Microstructural Parameters in Large Strain Deformed Ni Polycrystals as Investigated by Synchrotron Radiation

Dislocation densities, arrangements and long-range internal stresses in cold worked polycrystalline nickel were determined by X-ray diffraction profile analysis using synchrotron radiation. Torsionally deformed samples were scanned across the diameter of the specimens with a focal spot of 300 μm providing strain dependent results in good correlation with residual electrical resistivity data. On cold rolled specimens with different deformations, scanning measurements across single grains with a focal spot of less than 50 μm were carried out, in order to inform on the features of the deformation induced substructure. At small deformations including stage III, the dislocation densities and internal stresses are uniform within single grains while at higher deformations in stages IV and V, the dislocation density and internal stresses exhibit correlated fluctuations. In analogy to Cu, in stage IV these fluctuations could be identified as polarized tilt walls (PTW) formed from polarized dipole walls (PDW) whereas in stage V, as a difference to Cu, only PDWs and pile-ups of dislocations at the grain boundaries were found. It is suggested that the differences to Cu arise from the enhanced presence of deformation induced vacancies in Ni, enabling stronger effects of static recovery through dislocation climb to take place in between single passes of rolling deformation.