Mechanical behaviour and development of dislocation arrangements of f.c.c. single crystals fatigued at 77 K

The influence of low test temperatures on strain localization effects was studied on nickel and copper single crystals cycled at plastic strain amplitudes corresponding to the low end of the plateau in the cyclic stress-strain curve. For both materials the occurrence of extrusion-type surface slip markings after cyclic deformation at 77 K indicates that the cyclic strain becomes localized in layers parallel to the primary glide plane. In contrast to the situation at room temperature, there are no characteristic arrangements in the microstructure of dislocation walls which can be correlated with the extrusions on the surface. Nearly the entire volume of the saturated dislocation structure of crystals deformed at 77 K without predeformation consists of extended wall structures with irregularly arranged dislocation-dense regions. In particular, no regular persistent slip band ladders were found in nickel and copper crystals. There is a transition temperature below which no ladder-like dislocation arrangements are formed. Both the evolution of the dislocation arrangements and the mechanical behavior suggest that the structure at low temperatures develops in two stages: stage I in which the hardening behavior is qualitatively the same as at room temperature, followed by stage II in which a second hardening occurs and the extended wall structure is formed.