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Youngkook Lee - One of the best experts on this subject based on the ideXlab platform.

  • the effect of austenite grain size on deformation mechanism of fe 17mn steel
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2021
    Co-Authors: Jinyoung Lee, Jin Sung Hong, Seokhyeon Kang, Youngkook Lee
    Abstract:

    Abstract The transition of deformation mechanism from transformation-induced plasticity (TRIP) to Twinning-induced plasticity (TWIP) was investigated in a viewpoint of γ-austenite grain size (AGS) using Fe-17Mn steel. When the AGS decreased from 51.72 μm to 0.75 μm, the apparent stacking fault energy (SFE) value increased from 10.8 mJ/m2 to 23.5 mJ/m2. The transition of deformation mechanism from TRIP to TWIP occurred at the late stage of tensile strain in ultrafine-grained specimens with high apparent SFE values (>23.2 mJ/m2). This result matches well with the fact that a critical AGS for the transition of deformation mechanism is ∼0.52 μm, which was calculated from the relationships between AGS and critical resolved shear stresses for Mechanical Twinning (τtwin) and e-martensitic transformation (τe-mart). UFG specimens with the AGSs below 1.12 μm exhibited a two-step rise at the stage II of strain hardening rate curves due to active Mechanical Twinning and possessed high tensile strength without a great loss of elongation.

  • the mechanism of Mechanical Twinning near grain boundaries in Twinning induced plasticity steel
    Scripta Materialia, 2020
    Co-Authors: Jin Sung Hong, Singon Kang, Jaegil Jung, Youngkook Lee
    Abstract:

    Abstract The mechanism of Mechanical Twinning in the vicinity of a grain boundary, which has been considered as a twin nucleation site, was investigated using Fe-17.5Mn-0.58C-1.5Al Twinning-induced plasticity steel. When tensile strain was 0.02, multiple slips occurred near grain boundaries. The interaction between dislocations on multiple slip planes resulted in stacking faults composed of Shockley and Frank partial dislocations at the locations slightly away from grain boundaries. When the strain reached 0.05, Mechanical twins formed near grain boundaries to grow toward the interior of grains. This observation result matched well with Twinning mechanism proposed by Miura, Takamura, and Narita.

  • tensile properties and deformation mode of si added fe 18mn 0 6c steels
    Acta Materialia, 2018
    Co-Authors: Sang Min Lee, Seung Joon Lee, Sukjin Lee, Jae Hoon Nam, Youngkook Lee
    Abstract:

    Abstract The effects of Si concentration and austenite grain size (AGS) on the tensile properties, stacking fault energy (γ), and deformation mode of Fe-18Mn-0.6C (wt.%) steel were investigated to improve the yield strength (YS) of Twinning-induced plasticity (TWIP) steel. The 3% Si-added steel revealed the higher YS than previous TWIP steels at the same level of AGS. In particular when the AGS was ∼6.8 μm, its YS reached ∼593 MPa, which is comparable to the YS (613 MPa) of transformation-induced plasticity steel with a tensile strength of 980 MPa. The measured γ of 3% Si-added steel was exponentially decreased with grain coarsening primarily due to the reduction of micro-strain, finally reaching its intrinsic γ (γint) at the AGSs above ∼70 μm. This indicates that the γint measurement by means of X-ray diffractometry must be performed using coarse-grained specimens with the AGSs above ∼70 μm. Critical resolved shear stresses for Twinning (τtwin) and e-martensitic transformation (τe-mart) were evaluated as a function of AGS in (0-3%) Si-added steels. Whereas the τtwin value was slightly decreased with increasing AGS or Si concentration, the τe-mart value was more significantly reduced. This indicates that e-martensitic transformation precedes Mechanical Twinning with increasing AGS or Si concentration. As a result, a transition of deformation mode from Mechanical Twinning to e-martensitic transformation occurred with grain coarsening in Si-added steels. A critical AGS for the transition of deformation mode was reduced from ∼69 μm to ∼15 μm with increasing Si concentration from 0.5 wt.% to 3.0 wt.%.

