On the unified view of the contribution of plastic strain to cyclic crack initiation: Impact of the progressive transformation of shear bands to persistent slip bands

To examine the local conditions necessary to create cracks around cyclic slip bands, we studied the evolution of surface topography by atomic force microscopy and deformation localization by transmission electron microscopy for a wide structural variety (grain and precipitate sizes) in nickel-base alloys. We observed that the variance σ2 associated with height emergence of deformation bands correlated directly with the type of bands, i.e. shear bands (SBs) or persistent slip bands (PSBs). An increase in the diameter of shearing precipitates induced a progressive transformation of SBs to PSBs with a decrease in the variance σ2. Surface analyses showed that a critical value of the local irreversible plastic strain accumulated in the band (γirr,pl,loc) was necessary to for crack initiation. The value of γirr,pl,loc increased with a progressive transformation of SBs to PSBs and the increase in band thickness. This last parameter is an internal length scale that plays an important role in diffusion of vacancies in accordance with Repetto's model of crack initiation.