Surface and internal crack growth during tensile loading in Ni-free high-nitrogen austenitic steel

Abstract

The intergranular crack growth in an Fe-25Cr-1.1N austenitic steel (in wt.%) was examined by in situ scanning electron microscopy and three-dimensional tomographic reconstruction based on Xe-focused-ion beam serial sectioning. The intergranular crack growth exhibited discontinuity, crack deflection/branching along {111}, and crack tip blunting. These features could be interpreted by considering the effects of planar dislocation slip that causes stress concentration at grain boundaries and Lomer-Cottrell sessile dislocations. The models explaining the intergranular cracking and associated crack deflection were proposed based on an assumption of intense planar slip and no cross slip until near-fracture, which was observed by in situ electron channeling contrast imaging under mechanical loading in the present study. In this context, because crack tip deformation is significantly constrained in the specimen interior (plain strain condition), the dislocation-driven intergranular crack growth occurred preferentially in the specimen interior, and subsequently, surface crack propagation occurs in a ligament portion. After blunting of the main crack tip, the coalescence of the main crack and planar-slip-induced brittle crack allows further crack growth.