Microstrain and growth fault structures in electrodeposited nanocrystalline Ni and Ni-Fe alloys

Nanocrystalline Ni and Ni-Fe alloys produced by electrodeposition were characterized using high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). The grain sizes for these materials spanned a range of about 81-10 nm. HR-TEM analysis on a series of images revealed the presence of local strains at both high-angle and low-angle grain boundaries and twin boundaries. In addition to this, stacking faults and twins of the growth type (growth faults) were observed in both the nanocrystalline Ni and Ni-Fe alloys. The growth fault density increased with increasing Fe concentration, which is consistent with a decrease in the stacking fault energy. The microstrain for the samples was determined from XRD pattern analysis based on line broadening. A general increasing microstrain trend with decreasing grain size was observed and considered to be related to the local strains observed at grain boundaries in the HR-TEM image analysis. With respect to grain size, the microstrain values for the nanocrystalline Ni-Fe samples were noticeably higher than some of the Ni samples. Further XRD pattern analysis was performed to determine the growth fault probabilities for each of the samples and analyze their influence on the microstrain. Increasing Fe was accompanied by an increase in growth fault probability, which was consistent with the HR-TEM image analysis. In addition to the effect of grain size, there is likely a contributing effect on microstrain-induced XRD line broadening due to the presence of growth faults.