  • the advantage of grain refinement in the hydrogen embrittlement of fe 18mn 0 6c Twinning induced plasticity steel
    Corrosion Science, 2015
    Co-Authors: Il Jeong Park, Sang Min Lee, Hyun Hee Jeon, Youngkook Lee
    Abstract:

    Abstract The effect of grain size on hydrogen embrittlement (HE) was investigated using Fe–18Mn–0.6C Twinning-induced plasticity (TWIP) steel without Al through slow tensile tests and thermal desorption analyses. The grain refinement improved the resistance to HE of TWIP steel by suppressing a ductile to brittle transition. The high ductility of fine-grained specimens resulted from the low densities of twin boundaries, twin–twin junctions, and twin–grain boundary junctions, which are hydrogen-enriched during tensile tests, due to inactive Mechanical Twinning.

  • the effect of pre strain on hydrogen embrittlement in 310s stainless steel
    Journal of Alloys and Compounds, 2014
    Co-Authors: Hyunju Ji, Sang Min Lee, Il Jeong Park, Youngkook Lee
    Abstract:

    The effect of pre-strain on hydrogen embrittlement (HE) was investigated using STS 310S, considering its application to a ferrous cylinder liner for hydrogen storage. Whereas tensile strength was insignificantly influenced by hydrogen charging, elongation was decreased due to a ductile to brittle fracture transition. However, the degree of the reduction in elongation by hydrogen charging was decreased with increasing pre-strain, indicating that pre-straining improved the resistance to HE by suppressing the fracture transition. The cause for suppression of the fracture transition by pre-strain was investigated through the analyses of strain hardening behavior and thermal desorption of hydrogen. The strain hardening rate curves of annealed and pre-strained specimens was divided into several stages, which were greatly affected by primary and secondary Twinning, regardless of hydrogen charging. This strain hardening analysis showed that pre-straining suppressed Mechanical Twinning during tensile deformation. The thermal desorption analysis revealed that the migration of diffusible hydrogen atoms to twin boundaries was hindered by pre-strain. Therefore, the reason why pre-strain suppressed the fracture transition to improve the resistance to HE was because pre-strain hindered both Mechanical Twinning during tensile deformation and hydrogen delivery to twin boundaries not to form highly hydrogen-concentrated twin boundaries.

Il Jeong Park - One of the best experts on this subject based on the ideXlab platform.

  • the effect of si on hydrogen embrittlement of fe 18mn 0 6c xsi Twinning induced plasticity steels
    Acta Materialia, 2016
    Co-Authors: Il Jeong Park, Jaegil Jung
    Abstract:

    Abstract The hydrogen embrittlement (HE) of Fe-18Mn-0.6C-xSi (wt.%) Twinning-induced plasticity (TWIP) steels was investigated through slow strain rate tensile tests (SSRTs) and thermal desorption analyses of electrochemically H-charged specimens. Whereas the H-charged Si-free steel showed only the (Fe,Mn)O layer with a fcc crystal structure on the surface, the Si-added steels had double oxide layers; the outer layer was a mixture of (Fe,Mn)O and (Fe,Mn)2SiO4 with an orthorhombic crystal structure and the inner layer was only (Fe,Mn)2SiO4. When the Si concentration increased, the (Fe,Mn)2SiO4 layer became thicker and the charged H concentration decreased. This result indicates that the (Fe,Mn)2SiO4 layer is effective in suppressing the permeation of H. Both the elongation loss (Eloss) and the area fraction of the brittle-fractured region increased with increasing Si concentration in the H-charged TWIP steels, particularly in the 3 wt.% Si steel, although the H concentration slightly decreased with increasing Si concentration. The H-charged Si-free and 1.5 wt.% Si steels underwent Mechanical Twinning and the migration of H atoms from lattices, dislocations and grain boundaries to Mechanical twins during the SSRTs. The brittleness of both Si-free and 1.5 wt.% Si steels was caused by H-concentrated Mechanical twins. The H-charged 3 wt.% Si steel underwent e-martensitic transformation as well as Mechanical Twinning during the SSRT. H atoms migrated to Mechanical twins until a strain of 0.24, and then inherited primarily into e-martensite with further strain. The great Eloss of the H-charged 3 wt.% Si steel was caused mainly by H-concentrated e-martensite.

