Jin-Kyung Kim

• PhD
• Associate Professor at Hanyang University

Deformation twinning in coarse-grained fcc metals results from the highly coordinated glide of Shockley partial dislocations with the same Burgers vector on successive {111}-type twinning planes. The main issue of the formation mechanism of deformation twinning is how the arrangement of Shockley partials required for twinning evolves. Here, we pres...

We report initial and deformation microstructures of a Fe-6Mn-0.15C-1.5Si-1Al (wt%) medium Mn steel having an ultrafine duplex microstructure of ferrite and austenite. The cold-rolled and annealed material shows partially recrystallized ferrite grains. The ferrite grains contain screw dislocations formed by dislocation rearrangement upon recovery o...

We report the role of morphology and size of the σ-phase on mechanical properties of the Fe40Mn40Co10Cr10 non-equiatomic high entropy alloy. The dispersed and fine σ-precipitates formed after cold-rolling and annealing at 700 °C for 1 h lead to an increase in strength without severe ductility loss compared to the material without the σ-phase. Howev...

The FCC Fe40Mn40Co10Cr10 (at. %) high entropy alloy exhibits deformation twinning for room temperature deformation and deformation-induced HCP transformation for subzero deformation. Since the systematic investigation of temperature-dependent dislocation structures is not available, we present an in-depth characterization of the defects involved in...

We report the annealing time-dependent microstructures and deformation mechanisms of the novel face-centered cubic Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 HEA. Three types of precipitates, σ-phase, Cr-rich MC-type carbides, and nano-scale (Ti, V, Mo)C, are present in the annealed materials at 600°C. Such hierarchical precipitates could lead to sluggis...

Austenitic stainless steel, which exhibits superior cryogenic mechanical properties, is considered the most promising material for hydrogen transport applications. However, the cryogenic deformation of austenitic stainless steel often leads to Lüders-type yielding, which can induce formability issues in materials. This study compared the temperatur...

L12-precipitate-strengthened multi-component alloys (MCAs) possess exceptional combination of strength and ductility. However, the high production costs of L12-containing MCAs impede their industrial application. This study introduces a novel approach for designing cost-effective L12-containing MCAs by microalloying Al and Ti with a base compositio...

This study reports the grain-size-dependent deformation-induced face-centered cubic (FCC)–hexagonal closepacked (HCP) transformation behavior of Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 (at. %) high-entropy alloy in the medium grain size range (10–20 μm), their mechanical properties, and deformation mechanisms. The materials annealed at 900 ◦C for 1 h...

We investigated the temperature-dependent mechanical properties and deformation mechanisms of the Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 alloy from room temperature to cryogenic temperatures. As the deformation temperature decreased, both the strength and strain-hardening rate increased, while ductility tended to decrease. The higher strength of the...

We report the microstructures and mechanical properties of a novel Co-free Fe46.0Ni28.6Cr18.5Al3.9Ti3.0 multicomponent alloy containing L12 particles with a high density and heterogeneous distribution. The sample annealed at 600 °C for 3 h exhibited various types of L12 particles such as continuous, discontinuous, and incoherent precipitates. The e...

We report the chemical-composition-dependent precipitation of Cr-rich BCC particles and their role in the hardening of Fe–Cr–Ni medium entropy alloys (MEAs). Three alloys with different chemical compositions: 33Fe–32Cr–35Ni MEA (33Fe), 40Fe–30Cr–30Ni MEA (40Fe), and 45Fe–30Cr–25Ni MEA (45Fe) (at%), were investigated. After annealing at 800 and 950...

Materials displaying negative Poisson’s ratio, referred to as auxeticity, have been found in nature and created in engineering through various structural mechanisms. However, uniting auxeticity with high strength and high stiffness has been challenging. Here, combining in situ nanomechanical testing with microstructure-based modeling, we show that...

High Mn twinning-induced plasticity (TWIP) steels have been the focus of research in the past decade due to their superior combination of strength and ductility. In-situ TEM mechanical testing is an efficient tool to provide direct insights into fundamental deformation mechanisms of materials, and thus, key observations by in-situ TEM mechanical te...

