Nickel nanocrystalline thin films with various grain sizes were produced by electrodeposition using sulfamate solution with different brightener contents at two temperatures. The grain size of thin films determined by X-ray diffraction ranged between 9 and 68 nm. The effect of the grain size on the yield strength in tension tests can be divided into two regions: region A with the grain size larger than about 50 nm and region B with the grain size smaller than about 20 nm. The region with the grain size between 20 to 50 nm is the transitional region. The yield stress increased in proportion to the inverse square root of the grain size in region A, while in region B in proportion to the inverse of the grain size. In region A, ordinary slip deformation within the grain is operating, while grain-boundary generation of dislocations is predominant in region B. The change of X-ray diffraction profile during loading and unloading was examined for two kinds of films with the grain sizes of 9 and 68 nm. The full width at half maximum (FWHM) of the diffraction profile increased with plastic strain in both films. After unloading, it came back to the initial value for films with 9 nm grain size, while it recovered only partially for films with 68 nm grain size. Dislocations disappeared after unloading in the former film, and some dislocations remain in the latter film.
