B.B. He - Experimental investigation on a novel medium Mn steel combining transformation-induced plasticity and twinning induced.pdf

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Experimental investigation on a novel medium Mn steel
combining transformation-induced plasticity and twinning-
induced plasticity effects
ARTICLE in INTERNATIONAL JOURNAL OF PLASTICITY · NOVEMBER 2015
Impact Factor: 5.57 · DOI: 10.1016/j.ijplas.2015.11.004
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Binbin He
The University of Hong Kong
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Haiwen Luo
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M.X. Huang
The University of Hong Kong
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Experimental investigation on a novel medium Mn steel combining transformation-
induced plasticity and twinning-induced plasticity effects
B.B. He
1,2
, H.W. Luo
3*
, M.X. Huang
1,2*
1
Shenzhen Institute of Research and Innovation, The University of Hong Kong, Shenzhen, China
Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China
3
2
State Key Laboratory of Advanced Metallurgy & School of Metallurgical and Ecological
Engineering, University of Science and Technology Beijing, Xue Yuan Lu 30, Beijing 100083,
China
*Corresponding authors: M.X. Huang: mxhuang@hku.hk, Tel: +85228597906; Fax:
+85228585415
H.W.Luo: luohaiwen@ustb.edu.cn, Tel/Fax: +86 10 62332911
Abstract
The additional deep cryogenic treatment process prior to intercritical annealing was employed to
tailor the mechanical stability of austenite grains in a new medium Mn steel. As a consequence,
the medium Mn steel after intercritical annealing contains the austenite grains with different
mechanical stability due to the different grain size and C content. The large austenite grains with
low C content transform to martensite prior to the strain of 9.5% due to their low mechanical
stability and provide the transformation-induced plasticity (TRIP) effect during tensile test. On
the other hand, the small austenite grains with high C content have high mechanical stability and
proper stacking fault energy, therefore do not transform to martensite but offer the twinning-
induced plasticity (TWIP) effect from 9.5% strain up to fracture. Subsequently some of the
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twinned austenite grains provide the nucleation site for martensite formation from the strain of
26.2% up to fracture, providing the TRIP effect again in the large strain regime. In summary, the
present novel medium Mn steel has TRIP effect firstly, followed by TWIP effect and then
TWIP+TRIP effects during tensile test, therefore demonstrating enhanced work hardening
behaviour and excellent tensile properties.
Keywords: A. phase transformation; twinning; TRIP; TWIP; B. metallic material
1. Introduction
Different from the dislocation plasticity which takes place after the yielding of materials, the
transformation-induced plasticity (TRIP) is that the plastic deformation can occur at an external
equivalent stress lower than the yield stress of a material during phase transformation (Fischer et
al., 1998; Fischer et al., 2000). Two important mechanisms accompany the TRIP effect, namely
the Greenwood-Johnson and the Magee effects. The Greenwood-Johnson effect depicts the
accommodation process of the transformation eigenstrain by the elastic or plastic deformation in
both parent austenite phase and product phase (Greenwood and Johnson, 1965). While the
Magee effect describes the selection of preferred martensitic variants to minimize the total
transformation eigenstrain (Magee, 1966).
Many studies have been proposed to describe the TRIP effect in austenitic steels (Cherkaoui et
al., 2000; Cherkaoui et al., 1998; Das and Tarafder, 2009; Fischlschweiger et al., 2012; Hallberg
et al., 2007; Leblond et al., 1989; Leblond, 1989; Lee et al., 2010; Oberste-Brandenburg and
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Bruhns, 2004; Olson and Cohen, 1975; Petit-Grostabussiat et al., 2004; Prüger et al., 2014;
Stringfellow et al., 1992; Taleb et al., 2001; Taleb and Petit, 2006; Taleb and Sidoroff, 2003;
Zaera et al., 2012; Zaera et al., 2014). For example, the concise mathematical model describes
the Greenwood-Johnson effect with small applied stress for the ideal plastic and isotropic
hardening phases (Leblond et al., 1989; Leblond, 1989). This model was further modified by
introducing the plastic zone in the parent austenitic phase to remove the singularity of solutions
(Taleb and Sidoroff, 2003) and was also re-evaluated by different experimental setups (Petit-
Grostabussiat et al., 2004; Taleb et al., 2001; Taleb and Petit, 2006). Based on the kinetics of
strain induced martensitic transformation (SIMT) (Olson and Cohen, 1975), a new constitutive
model incorporating the stress state was developed and implemented in finite element program
(Stringfellow et al., 1992). The micromechanical model, which couples the classical plasticity
and TRIP, could describe the transformation kinetics of martensite in a single austenitic crystal
and the mechanical behavior of polycrystalline austenitic steel (Cherkaoui et al., 2000;
Cherkaoui et al., 1998). By incorporating the crystallography information of martensite into a
crystal plasticity finite element scheme (CPFEM) or a mean-field scheme, the orientation effect
of martensitic transformation on the macroscopic mechanical behavior of austenitic steel can be
captured (Fischlschweiger et al., 2012; Lee et al., 2010). The TRIP effect in the large strain
regime under different temperatures can be described by using the finite plasticity framework
combined with thermodynamic formulation (Hallberg et al., 2007; Oberste-Brandenburg and
Bruhns, 2004). The higher strain rates could suppress the martensite formation due to the
adiabatic heating, resulting in increased strength but reducing the ductility (Das and Tarafder,
2009). This strain rate sensitivity of martensitic transformation in austenitic steel was recently
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modeled by coupling the thermodynamics and kinetics of SIMT (Prüger et al., 2014; Zaera et al.,
2012; Zaera et al., 2014).
The TRIP effect has been employed to improve the mechanical properties of Advanced High
Strength Steels (AHSS) for an application in the automotive industry (Lee et al., 2013; Mohr et
al., 2010). These AHSSs include the low alloyed TRIP steels (Jacques, 2004; Kubler et al., 2011;
Mahnken et al., 2009), the quenching and partitioning (Q&P) steels (Speer et al., 2003; Xiong et
al., 2013) and the newly developed medium Mn TRIP steels (Shi et al., 2010; Yen et al., 2015).
The different type of above AHSSs may have a combination of different phases such as ferrite,
bainite, martensite and retained austenite, depending on the corresponding thermal mechanical
processing. But they all contain the retained austenite grains as their intrinsic component. These
austenite grains can transform to martensite during plastic deformation, generating the additional
plasticity which leads to a localized work hardening and delays the onset of necking process
(Jacques et al., 2001; Khan et al., 2012; Ma and Hartmaier, 2015). Therefore, the TRIP effect is
important for the excellent ductility of TRIP steels. It is noted that the TRIP effect is closely
related to the mechanical stability of retained austenite grains. Either the exhaustion of retained
austenite grains at the initial loading due to their low mechanical stability or the remaining of
most austenite grains at large deformation due to their high mechanical stability is detrimental to
the mechanical property (Ryu et al., 2010). It is therefore proposed that the austenite grains with
varied mechanical stability are beneficial for the mechanical property as they can gradually
transform to martensite and provide the TRIP effect until necking takes place.
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