Assessment of Internal Stresses Using Dislocation Dipole Heights in Cyclically Deformed [001] Copper Single Crystals

There have been a number of studies on dipole separations in cyclically deformed FCC single crystals in single slip while there are no such studies in multiple slip. The dipole heights provide insight into the presence of long-range internal stresses (LRIS). In this study, we investigated how LRIS compare with the single slip studies through measuring the dislocation of dipole heights. [001] oriented copper single crystals were cyclically deformed in strain-control to saturation at ambient temperature. Transmission electron microscopy (TEM) confirms a labyrinth dislocation microstructure with high dislocation density walls and low dislocation density channels. The maximum dipole heights under the saturation stress were approximately independent of location, being nearly equal in the walls and within the channels. This, by itself, supports a uniform stress across the microstructure and low long-range internal stresses. The maximum value for dipole heights suggests dipole strengths (local stresses) that are about a factor of 2.4 higher than the applied stress based on the usual athermal equations. Considering the small 'extra' stress that may be provided by tripoles or small dislocation pile-ups, a nearly homogenous stress distribution with only small internal stresses may be present, which is consistent with the observation of uniform dipole height across the heterogeneous dislocation microstructure. This observation that the stress state appears to be homogenous and higher than the applied stress has also been reported in the case of cyclically deformed metals in single slip. (authors)