Nanostructured Compound Layers in Nitrided Fe-Si- and Fe-C-Si-base Alloys
Gas nitriding in NH3/H2 atmospheres is a thermochemical surface treatment, which is frequently used to improve the tribological and corrosion properties of steels and cast irons. Nitriding of pure α-Fe and low-alloy steels leads to the formation of a dense compound layer in the surface-adjacent part of the material. This compound layer is composed of Fe (carbo)nitrides γ'-Fe4(N, C) and ε-Fe3(C, N)1+x. The type of Fe (carbo)nitride formed is thermodynamically related to the partial pressures of NH3 and H2 in the nitriding atmosphere. Typically, the columnar γ' and ε grains in the compound layer measure several microns in diameter and up to > 10 µm in length – depending on the nitriding conditions. The microstructure, the dimension of microstructural features and the phase composition in the compound layer are significantly affected when Si is present in the material, whose equilibrium solubility in γ' and ε is considered largely negligible.
The current contribution focusses on peculiar effects of Si on the compound layer formation during nitriding Si-rich steels and cast irons. Different Fe-Si-base steels and Fe-C-Si-base white-solidified cast irons have been produced from pure elements and gas nitrided at conditions known to trigger the formation of single-phase γ' layers in pure α-Fe. The samples have been characterized by X-Ray diffraction (XRD), scanning (transmission) electron microscopy (SEM/STEM), and atom probe tomography (APT).
In the ferritic Fe-Si steels, Si triggers an abnormal growth of γ'-plates (c.f. Meka et al., JOM 65, 2013, 769–775) with a thickness in the submicron range. Nanosized, amorphous Si nitride precipitates in the α-Fe. These precipitates may be subsequently overgrown and incorporated into the compound layer. Moreover, Si seems to promote the formation of ε with respect to the formation of γ' – particularly in the C-rich Fe-C-Si alloys. In the latter, a nanostructured compound layer is formed, which is characterized by amorphous Si-rich plates and rods, measuring < 10 nm in diameter, embedded in a crystalline ε matrix. That nanostructure is believed to result from an isothermal eutectoid reaction, α → ε+Si-rich regions, which is driven by a gradient in the N chemical potential introduced by the nitriding atmosphere. The remarkable volume fraction of interfaces in that unusual crystalline/amorphous nano-eutectoid might have a significant impact the thermodynamic stabilities of the involved N-containing phases.