Dislocation structures in 16MND5 pressure vessel steel strained in uniaxial tension at different temperatures from −196 °C up to 25 °C

16MND5 ferritic steel specimens are strained in uniaxial tension at different temperatures, from −196 °C up to 25 °C. The TEM observations made after the tests reveal that strain localization takes place at the scale of lath blocks, where the individual laths undergo bending straining. Noticeably, the lath boundary dislocations represent an important fraction of the initial sources. Within the deformed laths, dislocations are homogeneously distributed at low plastic strain and gradually condense into high density structures, with increasing deformation. Dislocation condensation takes place in the form of cell walls or tilt boundary structures at T ⩾ 0 °C; in the form of sub-lath size clusters at T ⩽ −90 °C. Unlike in pure iron, screw dislocation motion in 16MND5 steel is characterized by very active cross-slip and subsequent formation of cross-kinks. It is believed that this self-locking effect contributes to strain hardening, in addition to dislocation accumulation at the lath boundaries.