The dislocation structures of fatigued polycrystalline samples with small average grain sizes were investigated for plastic strains which ranged from 0.000015 to 0.01. It was found that the dislocation structures were arranged into 3 types of configuration which corresponded to the 3 regions of the cyclic stress-strain curve. Cylindrical loop patch structures were present in region A at low strain amplitudes, and were similar to those which had previously been observed in coarse-grained polycrystals. Irregular loop patches also formed in this region in small-grained polycrystals rather than in region B at intermediate strain amplitudes in coarse-grained polycrystals. In region B, persistent slip band structures were formed but occupied a low volume fraction when compared with the coarse-grained polycrystals. In region C, at high plastic strains, the dislocation structures were dominated by dipolar walls. In addition, labyrinth structures developed in region C instead of region B for coarse-grained polycrystals. All of the dislocation structures which were observed were considered to be forms of dipole-ized structures. Such a model dislocation arrangement was used to describe the formation of dislocation structures. It was shown that all of the dislocation configurations which formed in cycled polycrystalline Cu were low-energy structures.

C.D.Liu, M.N.Bassim, D.X.You: Acta Metallurgica et Materialia, 1994, 42[11], 3695-704