R. E. Schacherl

The kinetics of the nitriding of Fe–7Cr alloys; the role of the nitriding potential

Abstract The nitriding behaviour of the Fe–7Cr alloy was studied at 580°C in a gas mixture of ammonia and hydrogen. The nitriding potential was varied from 0.03 to 0.818 atm−1/2. Microstructural analysis of the nitrided specimens was performed by applying light microscopy, hardness measurements, X-ray diffraction, and electron probe microanalysis. The nitrided zone is composed of both regions with finely dispersed small chromium nitride (CrN) precipitates (continuous precipitates) in ferrite (α–Fe) grains and, mainly near the surface, regions where the precipitates have discontinuously coarsened leading to a lamellar CrN/α–Fe morphology. The nitrogen content within the nitrided zone is larger than expected on the basis of the chromium content and the solubility of nitrogen in (stress free) ferrite: excess nitrogen occurs. The hardness maximum in the nitrided zone and the nitriding depth increases with increasing nitriding potential as long as no iron nitride layer develops at the surface of the specimens. To describe the evolution of the nitrogen concentration depth profile of the nitrided layers, a numerical model was applied that has as important (fit) parameters: the surface nitrogen content, the solubility product of chromium and nitrogen dissolved in the ferrite matrix, and a parameter defining the composition of the precipitated chromium nitride. The nitriding depth depends roughly linearly on the square root of the nitriding potential. Analysis of the concentration depth profile data demonstrated that the amount of excess nitrogen considerably influences the nitriding kinetics.

Solubility of nitrogen in ferrite; the Fe–N phase diagram

Abstract To accurately define important phase boundaries in the iron–nitrogen (temperature–composition) phase diagram as well as the (temperature–potential) Lehrer diagram, the solubility of nitrogen in ferrite was determined as a function of the nitriding potential (which defines the chemical potential of nitrogen) and the temperature. To this end, thin iron foils were homogeneously nitrided in flowing gas mixtures composed of ammonia and hydrogen. Phase identification was performed by means of X-ray diffraction analysis. Further, from the data obtained, the absorption function and the enthalpy for dissolution of nitrogen into ferrite and the enthalpy of the reaction occurring at the α/(α + γ′)-phase boundary were determined. The data obtained were corrected for the occurrence of a stationary state instead of a local equilibrium at the surface of the specimens. It followed that parts of the phase boundaries in the Lehrer diagram do not represent equilibrium states but rather stationary states.

Phase transformation of mixed Cr1-xAlxN nitride precipitates in ferrite

Nitriding of specimens with the composition Fe–1.5 wt% Cr–1.5 wt% Al (Fe–1.6 at.% Cr–3.1 at.% Al) at 853 K leads to the formation of mixed, ternary Cr1− x Al x N nitride platelets precipitated in the cubic, rock-salt structure type obeying a Bain-type orientation relationship with the ferrite matrix. Upon subsequent annealing (at 973 K) the mixed, ternary nitrides transform into the two equilibrium, binary nitrides, namely CrN of cubic, rock-salt structure type in the Bain orientation relationship with the ferrite matrix and AlN of hexagonal, wurtzite structure type, obeying a Pitsch–Schrader orientation relationship with the ferrite matrix. At the same time, the mobile excess nitrogen, dissolved in the ferrite matrix, diffuses towards the originally not nitrided core, where relatively coarse, cubic CrN and hexagonal AlN precipitates develop. The microstructure and (local) composition changes were analysed using X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray and electron probe microanalysis techniques. It was found that the transformation proceeds by Al depletion of the original mixed Cr1− x Al x N precipitates.

The nitrogen-absorption isotherm for Fe-21.5 at. % Cr alloy : dependence of excess nitrogen uptake on precipitation morphology

Nitriding of Fe–21.5 at. % Cr alloy leads to a “discontinuously coarsened”, chromium-nitride/ferrite lamellar precipitation morphology in the nitrided zone. The nitrogen-absorption isotherm for this alloy with this precipitation morphology was determined at 560°C. To assure a constant precipitation morphology the Fe–21.5 at. % Cr specimen was first homogeneously pre-nitrided (at 580°C in an ammonia/hydrogen gas mixture of nitriding potential 0.103 atm−1/2) and then de-nitrided (at 470°C in hydrogen gas atmosphere). The amount of nitrogen remaining in the de-nitrided specimen indicated that the composition of the nitride precipitates is CrN and not (Fe, Cr)N. The measured nitrogen-absorption isotherm revealed the presence of excess nitrogen in the nitrided specimen, which is a surprise in view of the coarse, lamellar precipitation morphology. The occurrence of this excess nitrogen could be ascribed to an unexpected, minor fraction of the total chromium content in the alloy present as coherent, tiny nitride platelets within the ferrite lamellae of the “discontinuously coarsened” lamellar precipitation morphology, as evidenced by transmission electron microscopy. A possible kinetic background for this unusual phenomenon was discussed.

Microstructure of the "white layer" formed on nitrided Fe-7 wt.% Cr alloys

Abstract The formation of a compound, “white” layer on the surface of Fe-7wt.% Cr alloy, by exposure to a gas mixture of ammonia and hydrogen at 580°C, was investigated. The microstructure of the surface layer was analysed employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. For the first time a quantitative (phase) analysis of such a white layer was carried out, demonstrating the presence of a dispersed CrN phase within a γ| iron-nitride matrix.

Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe–4.75 at.% Al alloy

The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe4N1− x ) was investigated upon nitriding in NH3/H2 gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe2N1− z ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.

Normal and excess nitrogen uptake by iron-based Fe–Cr–Al alloys: the role of the Cr/Al atomic ratio

Upon nitriding ferritic iron-based Fe–Cr–Al alloys, containing a total of 1.50 at. % (Cr + Al) alloying elements with varying Cr/Al atomic ratio (0.21–2.00), excess nitrogen uptake occurred, i.e. more nitrogen was incorporated in the specimens than compatible with only inner nitride formation and equilibrium nitrogen solubility of the unstrained ferrite matrix. The amount of excess nitrogen increased with decreasing Cr/Al atomic ratio. The microstructure of the nitrided zone was investigated by X-ray diffraction, electron probe microanalysis, transmission electron microscopy and electron energy loss spectroscopy. Metastable, fine platelet-type, mixed Cr1− x Al x N nitride precipitates developed in the nitrided zone for all of the investigated specimens. The degree of coherency of the nitride precipitates with the surrounding ferrite matrix is discussed in view of the anisotropy of the misfit. Analysis of nitrogen-absorption isotherms, recorded after subsequent pre- and de-nitriding treatments, allowed quantitative differentiation of different types of nitrogen taken up. The amounts of the different types of excess nitrogen as function of the Cr/Al atomic ratio are discussed in terms of the nitride/matrix misfit and the different chemical affinities of Cr and Al for N. The strikingly different nitriding behaviors of Fe–Cr–Al and Fe–Cr–Ti alloys could be explained on this basis.

The microstructure of the diffusion zone of a gaseously nitrided Fe-1.5 wt-%Cr-1.5 wt-% Al alloy.

Abstract Gaseous nitriding experiments of an Fe–1·5 wt-%Cr–1·5 wt-%Al (i.e. Fe–1·6 at.-%Cr–3·1 at.-%Al) alloy were carried out as a function of time at 853 K. The microstructure of the diffusion zone was characterised by microhardness, electron probe microanalysis (EPMA), X-ray diffraction analysis (XRD), scanning transmission electron microscopy (STEM) in combination with energy dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES). Chromium and aluminium precipitate together as a mixed Cr1−xAlxN phase in the diffusion zone. The size of the (semi)coherent precipitates and the amount of excess nitrogen have a strong influence on the microstructure of the diffusion zone. Crack formation occurs after a certain nitriding time starting from the specimen surface and propagating along grain boundaries more or less perpendicularly to the surface towards larger depth. The grain boundary brittleness could be ascribed to the precipitation of excess nitrogen as nitrogen gas at the grain boundaries and to the segregation of Al at grain boundaries promoting precipitation of AlN at the grain boundaries. The residual stress–depth profile was determined and a simple model was proposed to explain the surprising initially occurrence of tensile stress parallel to the surface in the diffusion zone.

Coherency strain and precipitation kinetics: crystalline and amorphous nitride formation in ternary Fe–Ti/Cr/V–Si alloys

Specimens of iron-based binary Fe–Si alloy and ternary Fe–Me–Si alloys (with Me = Ti, Cr and V) were nitrided at 580 °C in a NH3/H2-gas mixture applying a nitriding potential of 0.1 atm−1/2 until nitrogen saturation in the specimens was attained. In contrast with recent observations in other Fe–Me1–Me2 alloys, no “mixed” (Me1, Me2) nitrides developed in Fe–Me–Si alloys upon nitriding: first, all Me precipitates as MeN; and thereafter, all Si precipitates as Si3N4. The MeN precipitates as crystalline, finely dispersed, nanosized platelets, obeying a Baker–Nutting orientation relationship (OR) with respect to the ferrite matrix. The Si3N4 precipitates as cubically, amorphous particles; the incoherent (part of the) MeN/α-Fe interface acts as heterogeneous nucleation site for Si3N4. The Si3N4-precipitation rate was found to be strongly dependent on the degree of coherency of the first precipitating MeN. The different, even opposite, kinetic effects observed for the various Fe–Me–Si alloys could be ascribed to the different time dependences of the coherent → incoherent transitions of the MeN particles in the different Fe–Me–Si alloys.

Compound layer formation on iron-based alloys upon nitriding; phase constitution and pore formation

Abstract The microstructural development associated with the formation of the compound (so-called “white”) layer on the surface of ferritic iron and iron-based alloys by nitriding has been presented in a comparative way on the basis of results obtained in the recent past by nitriding model systems. A tentative explanation of the strikingly different layer-growth mechanisms for iron and iron-based alloys has been offered. Attention has been paid in particular to (i) the possible occurrence in the compound layer of “mixed” iron-based nitrides, with the alloying element Me with affinity for nitrogen (e.g. Me = Cr, Al, V) dissolved substitutionally on the iron sublattice of the nitride, and to (ii) the various operative mechanisms for pore formation in the compound layer.