The present work reports the effect of initial microstructures (i.e., the material annealed at 600 °C (A600) with the fine grain single FCC phase vs. the material annealed at 900 °C (A900) with the coarse grain FCC/HCP dual phase) on deformation mechanisms, damage resistance, and mechanical properties of the Fe50Mn30Co10Cr10 HEA. The strength of A6...

We made the chemical heterogeneity in steel consisting of mostly martensite with retained austenite by applying intercritical annealing prior to full austenitization. It enables simultaneous improvement in strength and ductility, which have been regarded as mutually exclusive properties. Contribution from two types of chemical heterogeneity could b...

The present work reports microstructures and mechanical properties of the homogenized and cold-rolled (CoCrFeMnNi)95.2Al3.2Ti1.6 (at. %) high entropy alloy annealed at various temperatures. The material was cold-rolled and annealed in the temperature range from 600 °C to 1000 °C for 1 h. The σ phase was observed in the annealing temperature range f...

Twinning-induced plasticity (TWIP) steel is a second-generation advanced high strength steel grade developed for automotive applications. TWIP steels exhibit an excellent combination of strength and ductility, mainly originating from the activation of deformation twinning. However, TWIP steels generally exhibit a relatively low yield strength (YS),...

Three CoCrFeMnNi high-entropy alloys (HEAs) were produced by vacuum induction melting, induction melting under inert gas atmosphere, and melting under inert gas atmosphere followed by air exposure, respectively. The different manufacturing conditions for the three investigated alloys resulted in different levels and types of inclusions. The alloys...

Electrodeposited Cu film shows grain growth at room temperature, referred to as self-annealing. The present work reports changes in microstructures and properties of electroplated Cu film during self-annealing. The as-deposited Cu film shows an ultrahigh strength of 876 MPa due to a high density of defects such as grain boundaries and dislocations....

We investigated the carbothermic reduction process of ilmenite ore at 1873 K with flux addition. Without flux, the pseudobrookite phase with a high melting temperature was precipitated during ilmenite smelting. This could be the main reason for decreased reduction of iron in ilmenite. To accelerate reduction of ilmenite, two factors were considered...

The present study reports the effects of grain size on deformation microstuctures and stretch-flangeability of the fully austenitic cold-rolled and annealed Fe-17Mn-0.6C-1.5Al (wt. %) twinning-induced plasticity (TWIP) steel. Mechanical properties of the materials annealed at various annealing temperatures were investigated by uniaxial tensile test...

We report evidence of a displacive phase transformation from retained austenite to martensite during preparation of quenched and partitioned steel micro-pillars by using a focused ion beam (FIB) technique. The BCC phase produced by the FIB damage was identified as martensite. The invariant-plane strain surface relief associated with the martensitic...

The effect of the prior austenite grain size (PAGS) on the tensile properties and hole expansion ratio (HER) has been investigated. Starting from different PAGS values (4.7 μm and 15.2 μm) obtained by controlling the austenitizing temperature, microstructure consisting of martensite and austenite (Vγ of 0.09–0.18) was produced by the quenching and...

Mg alloys containing long-period stacking ordered (LPSO) structures are strong and ductile compared to conventional Mg alloys. We study here the compositional evolution of LPSO structures in a Mg97Y2Zn1 (at.%) alloy upon annealing at 500 °C using atom probe tomography. In the material annealed for 2.5 h, the Zn/Y ratio of the building blocks in the...

The present work provides an insight into the phase-specific mechanical properties of the microstructural constituents in a quenching and partitioning (Q&P) processed steel. The mechanical properties of carbon-enriched retained austenite and low-carbon primary martensite were studied by nano-indentation tests. The results show that the hardness of...

The present article proposes a constitutive model that includes the stacking fault energy (SFE)-dependence of the deformation behavior of Fe-Mn-C-(Al) TWIP steels. The different kinetics of the SFE-dependent strain hardening of twinned and twin-free grains are accounted for. The high flow stress of TWIP steels investigated is attributed to the comb...

