DISLOCATION-STRUCTURES IN FATIGUED POLYCRYSTALLINE COPPER

The dislocation structures in fatigued polycrystalline copper with sma ll average grain size were investigated over a plastic strain range fr om 1.5 x 10(-5) to 10(-2). It was found that the dislocation structure s are arranged into three types of configurations, which correspond to the three regions in the cyclic stress-strain curve. Cylindrical loop patch structure are present in region A for low strain amplitudes, si milar to those observed previously in coarse grained polycrystals. Mor eover, irregular loop patches are also formed in this region for small grains polycrystals rather than in region B at intermediate strain am plitudes for coarse grained polycrystals. In region B, persistent slip band (PSB) structures are formed but with a low volume content compar ed with the coarse grained polycrystals. In region C, at high plastic strains, the dislocation structures are dominated by dipolar walls. In addition, labyrinth structures are developed in region C instead of r egion B for coarse grained polycrystals. All the dislocation structure s observed are viewed as forms of dipolized structures. A dipolized di slocation arrangement model is proposed to describe the formation proc ess of dislocation structures. It is shown that all the dislocation co nfigurations formed in cycled polycrystalline copper are low energy st ructures.