Microstructural study of microbands in a Fe-30Mn-6.5Al-0.3C low-density steel deformed at cryogenic temperature by combined electron channeling contrast imaging and electron backscatter diffraction

Abstract

The microstructural characteristics of the microband (MB) structure in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel tensile deformed at -196 °C were investigated by combined electron channeling contrast imaging and electron backscatter diffraction. The nucleation mechanisms and grain-orientation dependence of the most relevant microstructural features of the MB structure, namely, morphology, internal dislocation configuration, and alignment were quantitatively analyzed on the main texture components, i.e. <111>//tensile axis and <001>//tensile axis directions. The present study shows several unrevealed microstructural characteristics of MBs in austenitic Fe-Mn-Al-C low-density steels deformed at cryogenic temperatures. The analysis of the interplay between the activated slip systems and the microstructural characteristics of the MB structure provides further insight into the nucleation mechanisms and the boundary alignment. Several in-grain and grain boundary-assisted MB nucleation mechanisms were identified. At low strain levels (ε = 0.3), MB formation is controlled by planar slip localization phenomena acting on closely spaced slip bands. The MB structure is localized in the grain interiors of areas containing intense plastic localization. At high strain levels (ε = 0.6), the MB structure only develops in grains that are not favorable oriented for deformation twinning, i.e. grains oriented close to <001>//TA directions. At this deformation stage, the deformation behavior becomes highly inhomogeneous due to the activation of an in-grain macroscopic localization phenomenon associated with moderated lattice rotations (∼ 3–5°). It leads to the formation of a complex deformation structure formed by macroscopic deformation bands containing arrays of crystallographic MBs. The analysis of the MB alignment reveals several interesting features such as the formation of non-crystallographic MBs aligned along (3 -1 5) and (3 5 -1) planes. Their formation can be explained in terms of the lattice rotations accommodated by the active slip systems predicted by Winther's model.