The influence of Nb addition on casting microstructures and high-temperature mechanical properties of Fe-Mn-C-Al-xNb TWIP steels was analyzed by phase-field modeling and experiments. Phase-field simulations showed that Mn, Nb, and C are enriched in inter-dendritic regions while Al is enriched in dendritic regions during solidification process of th...

The present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 90...

Determination of non-recrystallization temperature of Nb alloyed TWIP steels and 3D atom probe analysis of Nb precipitates in the TWIP steel

We investigated the effect of Nb micro-alloying in the range of 0.01 to 0.l wt.-% on the microstructures and mechanical properties of Fe17Mn0.6C1.5Al (wt. -%) TWIP steel. EBSD analysis shows that the Nb addition retards recrystallization in both the hot-rolled and cold-rolled and annealed TWIP steels. The Nb addition in the cold-rolled and annealed...

Microstructures and tensile properties of a Ti and Mo micro-alloyed ultrafine-grained medium Mn TWIP-TRIP steel were investigated. The steel exhibits a superior combination of strength and ductility, i.e. 46800 MPa∙%, achieved by a high phase fraction of room temperature austenite with enhanced stability. Grain refinement is obtained from the forma...

The data presented in this article are related to the research article entitled "On the plasticity mechanisms of lath martensitic steel" (Jo et al., 2017) [1]. The strain hardening behavior during tensile deformation of a lath martensitic press hardening steel was described using a dislocation density-based constitutive model. The Kubin-Estrin mode...

The plasticity mechanisms of press hardening steel with a fully lath martensite microstructure were examined experimentally by strain rate sensitivity measurements, repeated relaxation tests and internal friction measurements. The analysis of relaxation tests suggests that the micro-plasticity could be due to the motion of mobile non-screw dislocat...

The micro-scale plastic deformation behavior of an austenitic Fe-1.2%C-7.0%Mn (in wt.%) steel was studied by means of nano-indentation and in situ compression of micro-pillars with selected crystallographic orientations. Transmission electron microscopy analysis reveals that the partial dislocation mediated twinning is preferred in a [001]-oriented...

Mg is the most important lightweight engineering alloy enabling future weight-reduced and fuel-saving engineering solutions. Yet, Mg is soft. Long-period stacking ordered (LPSO) structures in Mg alloys have unique crystal structures, characterized by both complex chemical and stacking order. They are essential for strengthening of Mg alloys. The fo...

The effect of V addition on the microstructure and the tensile behavior of Fe-17Mn-0.45C-1.5Al-1Si-xV TWIP steel was investigated. The V additions contributed to a reduction of the grain size and an increase of the yield strength of the TWIP steels. The size distribution of various precipitates in the V-added steel, i.e. VC, TiC-VC co-precipitates...

The occurrence of dislocations containing a ⟨c⟩ component and deformation bands in ε martensite phase, is reported. The analysis of the deformation microstructure by means of transmission electron microscopy was carried out for the hcp ε martensite phase in Fe-16.6Mn-0.03C (wt.%). The observed accommodation of c-axis deformation is considered to be...

The deformation mechanisms and mechanical properties of Fe-Mn-C-Al twinning-induced plasticity (TWIP) steels with a chemical composition range of 12–18 wt% Mn and 0–3 wt% Al, are reviewed. The in-depth microstructural analysis revealed that all the investigated TWIP steels exhibit deformation twinning as the main deformation mechanism in addition t...

We present a systematic atomic scale analysis of the structural evolution of long-period-stacking-ordered (LPSO) structures in the (i) α-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg97Y2Zn1 (at. %) alloy annealed at 500°C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Vari...

We present a transmission electron microscopy study on the room temperature deformation mechanisms in a Mg 97 Y 2 Zn 1 (at.%) alloy with long-period-stacking-order (LPSO) phase. The alloy consists of a-Mg matrix with platelet-shaped LPSO precipitates 3–5 nm thick and interdendritic LPSO (18R structures) phase grains. The interdendritic LPSO phase w...

High Mn steels exhibit an exceptional combination of high strength and large ductility owing to their high strain-hardening rate during deformation. The addition of Al is needed to improve the mechanical performance of TWIP steel by means of the control of the stacking fault energy. In this study, a constitutive modeling approach, which can describ...