  • the advantage of grain refinement in the hydrogen embrittlement of fe 18mn 0 6c Twinning induced plasticity steel
    Corrosion Science, 2015
    Co-Authors: Il Jeong Park, Sang Min Lee, Hyun Hee Jeon, Youngkook Lee
    Abstract:

    Abstract The effect of grain size on hydrogen embrittlement (HE) was investigated using Fe–18Mn–0.6C Twinning-induced plasticity (TWIP) steel without Al through slow tensile tests and thermal desorption analyses. The grain refinement improved the resistance to HE of TWIP steel by suppressing a ductile to brittle transition. The high ductility of fine-grained specimens resulted from the low densities of twin boundaries, twin–twin junctions, and twin–grain boundary junctions, which are hydrogen-enriched during tensile tests, due to inactive Mechanical Twinning.

  • the effect of pre strain on hydrogen embrittlement in 310s stainless steel
    Journal of Alloys and Compounds, 2014
    Co-Authors: Hyunju Ji, Sang Min Lee, Il Jeong Park, Youngkook Lee
    Abstract:

    The effect of pre-strain on hydrogen embrittlement (HE) was investigated using STS 310S, considering its application to a ferrous cylinder liner for hydrogen storage. Whereas tensile strength was insignificantly influenced by hydrogen charging, elongation was decreased due to a ductile to brittle fracture transition. However, the degree of the reduction in elongation by hydrogen charging was decreased with increasing pre-strain, indicating that pre-straining improved the resistance to HE by suppressing the fracture transition. The cause for suppression of the fracture transition by pre-strain was investigated through the analyses of strain hardening behavior and thermal desorption of hydrogen. The strain hardening rate curves of annealed and pre-strained specimens was divided into several stages, which were greatly affected by primary and secondary Twinning, regardless of hydrogen charging. This strain hardening analysis showed that pre-straining suppressed Mechanical Twinning during tensile deformation. The thermal desorption analysis revealed that the migration of diffusible hydrogen atoms to twin boundaries was hindered by pre-strain. Therefore, the reason why pre-strain suppressed the fracture transition to improve the resistance to HE was because pre-strain hindered both Mechanical Twinning during tensile deformation and hydrogen delivery to twin boundaries not to form highly hydrogen-concentrated twin boundaries.

Singon Kang - One of the best experts on this subject based on the ideXlab platform.

  • the mechanism of Mechanical Twinning near grain boundaries in Twinning induced plasticity steel
    Scripta Materialia, 2020
    Co-Authors: Jin Sung Hong, Singon Kang, Jaegil Jung, Youngkook Lee
    Abstract:

    Abstract The mechanism of Mechanical Twinning in the vicinity of a grain boundary, which has been considered as a twin nucleation site, was investigated using Fe-17.5Mn-0.58C-1.5Al Twinning-induced plasticity steel. When tensile strain was 0.02, multiple slips occurred near grain boundaries. The interaction between dislocations on multiple slip planes resulted in stacking faults composed of Shockley and Frank partial dislocations at the locations slightly away from grain boundaries. When the strain reached 0.05, Mechanical twins formed near grain boundaries to grow toward the interior of grains. This observation result matched well with Twinning mechanism proposed by Miura, Takamura, and Narita.

