, which are considered as
preferential nucleation sites for the heterogeneous precipitation
of intermetallic compounds. The formation of sigma
phase in duplex stainless steels is described by the decomposition
of δ ferrite through an eutectoid transformation. After
the nucleation process, sigma phase particles grow into the
adjacent δ ferrite grains. Here, the sigma phase appears as
white areas. The figures show also that the sigma phase
presents a massive morphology and its particles appear very
large. The preferential precipitation of sigma phase from δ
ferrite is due to the richness of chromium and molybdenum
elements in the δ ferrite. The precipitation of Cr23C6 carbides
occurs first at the δ/γ interfaces (as shown in Fig. 6)