The influence of deformation localization during tensile deformation on the development of crystallographic texture in twinning-induced plasticity (TWIP) steel is reported. Whereas Al-added TWIP steel deforms homogeneously before the critical strain of onset of serrations, plastic deformation in Al-free TWIP steel occurs by propagation of isolated...

The twinning induced plasticity steel 0.6 C–18 Mn–1.5 Al (wt.%) employed in this study exhibited a strength of about 1GPa, combined with a large uniform elongation of over 60% at moderate strain rates. This excellent combination of tensile mechanical properties was shown to be a result of complex dynamic strain-induced microstructural reactions, in...

The effect of the addition of copper on the mechanical behavior of a Fe–12%Mn–0.7%C–1.0%Al twinning-induced plasticity steel was investigated by analysis of the mechanical properties obtained in uniaxial tensile tests by means of a physically based constitutive model. The addition of copper was found to retard the kinetics of twin formation and inf...

Room temperature dynamic strain aging (DSA) is often observed from the beginning of plastic deformation in high Mn Fe–18% Mn–0.6% C and Fe–22% Mn–0.6% C twinning-induced plasticitysteels. Although the phenomenon is in many cases very pronounced, there have up to now been no attempts to explain the phenomenon of room temperature DSA in TWIP steel co...

The effect of nitrogen on the dynamic strain again behavior of a Fe-18% Mn-0.6% C twinning-induced plasticity steel was investigated by means of in situ infrared thermography during tensile testing. The addition of nitrogen affected the initiation of the Portevin-Le Châtelier bands and the characteristic shape of the serrations on the stress-strain...

The effect of Al on the stacking fault energy (SFE) of Fe–18Mn–0.6C twinning-induced plasticity steel was investigated by means of weak-beam dark-field transmission electron microscopy. The SFE of Fe–18Mn–0.6C steel was measured to be 13 ± 3 mJ m−2 and the actual increase in SFE due to adding 1 wt.% Al was approximately +11.3 mJ m−2.

The stacking fault energy (SFE) of Fe-18pct Mn-0.6pct C-1.5pct Al twinning-induced plasticity (TWIP) steel was measured using weak-beam dark-field imaging of dissociated dislocations observed in transmission electron microscopy. The SFE was found to be 30mJ/m2. A relatively wide scatter was observed in the experimentally measured partial dislocatio...

High Mn steels demonstrate an exceptional combination of high strength and large ductility as a result of their high strain-hardening rate during deformation. The microstructure evolution and strain-hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uniaxial tension were examined. The purpose of this study was to determine the contribution of all...

High Mn steels demonstrate an exceptional combination of high strength and ductility due to their high work hardening rate during deformation. The microstructure evolution and work hardening behavior of Fe 1 8Mn0.6Cl.5Al TWIP steel in uni-axial tension were examined. The purpose of this study was to determine the contribution of all the relevant de...

The present study focused on the design and testing of new 3rd generation TWIP steels to find reduced Mn content alternatives to the existing high Mn FeMnC and FeMnAlC alloy systems. In order to investigate the effect of nitrogen addition, 12Mn0.6C-N was examined and 18Mn0.6C-N steel was used as a reference. Effects of nitrogen and manganese on the...

Edge stretching is an important formability issue when it comes to apply sheet steels to automotive industry. Anisotropy, strain hardening and toughness are closely related to hole expansion properties. In this paper, hole expansion properties of a high Mn fully austenitic Twinning Induced Plasticity (TWIP) steel are compared with three other types...

The mechanical properties of twinning-induced plasticity (TWIP) steels are often assumed to be solely due to the reduction of the mean free path of glide dislocations resulting from deformation twinning. Other mechanisms may also play an essential role: Mn-C cluster formation, planar glide, pseudo-twinning, short range ordering, and dynamic strain...

High-manganese FeMnC and FeMnAlC austenitic twinning-induced plasticity (TWIP) steel exhibits excellent strain-hardening properties due to the gradual reduction of the mean free path for dislocations glide resulting from deformation twinning. Serrated stress-strain curves are often obtained when this type of steel is tested in a uniaxial tensile te...