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Transmission electron micrographs of SPD-processed UFG-W, showing grains and dislocation cell structures refined to sizes of the order of 500 nm.
(Color online). True stress-strain curves of the UFG-W rectangular samples with square loading faces. The quasistatic tests used strain rates (MTS servohydraulic system). The specimens had a height to width ratio close to 2, following American Society for Testing and Materials standards. The dynamic (Kolsky bar) testing at high rates of used samples with a height to width ratio but , following well-established protocol (see Ref. 9).
(Color online). Strain rate sensitivity, , is reduced monotonically with microstructure refinement into the UFG regime, in several bcc metals (Fe, Ta, V, open symbols). The values for recrystallized and UFG-W are highlighted using solid symbols. Here the grain size is a measure of the microstructural length scale, for grains and subgrains separated by both high-angle and low-angle boundaries.
(a) Optical and (b) scanning electron microscope images showing the shear zones, and (c) the intense concentrated plastic flow with crack initiation inside the shear zone.
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