Configuration of loop patch at low strain amplitude in polycrystalline copper

The loop patch structure produced in polycrystalline copper, fatigued at low strain amplitudes below the quasiplateau region of the cyclic stress strain curves, reveals three types of configuration: cylindrical, irregular and cellular shapes. It is identified that the cylindrical loop patches consist of edge dipolar debris with primary Burgers vector and are formed from the deposition of edge segments by sweeping screw dislocations. The irregular loop patches are developed predominantly from stacking of two kinds of dislocation dipolar debris and loops in the grains oriented with double slip. The cellular loop patches are formed from the interaction of four types of dislocation in the grain oriented with multislip. In addition, cubic sessile dislocations are found to coexist with the irregular loop patches and the cellular loop patches. These cubic sessile dislocations are formed from the reaction between edge dislocations and screw dislocations which are perpendicular to each other. It is explained that all three types of structures are low-energy dislocation configurations and can be modelled by the Taylor lattice, the double Taylor lattice and the complex Taylor lattice respectively.