Fatigue properties and dislocation structures of polycrystalline copper film

Fatigue properties of polycrystalline copper films subjected to pulsating stresses are investigated on the basis of observations of surface topography, fracture surface and dislocation structure. Fatigue limit corresponding to 107 cycles of a rolled copper film is 2.3 times higher than that of an annealed one, but the fatigue limit ratio of the rolled specimen is lower than that of annealed one. From the observations of the surface crack and fracture surface, it is found that fatigue crack initiation and early stage crack growth mechanisms of annealed bulk copper are not necessarily applicable to those of film. No striations are formed in both films. In annealed films, cell structures are formed under high-cycle fatigue whereas no band/ladder structures are formed. Dislocation structures near the fatigue crack in annealed films are similar to those of bulk specimens except for the absence of intensely elongated cells observed in the shear flow regions in the bulk specimen. No appreciable change during fatigue was found in the dislocation structures of rolled films.