Cracking mechanisms of different grain boundaries in a high-nitrogen austenitic stainless steel under cavitation erosion

High-nitrogen austenitic stainless steels exhibit beneficial mechanical and chemical properties and are often used in environments involving a high risk of cavitation erosion (CE). These steels contain many types of grain boundaries with various CE resistances. In this study, 18Mn18Cr0.6N steel was subjected to CE, and cracks at different grain boundaries were investigated by electron backscatter diffraction, focused ion beam milling, and transmission electron microscopy. Step formation was found to be a key prerequisite for the crack initiation in CE. The step formation at the Σ3 boundary is unique owing to grain rotation, which is caused by the narrow deformation band composed of dense stacking faults and deformation twins in the Σ3 boundary region. For other types of boundaries, such as Σ9, Σ33c, and random high-angle grain boundaries, steps form because of the uncoordinated deformation of grains on both sides of the boundary. Cracks are prone to propagate along {111} planes; stacking fault regions, such as the Σ3 boundary region and slip band, provide the major paths for crack propagation. CE cracks have difficulty passing through the stacking fault regions laterally. Finally, approaches aiming to improve the CE resistance and factors affecting stacking fault formation were considered herein.