineering(Doctoral Program Credits Completed)

Professional Background

Body Engineer at Mazda Motor Corporation, Postdoc at Kitami Institute of Technology, Postdoc at Kanazawa University

Consultations, Lectures, and Collaborative Research Themes

Mechanical properties of metals

Main research themes and their characteristics

「Crystal plasticity analysis on ductility of Ferrite / Cementite Multilayers」

 Pearlite steels have been widely used as structural materials since they exhibit both high strength and a certain extent of ductility. These superior properties arise from the pearlite’s microstructure, which comprises ferrite and cementite layers, alternately arranged within submicron intervals. However, the mechanism that leads to these properties, particularly ductility, has not been fully understood yet.
 A recent analysis showed that the high strain-hardening ability of ferrite layers in pearlite suppresses the localization of plastic deformation in cementite layers, and then stabilizes elasto-plastic deformation of pearlite phase. This suggests that the strain hardening of the ferrite layers in pearlite microstructures plays a crucial role to improve the ductility of pearlite steels. Hence, revealing the dependence of the strain-hardening rate of the ferrite layers on the characteristic lengths of pearlite microstructures leads to elucidation of the mechanism resulting in the ductility of pearlite steels.
 Therefore, in this study, strain hardening behavior of ferrite layers in the microstructure of drawn pearlite wire is studied theoretically and numerically. It is shown that stress field associated to dislocations could diminish quickly if the dislocations enter the phase or grain boundaries and decompose into smaller segments to distribute along the boundary. Some atomistic simulations of single-phase media validate this phenomenon; dislocations show to pass, decompose or accumulate on tilt-type grain boundaries depending on their atomistic configuration (1). Mechanical responses of ninelayered pearlite models subjected to tensile load are analyzed by a strain gradient crystal plasticity analysis, where possible passage or absorption of dislocations is expressed in the model of dislocation mean free path (2). The critical resolved shear stress for slip systems consists of the lattice friction, the Taylor and Orowan terms and the strain hardening is given by the Taylor one. The density evolution of accumulated dislocations is evaluated by the model of Kocks and Mecking where the dislocation mean free path plays a major role. Results show that the smaller the dislocation absorption ability of the phase boundary and thinner the layer thickness, larger the strain hardening becomes (3). Slip localization in cementite layers is shown to be suppressed when the strain hardening of ferrite layers is higher, and this trend is consistent with results obtained in previous studies by molecular dynamics simulation and classical elasto-plasticity analyses.

Major academic publications

Y. Yasuda, T. Shimokawa, T. Ohashi , T. Niiyama
“Crystal Plasticity Analysis on Ductility of Ferrite/Cementite Multilayers:Effect of Dislocation Absorption Ability of the Hetero Interface”
Tetsu-to-Hagané ,105 (2019) 146-154.

Y. Yasuda, T. Ohashi, T. Shimokawa, T. Niiyama
“Strain-hardening characteristics of ferrite layers in pearlite microstructure”
Materials Science and Technology, 34 (2018) 772-779.