The strain amplitude controlled fatigue behavior of pure copper with ultra large grain size

The dislocation structural evolution in polycrystalline copper at constant strain amplitude during low cycle fatigue is well understood. However, the dislocation structural development at variable strain, which changes from high to low strain amplitude, has seldom been reported. Scanning electron microscope (SEM) back-scattering electron imaging was used in this study. Dislocation structure fatigue research was conducted on many copper materials in the literature. Hence, OFHC with an ultra-large grain was used in this work. The fatigue is completed using the Instron 8801 hydraulic fatigue test machine. After fatigue was instituted the microstructures were investigated using BEI of SEM and TEM. The results show that; (1) the S-N curve at reduced strain amplitude reveals softening after hardening at the initiation stage and up to fracture. No plateau area and secondary hardening occurred. (2) A special microstructure with mis-orientation cells embedded in a vein structure was observed in the ultra-large grain. (3) With small strain amplitude from high-low strain amplitude in the ultra-grain specimen, the fracture microstructure includes mis-oriented cells embedded in a loop patch structure. Under this load the micro-cracks are initiated at a small band with a mis-oriented cell structure.