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Sungjoon Kim - One of the best experts on this subject based on the ideXlab platform.

  • effect of austenitization of cold rolled 10 wt mn steel on microstructure and Discontinuous Yielding
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2020
    Co-Authors: Dong Hwi Kim, Jeehyun Kang, Joo Hyun Ryu, Sungjoon Kim
    Abstract:

    Abstract A cold-rolled 10 wt% Mn steel was austenitized before intercritical annealing. Austenite and ferrite fractions and austenite stability remained equivalent to those of the non-austenitized counterpart. However, martensite that was generated on quenching after austenitization did not recrystallize during intercritical annealing, and austenite formed along lath boundaries of martensite to yield a ferrite + austenite lamellar structure. The transition from interface strengthening to strain hardening in the lamellar structure was not as evident as in equiaxed structure, owing to the higher dislocation density and lower interface strengthening in the lamellar structure than in the equiaxed structure. Consequently, Discontinuous Yielding is effectively prohibited in the lamellar structure.

  • Effect of austenitization of cold-rolled 10 wt% Mn steel on microstructure and Discontinuous Yielding
    Materials Science and Engineering: A, 2020
    Co-Authors: Dong Hwi Kim, Jeehyun Kang, Joo Hyun Ryu, Sungjoon Kim
    Abstract:

    Abstract A cold-rolled 10 wt% Mn steel was austenitized before intercritical annealing. Austenite and ferrite fractions and austenite stability remained equivalent to those of the non-austenitized counterpart. However, martensite that was generated on quenching after austenitization did not recrystallize during intercritical annealing, and austenite formed along lath boundaries of martensite to yield a ferrite + austenite lamellar structure. The transition from interface strengthening to strain hardening in the lamellar structure was not as evident as in equiaxed structure, owing to the higher dislocation density and lower interface strengthening in the lamellar structure than in the equiaxed structure. Consequently, Discontinuous Yielding is effectively prohibited in the lamellar structure.

Nobuhiro Tsuji - One of the best experts on this subject based on the ideXlab platform.

  • Yielding behavior and its effect on uniform elongation in IF steel with various grain sizes
    Journal of Materials Science, 2014
    Co-Authors: Si Gao, Mohit Joshi, Akinobu Shibata, Meichuan Chen, Nobuhiro Tsuji
    Abstract:

    In the present study, IF steel specimens with different grain sizes ranging from 12 to 0.45 μm were fabricated by accumulative roll-bonding process and subsequent annealing. Tensile tests revealed that by decreasing the mean grain size down to an ultrafine range smaller than approximately 1.5 μm, Yielding behavior of the IF steel gradually changed from continuous Yielding to Discontinuous Yielding. An abrupt loss in the uniform elongation occurred, when the average grain size was smaller than about 1 μm. Hall–Petch analysis on the yield stress and uniform elongation implied that the abrupt loss in the uniform elongation in the UFG grain size range corresponded to the appearance of the Discontinuous Yielding behavior. As it has been found in many UFG materials, Discontinuous Yielding is believed to be a unique mechanical behavior of UFG materials, and it has significant importance on the uniform elongation of UFG materials.

  • Fabrication of fine recrystallized grains and their mechanical property in HPT processed pure magnesium
    IOP Conference Series: Materials Science and Engineering, 2014
    Co-Authors: Mohit Joshi, Yuko Fukuta, Nokeun Park, Daisuke Terada, Nobuhiro Tsuji
    Abstract:

    To clarify the grain size effect on mechanical property in fine grained pure Mg, mechanical properties of fully recrystallized Mg with fine grain sizes were investigated. The specimens having fully recrystallized microstructures with mean grain sizes of 2.8 μm and 7.8 μm were fabricated by HPT processing and subsequent annealing. The 2.8 μm specimen showed Discontinuous Yielding with a high yield stress and low strain hardening while the 7.8 μm specimen showed continuous Yielding with a low yield stress and high strain hardening. The as-HPT processed specimen showed a larger ductility than that in the subsequently annealed specimens.

