Cyclic slip activity of PSBs in bulk and surface grains

The slip behaviour of PSBs and their half-cycle slip activity in bulk grains of a nickel polycrystal cycled at room temperature with medium plastic strain amplitudes up to an advanced stage of the fatigue life (0.6 Nf) was investigated using both electron channelling contrast imaging (ECCI) in the scanning electron microscope (SEM) and atomic force microscopy (AFM). Comparison of the present experimental data with earlier results for PSBs in surface grains reveals that PSBs are not only a surface phenomenon; they also occur quite frequently in the bulk material. The half-cycle slip activity of PSBs in bulk grains shows the same large scatter as PSBs in surface grains. However, a significant difference was found in the degree of localization of half-cycle slip – the thickness of half-cycle active parts of PSBs in bulk grains is much higher than for PSBs in surface grains. The local shear strain amplitudes for PSBs in bulk grains and surface grains lies in the same order of magnitude. In bulk grains, grain boundary displacements due to adjoining PSBs were observed indicating the possible high local shear strain within PSBs in bulk grains.