先端透過電子顕微鏡法を用いた双晶誘起塑性鋼の塑性変形機構の解明

概要

九州大学学術情報リポジトリ
Kyushu University Institutional Repository

Investigating the plastic deformation
mechanisms of a twinning-induced plasticity
steel using advanced transmission electron
microscopy
ジェサダ, プンヤフ

https://hdl.handle.net/2324/7157367
出版情報：Kyushu University, 2023, 博士（工学）, 課程博士
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（様式３）Form 3

氏

名 ： Jesada Punyafu

Name

論 文 名 ： Investigating the plastic deformation mechanisms of a twinning-induced
plasticity steel using advanced transmission electron microscopy（先端透過電子

Title

顕微鏡法を用いた双晶誘起塑性鋼の塑性変形機構の解明）

区

分 ：甲

Category

論 文 内 容 の 要 旨
Thesis Summary
The dissertation aims to enhance the current knowledge of plastic deformation mechanisms in an Fe22Mn-0.6C (wt.%) twinning-induced plasticity (TWIP) steel using advanced transmission electron
microscopy (TEM). The three main topics treated are 1) the microstructural factors dictating deformation
mechanisms in this TWIP steel with ultrafine-grained (UFG) structures, 2) the effect of grain size on the
dissociation behavior of dislocations, and 3) the seeking for a proof of dislocation pinning. A combination of
ex-situ and in-situ TEM deformation techniques, along with post-mortem dislocation analysis was employed
throughout the dissertation.
Structural defects in this TWIP steel with different average grain sizes are investigated using an ex-situ
deformation approach (post-mortem) utilizing scanning-TEM (STEM) and TEM techniques to elucidate the
microstructural factors influencing deformation mechanisms. The alternation of deformation mechanisms
from dislocation gliding to deformation twinning when the grain size is smaller than 1 µm was observed. The
nucleation of stacking faults and deformation twins in under 1 µm sized grains exhibited grain orientation
(Schmid factor) dependence.
The effect of grain size on deformation twinning was further elucidated by looking into the dissociation
behavior of dislocations using an in-situ TEM deformation technique. Wide stacking faults emitted from
grain boundaries were observed in grains smaller than 1 µm. The influence of grain size on dislocation
dissociation was quantitatively discussed based on the forces acting on Shockley partial dislocations, leading
to a novel formula estimating the emission stress of a partial from a grain boundary. The emission stress
exhibits Schmid factor dependence. The twin initiation stresses estimated from the Hall-Petch relationship
and the emission stress are compared to explain twinning behavior in TWIP steels across all grain sizes.
To obtain a proof of dislocation pinning in the current TWIP steel, the kinetic behavior of dislocations under
external tensile stress was investigated using the in-situ TEM deformation technique. A unique dislocation
glide observed in the present alloy differs from the dislocation glide observed in other alloys, indicating the
presence of dislocation pinning. The majority of pinned segments have a large edge character and are located
near the specimen’s foil surface, suggesting that the pinning is caused by a Cottrell atmosphere through
dislocation pipe diffusion of interstitial C atoms. The potential mechanisms for plastic instabilities resulting
from dislocation pinning are discussed.
The novelty of this work is the comprehensive demonstration of deformation mechanisms in UFG TWIP

steel using advanced TEM. The findings deepen the current knowledge of plastic deformation mechanisms
in TWIP steel and have substantial implications for the development of high-strength materials with
exceptional ductility.