  • Correlation between Continuous/Discontinuous Yielding and Hall­Petch Slope in High Purity Iron
    MATERIALS TRANSACTIONS, 2014
    Co-Authors: Si Gao, Nokeun Park, Akinobu Shibata, Meichuan Chen, Nobuhiro Tsuji
    Abstract:

    High purity iron specimens containing 11 ppm carbon and 8 ppm nitrogen with different grain sizes were fabricated by cold rolling and subsequent annealing. It was found that the specimens exhibited entirely different Yielding behavior in tensile tests depending on different cooling processes after annealing. The water-cooled specimens exhibited continuous Yielding while the air-cooled ones exhibited Discontinuous Yielding. It was found that the Hall­Petch slope, ky, significantly changed depending on the different Yielding behaviors. [doi:10.2320/matertrans.MA201326]

  • correlation between continuous Discontinuous Yielding and hall petch slope in high purity iron
    Materials Transactions, 2014
    Co-Authors: Si Gao, Nokeun Park, Akinobu Shibata, Meichuan Chen, Nobuhiro Tsuji
    Abstract:

    High purity iron specimens containing 11 ppm carbon and 8 ppm nitrogen with different grain sizes were fabricated by cold rolling and subsequent annealing. It was found that the specimens exhibited entirely different Yielding behavior in tensile tests depending on different cooling processes after annealing. The water-cooled specimens exhibited continuous Yielding while the air-cooled ones exhibited Discontinuous Yielding. It was found that the Hall­Petch slope, ky, significantly changed depending on the different Yielding behaviors. [doi:10.2320/matertrans.MA201326]

  • Yielding Behavior and Its Effect on Uniform Elongation of Fine Grained IF Steel
    MATERIALS TRANSACTIONS, 2014
    Co-Authors: Si Gao, Naoya Kamikawa, Akinobu Shibata, Meichuan Chen, Shuai Chen, Nobuhiro Tsuji
    Abstract:

    Interstitial free (IF) steel specimens with different mean grain sizes ranging from 0.4 to 12µm were fabricated by the accumulative roll bonding (ARB) process and subsequent annealing. Tensile tests at room temperature have revealed that by decreasing the mean grain size down to an ultra-fine range, the Yielding behavior gradually changes from the continuous Yielding to the Discontinuous Yielding, accompanying a yield drop phenomenon. It has been found that the yield stress of specimens having fine grain sizes shows extra-hardening, deviated from the original Hall­Petch relation for coarse-grained specimens in accordance with the Discontinuous Yielding. The Hall­Petch analysis also has indicated that the loss in the uniform elongation in the ultrafine grain size range is related to the appearance of the Discontinuous Yielding behavior. [doi:10.2320/matertrans.MA201317]

Dong Hwi Kim - One of the best experts on this subject based on the ideXlab platform.

  • effect of austenitization of cold rolled 10 wt mn steel on microstructure and Discontinuous Yielding
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2020
    Co-Authors: Dong Hwi Kim, Jeehyun Kang, Joo Hyun Ryu, Sungjoon Kim
    Abstract:

    Abstract A cold-rolled 10 wt% Mn steel was austenitized before intercritical annealing. Austenite and ferrite fractions and austenite stability remained equivalent to those of the non-austenitized counterpart. However, martensite that was generated on quenching after austenitization did not recrystallize during intercritical annealing, and austenite formed along lath boundaries of martensite to yield a ferrite + austenite lamellar structure. The transition from interface strengthening to strain hardening in the lamellar structure was not as evident as in equiaxed structure, owing to the higher dislocation density and lower interface strengthening in the lamellar structure than in the equiaxed structure. Consequently, Discontinuous Yielding is effectively prohibited in the lamellar structure.

  • Effect of austenitization of cold-rolled 10 wt% Mn steel on microstructure and Discontinuous Yielding
    Materials Science and Engineering: A, 2020
    Co-Authors: Dong Hwi Kim, Jeehyun Kang, Joo Hyun Ryu, Sungjoon Kim
    Abstract:

    Abstract A cold-rolled 10 wt% Mn steel was austenitized before intercritical annealing. Austenite and ferrite fractions and austenite stability remained equivalent to those of the non-austenitized counterpart. However, martensite that was generated on quenching after austenitization did not recrystallize during intercritical annealing, and austenite formed along lath boundaries of martensite to yield a ferrite + austenite lamellar structure. The transition from interface strengthening to strain hardening in the lamellar structure was not as evident as in equiaxed structure, owing to the higher dislocation density and lower interface strengthening in the lamellar structure than in the equiaxed structure. Consequently, Discontinuous Yielding is effectively prohibited in the lamellar structure.

Akihiko Chiba - One of the best experts on this subject based on the ideXlab platform.

