A three-dimensional model of non-slipping stress corrosion cracking under low loads

Stress corrosion cracking (SCC) of 316L single-crystal austenitic stainless steel subjected to low loads ( σ nom = 20-40 MPa) in a 45% boiling MgCl 2 solution was studied using synchrotron X-ray computed tomography, finite element analysis and so on. Results show that there was no surface slip band around the nucleation sites and the tips of short cracks, and only several slip bands turned up near the long crack tip. Serial slices and three-dimensional reconstruction of the discontinuous zig-zag surface SCC cracks indicate that these cracks were continuous inside the specimens, coinciding with the macroscopic propagation direction (MPD) and the disconnected steps in the river-like fractograph. The obtaining through the two-surface trace method manifests, rather than {111} slip planes, the cracks extended along {100} crystal planes with the lowest surface energy. The numerous microsteps distributed in the fractograph with and without the surface slip bands, and some of the slip bands were corroded chemically, hence the microcleavage and the local dissolution synergistically led to the SCC advance. When the slip bands appeared on specimen surfaces, lots of tear ridges were also found in the fractograph. As a result, the microshear was one of the primarily microscopic SCC mechanisms under the high load. The three-dimensional SCC model was created at the low stress level, where a main SCC crack grew along MPD due to anodic dissolution, and a microdefect was formed on the crack front. When the microdefect enlarged to a critical size, secondary microcracks nucleated at the stress-concentrated microdefect shoulders. Then, the microcracks propagated to two sides of MPD through anodic dissolution, microcleavage or mciroshear, resulting in the formation of the river-like fractography and the discontinuous surface SCC cracks with or without surface slipping.