  • the effects of grain size on yielding strain hardening and Mechanical Twinning in fe 18mn 0 6c 1 5al Twinning induced plasticity steel
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2016
    Co-Authors: Singon Kang, Jaegil Jung, Mihyun Kang
    Abstract:

    Abstract The objective of the present study was to investigate the influences of grain refinement on yielding, strain hardening, and Mechanical Twinning during tensile deformation in Fe-high Mn Twinning-induced plasticity (TWIP) steel. For this purpose, Fe–18Mn–0.6C–1.5Al TWIP steels with average grain sizes of 2, 10, and 50 μm were tensile tested at room temperature, and their stress–strain and strain hardening rate curves, dislocation densities, and microstructures were measured and analyzed by means of transmission electron microscopy and neutron diffractometry. The stress–strain curves showed a transition from continuous to discontinuous yielding with grain refinement, which was due to a lack of mobile dislocations, not due to Mechanical Twinning or martensitic transformation. The grain refinement increased the dislocation density, caused the planar to non-planar slip, and retarded primary and secondary Mechanical Twinning. The strain hardening rate–strain curves of TWIP steels used were able to be divided into five stages by the slope change. Until the stage III, dislocation hardening was predominant; at the stages IV and V Mechanical Twinning became more contributive to strain hardening. The suppression of both planar dislocation slip and Mechanical Twinning by grain refinement is most likely due to the increase in the back stress of dislocations on a slip plane, which was caused by the rapid accumulation of dislocations by plastic deformation in the fine-grained TWIP steel. A high level of back stress narrows the width of stacking faults, facilitates the cross slip of dislocations, and reduces the interactions between partial dislocations required for Mechanical Twinning.

  • The effects of Si on the Mechanical Twinning and strain hardening of Fe-18Mn-0.6C Twinning-induced plasticity steel
    Acta Materialia, 2013
    Co-Authors: Kookhyun Jeong, Yeon Seung Jung, Singon Kang
    Abstract:

    Abstract The stacking-fault energy (SFE), dislocation slip, Mechanical Twinning, strain hardening, and yield and tensile strengths were systemically investigated in Fe–18Mn–0.6C–1.5Si Twinning-induced plasticity (TWIP) steel. The results were also compared with those for Fe–18Mn–0.6C and Fe–18Mn–0.6C–1.5Al TWIP steels. The SFE decreased by 4 mJ m −2 per 1 wt.% Si. The addition of Si increased both the yield strength, due mainly to solid solution hardening, and the tensile strength, owing to the high strain hardening that occurred while maintaining a large elongation of over 60%. To examine this high strain hardening, especially at low strains, the volume fractions of the primary and secondary Mechanical twins were quantitatively evaluated by combining the merits of electron backscattered diffractometry and transmission electron microscopy. The volume fractions of both the primary and secondary twins were the highest in the Fe–18Mn–0.6C–1.5Si TWIP steel, which had the lowest SFE of the three TWIP steels. In particular, the volume fraction of the secondary Mechanical twins increased rapidly with the addition of Si. The contributions of dislocation storage, Mechanical Twinning and dynamic strain aging (DSA) to the strain hardening were also quantitatively evaluated in the three TWIP steels. The Si-added TWIP steel had the highest strain hardening, due mainly to the active primary and secondary Twinning, and experienced negligible DSA. In contrast, the Al-added TWIP steel exhibited the lowest strain hardening due to the reductions in both the Mechanical Twinning and DSA.

  • effects of niobium on Mechanical Twinning and tensile properties of a high mn Twinning induced plasticity steel
    Materials Transactions, 2012
    Co-Authors: Singon Kang, Jaegil Jung, Youngkook Lee
    Abstract:

    The effects of Nb on Mechanical Twinning and tensile deformation behavior were investigated in hot-rolled Fe­18Mn­0.6C­1.5Al Twinning-induced plasticity (TWIP) steels without and with 0.1Nb. In comparison to the Nb-free TWIP steel with fully-recrystallized and equiaxed grains, the Nb-added TWIP steel showed non-recrystallized and elongated grains with well-dispersed NbC particles and a high density of dislocations. The increased yield and tensile strengths in the Nb-added TWIP steel were mainly caused by the hardening of dislocations in non-recrystallized grains rather than the precipitation hardening of NbC particles or the solid solution hardening of Nb atoms. Meanwhile, the uniform elongation and strain hardening rate of the Nb-added TWIP steel were decreased due to inactive Mechanical Twinning, which is probably because the dislocations interfere with the movement and dissociation of dislocations necessary for Mechanical Twinning. [doi:10.2320/matertrans.M2012273]

Jaegil Jung - One of the best experts on this subject based on the ideXlab platform.

