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 inv estigated by transmission electron microscopy (TEM) after thermal aging. Be sides carbides (M23C6 and M6C) and intermetallic phases, an unknown phase o f an Fe-Mo-Cr(-Si) system in a filmlike morphology precipitated at grain bo undaries. In spite of the similarity in its chemical composition to that of the Laves phase, the phase of the Fe-Mo-Cr(-Si) system exhibited five-, th ree-, 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 con tent to that of commercial type 316L grade suppressed the formation of the filmlike fivefold phase. This was attributed to the dissipation of small La ves clusters by M23C6 carbides, which increased as a result of the decrease d nitrogen content.