LOCAL VARIATIONS OF LATTICE-PARAMETER AND LONG-RANGE INTERNAL-STRESSES DURING CYCLIC DEFORMATION OF POLYCRYSTALLINE COPPER

X-ray line profiles and line positions were measured on single grains of cyclically deformed polycrystalline copper specimens unloaded from characteristic points of the stress-strain hysteresis loop. A high-resolution double crystal diffractometer was used to obtain local lattice parameter changes, from which long-range internal stresses and mean stresses were evaluated. In the case of samples unloaded from the load-reversal points the asymmetry of the characteristically broadened X-ray line profiles indicated forward and backward internal stresses prevailing in the dislocation cell walls and cell interiors of the dislocation cell structure, respectively. The changes of the long-range internal stresses after unloading from different points of the stress-strain hysteresis loop can be explained in terms of the composite model. In addition, mean strains and stresses, varying from grain to grain, could be determined from the shifts of the Bragg reflections. A detailed microstructure-based discussion of the elastic stored energy, evaluated from the analysis of the stress-strain hysteresis loop, is given.