Elastic and Plastic Deformation Behavior Studied by In-Situ Synchrotron X-ray Diffraction in Nanocrystalline Nickel

In situ XRD measurements were conducted during the tensile deformation of both submicron-grained Ni specimens fabricated by accumulative roll bonding (having a grain size of 270 nm) and nanocrystalline Ni fabricated by electrodeposition (having a grain size of 52 nm). Variations in the dislocation density and the extent of elastic deformation could be determined with a time resolution of 1.0 s based on changes in the full width at half maximum of ten Bragg peaks and in the Bragg peak shifts, respectively. The dislocation density was found to vary in four different stages. Regions I and III were the elastic and plastic deformation regions, respectively, while Region II was the transition region. Here, the dislocation density rapidly increased to a value, rho(II), necessary for plastic deformation. Since the increase in rho(II) was inversely proportional to grain size, it is evident that nanocrystalline materials require extremely high dislocation densities for deformation to progress solely by plastic deformation. In Region IV, the multiple dislocations were rapidly annihilated by unloading associated with fracture. In the case of the nanocrystalline Ni, there was little difference in the stress distribution in the grains depending on the crystal direction during plastic deformation and, accordingly, there was only minimal variation in the residual stress in the grain with different crystal directions after unloading.