EFFECT OF THE LOADING RAMP LENGTH ON THE EVOLUTION OF SURFACE AND DISLOCATION-STRUCTURES IN CYCLICALLY DEFORMED POLYCRYSTALLINE COPPER

The effect of the start-up procedure on the cyclic stress strain behav iour, surface topography and dislocation substructures of polycrystall ine copper were studied by SEM and TEM techniques. Two different loadi ng ramps were used with N-R = 1 and 100 cycles, respectively. They res ult in different magnitudes of unidirectional strain, saturation plast ic strain ranges, density of surface slip markings, PSB formation and different dislocation substructures. For specimens with N-R = 1 it was found that the PSB nucleated preferentially at tensile deformation ba nds. Based on TEM observations a microscopic model can be worked out. The stabilization of the vein structure occurs by secondary dislocatio ns gliding on primary planes; destabilization of the veins and nucleat ion of PSB can be caused by local internal stresses arising from secon dary dislocations gliding on secondary planes. This model yields consi stent explanations for: (a) the formation of coarse slip lines for N-R = 1; (b) the correlation between slip lines and PSB; (c) the larger d ensity of PSB and therefore a lower cyclic hardening for N-R = 1; (d) the occurrence of different cyclic hardening for different ramp length s and its maximum near the PSB nucleation stress.