Effect of grain size and deformation temperature on the dislocation structure in cyclically deformed polycrystalline nickel

The effect of grain size and deformation temperature on the stabilised dislocation structure was studied in individual grains of fine- and coarse-grained nickel polycrystals after cyclic deformation at room temperature and at 77 K. It has been found that, independent of the grain size and deformation temperature, the dislocation patterns in the individual grains are substantially determined by the axial orientation of the grains. A typical feature of the structure in the coarse-grained material fatigued at room temperature is a fragmentation of the grain dislocation configuration into structure sections with different mesoscopic structure parameters. In fine-grained material after cycling at room temperature and in coarse-grained material cycled at 77 K, no subdivision of the grain structure could be detected. Low-temperature deformation of coarse-grained nickel leads to the formation of a ‘two-phase’ structure, that is characteristically different to the extended wall structure observed in single-slip oriented nickel monocrystals cyclically deformed at 77 K.