Precipitation of (Fe, Cr) sigma phase from austenite

Abstract A study has been made by thin foil electron microscopy of the precipitation of (Fe, Cr) sigma phase from an austenitic Ni-Cr steel containing Ti and Al. Of the phases which precipitate before sigma, (Ni 3 (TiAl) and TiC), only TiC is observed to directly influence its nucleation. Sigma grows in the form of thin plates which appear to be nucleated around TiC particles in the matrix. The matrix habit plane is of the type (111) which has a similar atomic configuration to the sigma interface plane (001) of the tetragonal unit cell. The complete orientation relation between matrix and sigma was found to be: γ(111) ∥σ(001), γ[011] ∥σ[140]. A high density of stacking faults are present in the sigma plates which form mainly on (100) and (010) planes and may be interpreted as being low energy sequence faults which form during the process of plate growth. Grain boundary sigma forms at an earlier stage of ageing and at the expense of the previously precipitated large TiC particles. It exhibits no definite orientation relation with either grain and contains a low density of stacking faults in comparison with plates precipitating within the grains.