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

Abstract

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.

Initiation and growth of short fatigue cracks in austenitic Sanicro 25 steel

“…As reported earlier in austenitic stainless steels the cracks prefer to grow in the grains with the Schmid factor higher than 0.4 along one of the {111} <110> slip systems. This statement is in agreement with the previous observations based on crack initiation in PSMs . In AISI 316L steel, the increase of the applied plastic strain amplitude enhances the number of cracks initiated in PSMs.…”

“…Figure A shows typical relief of surface grain after fatigue where 5 well‐developed parallel PSMs are present. As reported earlier by Polák et al, PSMs correspond to PSBs along the {111} slip planes. Narrow PSBs of the thickness of approximately 250 nm lead to the formation of the surface relief, which is composed of 1 or more extrusions and parallel intrusions.…”

“…The localization of plastic deformation into the persistent slip bands leads to the formation of PSMs at the specimen surface in the form of extrusions and intrusions as thoroughly reported and discussed in previous studies . Fatigue cracks preferably start from PSMs and continue growing along the PSM/matrix interface.…”

“…Contrary to 316L steels, Sanicro 25 has higher tendency to formation of planar dislocation structures during fatigue at ambient temperature mainly as a result of its chemical composition . The thickness of the PSBs is around 250 nm, what is 4 times less when compared with the thickness of PSBs in 316L steel (1 μm) and 5 times less when compared with the thickness of PSB of copper (more than 1 μm) . The strain localization into the narrow bands implies the primary role of PSMs in initiation of stage I fatigue cracks.…”

“…The initiation of fatigue cracks at ambient temperatures has been studied intensively on copper single crystals, [8][9][10][11][12] copper bicrystals, 13,14 and copper polycrystals. [15][16][17][18] Different views on the mechanisms of the initiation of fatigue cracks were expressed. 13,[19][20][21][22][23][24] Physically based models propose an important role of point defects in fatigue crack initiation.…”