Eutectoid growth of nanoscale amorphous Fe-Si nitride upon nitriding

Abstract

Fe nitrides are highly important for the surface engineering of steels and cast irons, and interesting functional materials. The effect of Si on the formation and growth of Fe nitrides is insufficiently understood. No quantitative data on the Si solubility in the most prominent ε and γ' Fe nitrides exist. In this study, the formation and growth of Fe nitrides and Si nitrides in Si-rich α-Fe in ferrite and pearlite has been studied upon nitriding white-solidified Fe-C-Si alloys. Two unreported isothermal eutectoid reactions occurred under the continuous supply of N to the material, involving the cooperative growth of a crystalline phase, α-Fe or ε, and nanosized amorphous Si-rich nitride (XSC/ε): Fe23Si5C4 → α-Fe+XSC and α-Fe(Si) → ε+Xε. For the first time, the incorporation of Si into ε was proven by atom probe tomography of a specifically designed and nitrided Fe-3.5wt.%C-3wt.%30Si alloy, containing 30Si instead of naturally dominant 28Si. The Si/Fe atomic ratio in the eutectoid ε is < 0.02, corresponding to approx. 1.5 at.% Si for ε-Fe3(N, C). Moreover, the above-mentioned Xε is an unreported, amorphous mixed Fe-Si nitride, with a composition between Fe2Si5N8 and FeSiN2, being compatible with Si4+, N3− and Fe2+ constituents, relating to charge neutrality constraints. Similar to silica glasses containing metal oxide modifiers, the composition of Xε can be formulated as (Si3N4)1-x(Fe3N2)x, with 0.33 < x < 0.43. The current results reveal the inevitable need to extend existing thermodynamic descriptions of the Fe(-C)-Si-N phase diagram. A schematic section of the Fe-C-Si-N diagram was deduced.