Transgranular dislocation activities and substructural evolutions accommodating two-dimensional grain boundary sliding in ODS ferritic steel

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 micro structural 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 theta between GBS and dislocation slippage. When theta was small, GBS tended to be accommodated by a group motion of dislocations belonging to <111> {110} or <111> {112} slip systems (slip-band type). When theta 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. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.