The interplay between grain boundary sliding (GBS) and atomic diffusion was studied for understanding the fundamental mechanisms of superplasticity and diffusion creep. Two-dimensional GBS was ...achieved during shear deformation at 900 °C with strain rates of 1.1 × 10−5–3.3 × 10−5 s−1 in oxide dispersion strengthened ferritic steel with an anisotropic grain structure, which was designed to minimize the free surface effects including floating grains. Microstructural development during the deformation was observed via electron backscatter diffraction and surface fiducial markers drawn by Ga+ focused ion beam. The plastic flow was predominantly mediated by the cooperative process of GBS and grain boundary diffusion, while other mechanisms including intragranular deformation was hardly recognized. The diffusional flux was typically triggered by local principal stress induced at grain boundaries; the matters flew from overlapping (compressive) to splitting (tensile) grain boundaries. In addition, grain boundary morphology changed from wavy to flat patterns via mass flux from convex to concave sides of grain boundaries to minimize the grain boundary energy. Two distinct interplays between GBS and atomic diffusion were confirmed; the most predominant mode was GBS along the shear strain (i.e. Rachinger sliding) and diffusional accommodation via grain boundaries, while a less amount of Coble diffusion creep along macroscopic principal stress was confirmed with GBS accommodation uncorrelated with the shear strain (i.e. Lifshitz sliding).
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Two-dimensional (2D) grain boundary sliding (GBS), which is useful for phenomenological understanding of superplastic and near-superplastic deformation, was achieved during a high-temperature shear ...test in oxide-dispersion-strengthened ferritic steel exhibiting anisotropic microstructure with largely elongated and aligned grains. In this study, 2D GBS, dislocation slip and subsequent microstructural evolutions were examined using surface markers drawn by focused ion beam and electron back-scattered diffraction analysis. In the near-superplastic state (region III), GBS was accommodated by transgranular dislocation activities initiating from grain protrusions or triple junctions into core areas, as described by the Ball–Hutchison model. The accommodation mechanisms were determined by the microstructural correlation between GBS-triggered stress concentration and available slip orientation and were closely related to the angle θ between GBS and dislocation slippage. When θ was small, GBS tended to be accommodated by a group motion of dislocations belonging to {110} or {112} slip systems (slip-band type). When θ was large, GBS tended to be accommodated by intragranular dislocation accumulation, which led to the development of sub-boundaries along {110} planes via dynamic recovery (sub-boundary type); this would be the origin of continuous dynamic recrystallization.
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The high-temperature deformation mechanism of the FeCrAl-ODS ferritic steel was investigated at 1000 °C for the creep loading perpendicular to the elongated and aligned grains. The strain rate was ...varied in the range from the order of 10−2 to 10−7s−1. With decreasing strain rate from 10−2 to 10−5s−1, creep mechanism shifts from conventional dislocation creep pinned by oxide particles to grain boundary sliding (GBS) assisted concomitantly by diffusional creep. With further decreasing strain rate to 10−7s−1, deformation mechanism is drastically changed; group of three grains can move cooperatively, and cooperative GBS (CGBS) was originally recognized. The threshold stress for onset of CGBS was designated as σthI(CGBS). Rate limiting process of CGBS is dominated by dislocation movement over the oxide particles so as to relieve the stress accumulation due to CGBS. The σthI(CGBS) for CGBS corresponds to one third of the conventional threshold stress for dislocation creep.
•Creep deformation mechanism was studied for FeCrAl-ODS ferritic steels.•At the strain rate of 10−7s−1, cooperative grain boundary sliding (GCGBS) was discovered.•CGBS is dominated by dislocation movement over the oxide particles.•The threshold stress for CGBS was found to be one third of the conventional threshold stress for dislocation creep.
A tensile test was performed at a loading direction perpendicular to elongated cold-rolled grains, and it confirmed the step of the scratched lines across the grain boundaries on the specimen ...surface, providing evidence for grain boundary sliding in oxide dispersion strengthened (ODS) steels. Dynamic recovery within the grains was also observed, and a simple model was constructed that consisted of grain boundary sliding and mismatch accommodation induced by vacancy flow. It was confirmed that such grain boundary sliding was suppressed in ODS steel relative to iron as a result of the pinning of the dislocation movement by the dispersed oxide particles.
Two-dimensional grain-boundary sliding (GBS) was achieved microscopically in an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure, which was deformed ...perpendicular to the long axis. At the border between superplastic regions II and III, microscopic deformation was observed using sub-micron grids drawn on the material surface using a focused ion beam. GBS was accommodated by intragranular deformations in narrow areas around grain boundaries, which has been predicted by earlier researchers as characteristics of the core-mantle model. These observations suggest that dislocations slip only in the mantle regions around wavy boundaries to relax the stress concentration caused by GBS during superplasticity.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The ODS ferritic steels realize potentially higher operating temperature due to structural stability by the dispersed nano-size oxide particles. The deformation process and mechanism of 15CrODS ...ferritic steels were investigated at 1073 K and 1173 K for the cold-rolled and recrystallized conditions. Tensile and creep tests were conducted at the stress in parallel (LD) and perpendicular (TD) directions to the grain boundaries. Strain rate varied from 10−1 to 10−9 s−1. For the LD specimens, deformation in the cold rolled and recrystallized conditions is reinforced by finely dispersed oxide particles. The dominant deformation process for the recrystallized TD specimen is controlled through the grain boundary sliding and stress accommodation via diffusional creep at temperature of 1173 K and lower strain rate less than 10−4 s−1. The grain boundary sliding couldn't be rate-controlling process at 1073 K for the as-cold rolled TD specimen, where a dynamic recovery of the dislocation produced by cold-rolling is related to the deformation process.
The recrystallization behavior of a 88% cold-rolled 15Cr–ODS ferritic steel was investigated. Specimens annealed at low and high temperatures show two different recrystallization modes. Annealing at ...1000°C generates a structure consisting of coarse grains with {110} texture, while annealing at 1150°C and 1300°C produce fine grains with {111} texture. This phenomenon is ascribed to that the mobility of boundaries between {110} nuclei and {001} deformed matrix are higher than between {111} nuclei and {001} deformed matrix. Also it is found that a recovery annealing at 900°C prior to recrystallization annealing will retard recrystallization, which results in a structure of coarse grains with {110} texture even after the following annealing at 1300°C.
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