Unusual iron nitride formation upon nitriding Fe-Si alloy

Abstract

The influence of Si, substitutionally dissolved in ferritic Fe-2 at. pct Si and Fe-4.5 at. pct Si alloys, on the nucleation and growth of γ′ iron nitride upon controlled gaseous nitriding was investigated. The nitrided specimens were characterized by XRD, light microscopy, TEM, EELS, EPMA and EBSD. The combination of difficult and thus delayed precipitation of (1) silicon nitride, because of a large misfit with the ferrite matrix, and (2) γ′ iron nitride, because of the negligible solubility of Si, was shown to lead to a series of unusual, nonequilibrium phenomena: high nitrogen supersaturation, development of Si-containing metastable γ′ phase of peculiar morphology, precipitation of amorphous silicon nitride in γ′ and development of ε nitride at thermodynamic conditions unlikely to allow its formation.