Mechanism of Si3N4 precipitation in nitrided Fe-Si alloys: A novel example of particle-stimulated-nucleation

Atom probe tomography (APT) was employed to investigate the precipitation process of Si3N4 particles after plasma assisted nitriding and subsequent annealing of Fe-Si alloy. The nitriding was done using 15N2 medium to enable quantitative analysis of Si and N by APT. Al, as a minor alloying element, is found to play an important role in the precipitation sequence. It is shown that Al forms Al-N clusters before Si3N4 precipitation takes place, and provides nucleation sites for the precipitation of Si3N4. Composition measurements show that silicon nitrides are enriched in Al, Ti and Mn as compared to the matrix.