Profiles of persistent slip markings and internal structure of underlying persistent slip bands

Focused ion beam sections and lamellae for transmission electron microscopy were prepared from fatigued specimens of polycrystalline copper and austenitic Sanicro 25 steel. The profiles of persistent slip markings developed on the surface were observed and documented simultaneously with the underlying dislocation structure. In copper fatigued at room temperature and close to liquid nitrogen temperature, persistent slip markings consisting of pronounced extrusions and parallel intrusions appeared at locations where persistent slip bands having the ladder-like dislocation structure egress on the surface. Stage I cracks initiated from the tip of the intrusions. In Sanicro 25 steel, more planar character of the dislocation structure led to thin extrusions and intrusions and several stage I cracks running parallel to the primary slip plane. Exceptionally, ladder-like structure and generally alternating dislocation-rich and dislocation-poor volumes were observed in the PSBs. Dislocations bands of secondary slip systems in the matrix disappeared on intersections with PSBs. Experimental findings were compared with predictions of recent physically based models of fatigue crack initiation.