Microstructural characterization of ultrafine-grained nickel

Peak profile analysis based on high-resolution X-ray diffractometry and transmission electron microscopy (TEM) were used to measure the distribution of grain sizes, the dislocation density, and the corresponding root mean square strain in ultrafine-grained (UFG) nickel processed by different methods: electrodeposition, equal channel angular pressing (ECAP), cold rolling (CR), high pressure torsion (HPT), and their combinations. The Fourier transforms of the experimental X-ray peak profiles were fitted by theoretical functions calculated on the basis of the model of the microstructure. In this model the crystallites are assumed to have spherical shape and log-normal size distribution. It is also supposed that the strain broadening of the profiles is caused by <110>{111} type dislocations. The results obtained from X-ray diffraction are compared with TEM micrographs. It is found that additional deformation following ECAP further decreases the crystallite size and increases the dislocation density. However, in the electrodeposited specimen the crystallite size is lower and the dislocation density is higher than in the samples obtained by any of the combinations of the severe plastic deformation (SPD) procedures.