Fe Nitride Formation in Fe–Si Alloys: Crystallographic and Thermodynamic Aspects

Abstract

A Fe–3wt pctSi alloy was gas nitrided to study the effect of Si on the Fe nitride formation. Both ε-Fe3N1+x and γ′-Fe4N were observed at nitriding conditions only allowing to form single-phase γ′ layers in pure α-Fe. During short nitriding times, ε and γ′ simultaneously grow in contact with Si-supersaturated α-Fe(Si). Both nitrides almost invariably exhibit crystallographic orientation relationships with α-Fe, which are indicative of a partially displacive transformation of α-Fe being involved in the initial formation of ε and γ′. Due to Si constraining the Fe nitride growth, such transformation mechanism becomes highly important to the nitride layer formation, causing α-Fe-grain-dependent variations in the nitride layer morphology and thickness, as well as microstructure refinement within the nitride layer. After prolonged nitriding, α-Fe is depleted in Si due the pronounced precipitation of Si-rich nitride in α-Fe. The growth mode of the compound layer changes, now advancing by conventional planar-type growth. During nitriding times of 1 to 48 hours, ε exists in contact with the NH3/H2-containing nitriding atmosphere at a nitriding potential of 1 atm−1/2 and 540 °C, only allowing for the formation of γ′ in pure Fe, indicating that Si affects the thermodynamic stability ranges of ε and γ′.

Nitriding of White‐Solidified Fe–C–Si Alloys: Diffusion Path Concept Applied to Inhomogeneous Microstructures

“…It is inferred from this that the phase fraction of X precipitated in α-Fe in front of the compound layer depends on the local value of μ N in α-Fe, that the phase fraction of X in α-Fe decreases with decreasing μ N (increasing z), and that the phase fraction of X in α-Fe has taken an equilibrium value at a given μ N in front of the compound layer after prolonged nitriding. [12,37] In contrast to t 1 , thus, α-Fe is unlikely supersaturated by Si and N anymore at given μ N , but not Si-free. The reaction α !…”

“…À∞; see Section 3) in Figure 8 should relate to the Fe-Si-N boundary system, which is experimentally little explored. [37] The α/ε and α/ε/X phase boundaries observed at the growth front of the compound layer in u Si,α and u Si,Fe 23 Si 5 C 4 regions exist in neither the Fe-C-N system [44,47] nor the Fe-Si-N [31,41] system at the used nitriding temperature. The present authors have previously suggested [12] that these α/ε and α/ε/X phase boundaries indicate the existence of α þ ε and α þ ε þ X phase fields in the Fe-C-Si-N phase diagram at nonzero contents of both C and Si, which are not considered in the available thermodynamic databases.…”

“…[33] Thus, α-Fe may be considered as supersaturated with N and Si at a given distance from the surface, z, i.e., at a given local value of μ N (cf., Section 1.2). [27,37] The Si supersaturation of α-Fe obstructs the ε formation in α-Fe, as the low Si solubility in ε [12] requires redistributing Si to allow for the growth of ε(þX). By contrast, the formation of ε from Si-free θ can readily occur.…”

“…This relates to the known sluggish precipitation kinetics of X in α-Fe. [33,37] Due to the latter, it appears that a large mole fraction of Si should be still dissolved in α-Fe of the diffusion zone after short nitriding times, despite a large driving force to precipitate X. [33] Thus, α-Fe may be considered as supersaturated with N and Si at a given distance from the surface, z, i.e., at a given local value of μ N (cf., Section 1.2).…”

“…The curves are linearized. The line representing the ε þ γ 0 field in the Fe-Si-N boundary system is drawn with a negative slope, not evident from the thermodynamic database,[47] as previous studies[12,37] have indicated that Si lowers the chemical potential of N at which ε can form. The Fe-C-Si boundary system,[47] a Si -a C plane, is considered in the region of the phase diagram in which α and θ are the only stable phases.…”