Influence of size effect on microstructural changes in cyclically deformed polycrystalline nickel

Abstract

Abstract Scanning and transmission electron microscopy investigations and X-ray diffraction measurements were performed on cyclically deformed nickel samples showing average grain sizes D between 500 nm and 5 μm and containing different initial dislocation structures. The results are discussed in comparison with main microstructure features observed in fatigued nackel with D = 30 μm and D = 250 μm. A lower threshold value DTH a 1 μm was found for the development of typical fatigue-induced dislocation patterns (FIDPs). The spacing dPR between dislocation-dense regions in the FIDPs is nearly independent of D down to DTH and depends only weakly on the strain amplitude ϵpa (dPR a 500 nm at ϵpa = 2 × 10−3). While for D > 3 μm no D dependence on the cyclic stress–strain curves could be observed, at D > 3 μm the stress level of the cyclic stress–strain curves seems to follow a Hall-Petch relation.