Dislocation structure and strain localisation in nickel single crystals cyclically deformed at 77 K

An experimentally based investigation is presented of the dislocation struc ture and of glide effects occurring in single slip oriented nickel crystals cyclically deformed at 77 K until saturation of the stress amplitude. Spec ial attention is paid to a comparison of slip and structure phenomena obser ved in fatigue tests at 77 K and those found after cycling at room temperat ure (RT) and elevated temperatures. At strain amplitudes within the plateau region of the cyclic stress-strain curve, where at higher temperatures in the crystal two structure types co-exist, at 77 K nearly the entire specime n volume is occupied by one structure "phase", a dislocation-"condensed" wa ll configuration. On different scale levels the main characteristics of thi s extended wall structure were found to be independent of the imposed ampli tude, and they turned out to fit in the temperature dependence of the struc ture features of the ladder-like wall phase characterising the zones of int ense slip (persistent slip bands) at RT and elevated temperatures. At 77 K the strain is localised in narrow slip bands (SBs) in the same way as at hi gher temperatures, although there is no indication of a "two-phase" structu re. From the experimental findings it is concluded that WINTERS "two-phase" model remains valid, when averaging the plastic strain values over all SBs and over a sufficient number of cycles.