  • the mechanism of Mechanical Twinning near grain boundaries in Twinning induced plasticity steel
    Scripta Materialia, 2020
    Co-Authors: Jin Sung Hong, Singon Kang, Jaegil Jung, Youngkook Lee
    Abstract:

    Abstract The mechanism of Mechanical Twinning in the vicinity of a grain boundary, which has been considered as a twin nucleation site, was investigated using Fe-17.5Mn-0.58C-1.5Al Twinning-induced plasticity steel. When tensile strain was 0.02, multiple slips occurred near grain boundaries. The interaction between dislocations on multiple slip planes resulted in stacking faults composed of Shockley and Frank partial dislocations at the locations slightly away from grain boundaries. When the strain reached 0.05, Mechanical twins formed near grain boundaries to grow toward the interior of grains. This observation result matched well with Twinning mechanism proposed by Miura, Takamura, and Narita.

  • the effects of grain size on yielding strain hardening and Mechanical Twinning in fe 18mn 0 6c 1 5al Twinning induced plasticity steel
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2016
    Co-Authors: Singon Kang, Jaegil Jung, Mihyun Kang
    Abstract:

    Abstract The objective of the present study was to investigate the influences of grain refinement on yielding, strain hardening, and Mechanical Twinning during tensile deformation in Fe-high Mn Twinning-induced plasticity (TWIP) steel. For this purpose, Fe–18Mn–0.6C–1.5Al TWIP steels with average grain sizes of 2, 10, and 50 μm were tensile tested at room temperature, and their stress–strain and strain hardening rate curves, dislocation densities, and microstructures were measured and analyzed by means of transmission electron microscopy and neutron diffractometry. The stress–strain curves showed a transition from continuous to discontinuous yielding with grain refinement, which was due to a lack of mobile dislocations, not due to Mechanical Twinning or martensitic transformation. The grain refinement increased the dislocation density, caused the planar to non-planar slip, and retarded primary and secondary Mechanical Twinning. The strain hardening rate–strain curves of TWIP steels used were able to be divided into five stages by the slope change. Until the stage III, dislocation hardening was predominant; at the stages IV and V Mechanical Twinning became more contributive to strain hardening. The suppression of both planar dislocation slip and Mechanical Twinning by grain refinement is most likely due to the increase in the back stress of dislocations on a slip plane, which was caused by the rapid accumulation of dislocations by plastic deformation in the fine-grained TWIP steel. A high level of back stress narrows the width of stacking faults, facilitates the cross slip of dislocations, and reduces the interactions between partial dislocations required for Mechanical Twinning.

  • the effect of si on hydrogen embrittlement of fe 18mn 0 6c xsi Twinning induced plasticity steels
    Acta Materialia, 2016
    Co-Authors: Il Jeong Park, Jaegil Jung
    Abstract:

    Abstract The hydrogen embrittlement (HE) of Fe-18Mn-0.6C-xSi (wt.%) Twinning-induced plasticity (TWIP) steels was investigated through slow strain rate tensile tests (SSRTs) and thermal desorption analyses of electrochemically H-charged specimens. Whereas the H-charged Si-free steel showed only the (Fe,Mn)O layer with a fcc crystal structure on the surface, the Si-added steels had double oxide layers; the outer layer was a mixture of (Fe,Mn)O and (Fe,Mn)2SiO4 with an orthorhombic crystal structure and the inner layer was only (Fe,Mn)2SiO4. When the Si concentration increased, the (Fe,Mn)2SiO4 layer became thicker and the charged H concentration decreased. This result indicates that the (Fe,Mn)2SiO4 layer is effective in suppressing the permeation of H. Both the elongation loss (Eloss) and the area fraction of the brittle-fractured region increased with increasing Si concentration in the H-charged TWIP steels, particularly in the 3 wt.% Si steel, although the H concentration slightly decreased with increasing Si concentration. The H-charged Si-free and 1.5 wt.% Si steels underwent Mechanical Twinning and the migration of H atoms from lattices, dislocations and grain boundaries to Mechanical twins during the SSRTs. The brittleness of both Si-free and 1.5 wt.% Si steels was caused by H-concentrated Mechanical twins. The H-charged 3 wt.% Si steel underwent e-martensitic transformation as well as Mechanical Twinning during the SSRT. H atoms migrated to Mechanical twins until a strain of 0.24, and then inherited primarily into e-martensite with further strain. The great Eloss of the H-charged 3 wt.% Si steel was caused mainly by H-concentrated e-martensite.