  • Discontinuous Yielding and microstructural evolution of ti 40 at al alloy compressed in single α hcp phase region
    Journal of Alloys and Compounds, 2017
    Co-Authors: Daixiu Wei, Yuichiro Koizumi, Akihiko Chiba
    Abstract:

    Abstract We examined the hot deformation behaviors of Ti-40 at.% Al alloy via hot compression tests at temperatures in the range of 1423–1523 K (single α-hcp phase) at strain rates in the range of 1–0.001 s−1. Specifically, we investigated the Discontinuous Yielding phenomenon and the strain rate dependence of the microstructural evolution of the alloy. Sharp drops in flow stresses occurred during hot compression at strain rates of 1 s−1 and 0.1 s−1, which were attributed mainly to both the multiplication of dislocations and the formation of deformation twins. The twins were determined as being of the compound type with the following twin elements: K1: { 11 2 ¯ 1 } , K2: { 11 2 ¯ 2 ¯ } , η1: 11 2 ¯ 6 ¯ > , η2: 11 2 ¯ 3 > , s = 0.232. The grains of the α2-Ti3Al single phase were more significantly refined and homogenized by hot compression at a high strain rate of 1 s−1 than at a low strain rate of 0.001 s−1, with the assistance of the deformation twins at the high strain rate.

  • Discontinuous Yielding and microstructural evolution of Ti-40 at.% Al alloy compressed in single α-hcp phase region
    Journal of Alloys and Compounds, 2017
    Co-Authors: Daixiu Wei, Yuichiro Koizumi, Akihiko Chiba
    Abstract:

    Abstract We examined the hot deformation behaviors of Ti-40 at.% Al alloy via hot compression tests at temperatures in the range of 1423–1523 K (single α-hcp phase) at strain rates in the range of 1–0.001 s−1. Specifically, we investigated the Discontinuous Yielding phenomenon and the strain rate dependence of the microstructural evolution of the alloy. Sharp drops in flow stresses occurred during hot compression at strain rates of 1 s−1 and 0.1 s−1, which were attributed mainly to both the multiplication of dislocations and the formation of deformation twins. The twins were determined as being of the compound type with the following twin elements: K1: { 11 2 ¯ 1 } , K2: { 11 2 ¯ 2 ¯ } , η1: 11 2 ¯ 6 ¯ > , η2: 11 2 ¯ 3 > , s = 0.232. The grains of the α2-Ti3Al single phase were more significantly refined and homogenized by hot compression at a high strain rate of 1 s−1 than at a low strain rate of 0.001 s−1, with the assistance of the deformation twins at the high strain rate.

Dierk Raabe - One of the best experts on this subject based on the ideXlab platform.

  • Phase boundary segregation-induced strengthening and Discontinuous Yielding in ultrafine-grained duplex medium-Mn steels
    Acta Materialia, 2020
    Co-Authors: Binhan Sun, Alexander Schökel, Wenwen Song, Dirk Ponge, Dierk Raabe, Wolfgang Bleck
    Abstract:

    Abstract The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted Discontinuous Yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus Discontinuous Yielding. These findings extend the current understanding of the Yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials.

  • Macroscopic to nanoscopic in situ investigation on Yielding mechanisms in ultrafine grained medium Mn steels: Role of the austenite-ferrite interface
    Acta Materialia, 2019
    Co-Authors: Binhan Sun, Wenwen Song, Dirk Ponge, Nicolas Vanderesse, Rama Srinivas Varanasi, Philippe Bocher, Dierk Raabe
    Abstract:

    Abstract Ultrafine austenite-ferrite duplex medium Mn steels often show a Discontinuous Yielding phenomenon, which is not commonly observed in other composite-like multiphase materials. The underlying dislocation-based mechanisms are not understood. Here we show that medium Mn steels with an austenite matrix (austenite fraction ∼65 vol%) can exhibit pronounced Discontinuous Yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. We observe that both austenite and ferrite are plastically deformed before the macroscopic yield point. In this microplastic regime, plastic deformation starts in the austenite phase before ferrite yields. The austenite-ferrite interfaces act as preferable nucleation sites for new partial dislocations in austenite and for full dislocations in ferrite. The large total interface area, caused by the submicron grain size, can provide a high density of dislocation sources and lead to a rapid increase of mobile dislocations, which is believed to be the major reason accounting for Discontinuous Yielding in such steels. We simultaneously study the Luders banding behavior and the local deformation-induced martensite forming inside the Luders bands. We find that grain size and the austenite stability against deformation-driven martensite formation are two important microstructural factors controlling the Luders band characteristics in terms of the number of band nucleation sites and their propagation velocity. These factors thus govern the early Yielding stages of medium Mn steels, due to their crucial influence on mobile dislocation generations and local work hardening.