Type 316l Stainless Steels Exhibiting Five-Fold Symmetry Mo-Rich Phase

Abstract The existence of phases exhibiting five-fold symmetry patterns has recently been reported for conventional heat treated alloys as well as rapidly solidified alloys. The present paper reports initial results on the analysis of a Mo-rich phase in 316L stainless steel which exhibits this symmetry. The formation and growth of this Mo-rich phase were investigated as a function of aging time. In addition, the effect of nitrogen content on the formation of this five-fold phase was also investigated. The materials investigated were type 316L stainless steels with nitrogen contents of 0.024% and 0.150%. The materials were solution-annealed at 1100 °C for 30 minites followed by an aging treatment at 700 °C for 10–50 hours in a vacuum sealed quartz tube. TEM samples were prepared by conventional twin-jet polishing and carbon replications extracted by use of a bromine-etch. Irrespective of nitrogen content, the grain boundary phases after 50 hours of aging were totally observed as five different types of precipitates; M23C6 and M6C carbides, intermetallic η and χ phases, and the five-fold symmetry phase.