Grain boundary filmlike Fe–Mo–Cr phase in nitrogen-added type 316L stainless steels

The precipitates in nitrogen-added type 316L stainless steels (SS) were investigated by transmission electron microscopy (TEM) after thermal aging. Besides carbides (M₂₃C₆ and M₆C) and intermetallic phases, an unknown phase of an Fe–Mo–Cr(–Si) system n a filmlike morphology precipitated at grain boundaries. In spite of the similarity in ts chemical composition to that of the Laves phase, the phase of the Fe–Mo–Cr(–Si) system exhibited five-, three-, and twofold symmetries, which are generally observed in quasicrystals having icosahedral symmetry. This phase was formed from the intergrowth of small crystalline clusters of the Laves phase. Decreasing the nitrogen content to that of commercial type 316L grade suppressed the formation of the filmlike fivefold phase. This was attributed to the dissipation of small Laves clusters by M₂₃C₆ carbides, which increased as a result of the decreased nitrogen content.