CYCLIC DEFORMATION OF FERRITIC STEEL .1. STRESS-STRAIN RESPONSE AND STRUCTURE EVOLUTION

The complete CSS-curve has been established for a low alloyed structural steel tested under plastic strain control. The curve which can be divided into three separate regimes has a plateau region between cyclic strain levels of 10(-4) and 8.10(-4). PSBs are formed on the specimen surface when the plastic strain range corresponds to the plateau regime. The PSBs are sites for crack nucleation. The substructure evolution as seen going along the CSS-curve and with accumulating numbers of cycles is documented in detail and includes: dislocation loops, veins, walls including the ladder-like walls usually associated with PSBs, labyrinths, cells, subgrains, banded cells and subgrains. At low and intermediate plastic strain ranges the surface grains contain a more fine-scaled substructure and develop features which appear in the interior at higher plastic strain ranges. At larger cyclic strain levels microbands and noncrystallographic deformation bands become dominating features. Heavily displaced and serrated grain boundaries are observed at intermediate and high plastic strains both in interior- and surface grains containing microbands, banded cells or banded subgrains.