MECHANICAL-BEHAVIOR AND DEVELOPMENT OF DISLOCATION ARRANGEMENTS OF FCC 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 str ain amplitudes corresponding to the low end of the plateau in the cycl ic 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 pr imary glide plane. In contrast to the situation at room temperature, t here are no characteristic arrangements in the microstructure of dislo cation walls which can be correlated with the extrusions on the surfac e. 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. I n particular, no regular persistent slip band ladders were found in ni ckel and copper crystals. There is a transition temperature below whic h no ladder-like dislocation arrangements are formed. Both the evoluti on 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.