  • effects of niobium on Mechanical Twinning and tensile properties of a high mn Twinning induced plasticity steel
    Materials Transactions, 2012
    Co-Authors: Singon Kang, Jaegil Jung, Youngkook Lee
    Abstract:

    The effects of Nb on Mechanical Twinning and tensile deformation behavior were investigated in hot-rolled Fe­18Mn­0.6C­1.5Al Twinning-induced plasticity (TWIP) steels without and with 0.1Nb. In comparison to the Nb-free TWIP steel with fully-recrystallized and equiaxed grains, the Nb-added TWIP steel showed non-recrystallized and elongated grains with well-dispersed NbC particles and a high density of dislocations. The increased yield and tensile strengths in the Nb-added TWIP steel were mainly caused by the hardening of dislocations in non-recrystallized grains rather than the precipitation hardening of NbC particles or the solid solution hardening of Nb atoms. Meanwhile, the uniform elongation and strain hardening rate of the Nb-added TWIP steel were decreased due to inactive Mechanical Twinning, which is probably because the dislocations interfere with the movement and dissociation of dislocations necessary for Mechanical Twinning. [doi:10.2320/matertrans.M2012273]

Frederic Prima - One of the best experts on this subject based on the ideXlab platform.

  • a new titanium alloy with a combination of high strength high strain hardening and improved ductility
    Scripta Materialia, 2015
    Co-Authors: Jing Zhang, Cedrik Brozek, Philippe Vermaut, Matthieu Marteleur, Patrice Jacques, Mathieu Veron, E.f. Rauch, Frederic Prima
    Abstract:

    A ternary β-metastable titanium Ti–9Mo–6W (wt.%) was designed. A very high work hardening rate close to 2100 MPa and a uniform deformation larger than 35% were recorded, thanks to combined transformation-induced plasticity and Twinning-induced plasticity effects. In this paper, detailed microstructural analysis was performed to understand the deformation process. Various mechanisms, {3 3 2}〈1 1 3〉 Mechanical Twinning, stress-induced ω phase and stress-induced α″ martensite were identified after Mechanical testing, resulting in a complex network of deformed microstructures with very special synergetic features.

  • investigation of early stage deformation mechanisms in a metastable β titanium alloy showing combined Twinning induced plasticity and transformation induced plasticity effects
    Acta Materialia, 2013
    Co-Authors: Fan Sun, Philippe Vermaut, Matthieu Marteleur, Jing Zhang, Pascal Jacques, Thierry Gloriant, Denis Laille, P Castany, Caroline Curfs, Frederic Prima
    Abstract:

    As expected from the alloy design procedure, combined Twinning-induced plasticity and transformation-induced plasticity effects are activated in a metastable β Ti–12 wt.% Mo alloy. In situ synchrotron X-ray diffraction, electron backscatter diffraction and transmission electron microscopy observations were carried out to investigate the deformation mechanisms and microstructure evolution sequence. In the early deformation stage, primary strain/stress-induced phase transformations (β → ω and β → α″) and primary Mechanical Twinning ({3 3 2}〈1 1 3〉 and {1 1 2}〈1 1 1〉) are activated simultaneously. Secondary martensitic phase transformation and secondary Mechanical Twinning are then triggered in the twinned β zones. The {3 3 2}〈1 1 3〉 Twinning and the subsequent secondary mechanisms dominate the early-stage deformation process. The evolution of the deformation microstructure results in a high strain-hardening rate (∼2 GPa), bringing about high tensile strength (∼1 GPa) and large uniform elongation (>0.38).