Evolution of shear banding in fully dense nanocrystalline Ni sheet

Compared with the coarse-grained counterpart, nanocrystalline (NC) metals have higher strength simultaneously with a decrease in ductility, strain localization is a main factor contributed to the early failure of NC metals during plastic deformation. This work deals with the study of shear banding in fully dense electrodeposited NC Ni sheet with sample dimensions at tens of millimeters under quasi-static uniaxial tensile load through the use of a strain gage calculated by digital image correlation technique. Shear band nucleation, broadening process and failure point were recognized. It is identified that maximum shear strain happens in the middle of the shear band where crack initiates first in this experiment. This indicates that the shear banding induces the failure of the NC Ni sample. Meanwhile, physical characteristics of the shear band, such as inclination and width of single full-developed shear band, were determined quantitatively. The results show that the inclination of shear band is about 63°, as well as the width of shear band is in sub-micrometer range. To investigate the micro-mechanisms during the shear banding process in the NC Ni sample, in situ tensile testing in a transmission electron microscope was conducted, the results suggest that grain boundary migration and grain coalescence are the main carriers during the propagation of shear band.