g’-Fe₄N formation upon annealing e-Fe₃N: A Powder Diffraction study using Synchrotron Radiation

T. Liapina¹, A. Leineweber¹, E. J. Mittemeijer¹, M. Knapp², C. Baehtz², Z.Q. Liu³, K. Mitsuishi³, and K. Furuya³

¹Max Planck Institute for Metals Research, Heisenbergstraße 3, 70569 Stuttgart, Germany.

²Institute for Materials Science, Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt, Germany.

³Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba 305-0003, Japan

The investigation of iron nitrides is largely motivated due to their role in metallurgy and also their possible potential as magnetic recording materials. The most important iron nitride phases are e-FeN_y (y = 0.22-0.49) and g’-Fe₄N. The crystal structure of e-FeN_y is based on a hcp arrangement of Fe whereas in g’-Fe₄N the arrangement of Fe is fcc. In both cases N occupies octahedral interstitial sites. Most studies on iron nitrides were performed on compound layers generated on the surfaces of iron or steel. The iron nitrides in such layers contain strong nitrogen concentration gradients caused by the inward diffusion of nitrogen during nitriding. Homogeneous iron nitrides can be prepared e.g. by nitriding of thin iron foils or powders for moderate times. Such iron nitride powders are well suited for structural analysis by powder diffraction methods. X-ray powder diffraction is able to determine quantitatively very small changes in composition [1] and the presence of compositional inhomogeneities [2].

Previous experiments on the behaviour of e-iron nitride powders upon annealing showed [3] that at 350°C e-FeN_0.33 forms precipitates of g’-Fe₄N, which leads to an enrichment of the remaining e-phase with nitrogen (final composition FeN_0.36), in accordance with the phase diagram (Figure 1) and as also observed for bulk specimens [4]. However, details of the transformation mechanism are unknown at present.

Figure 1. Simplified section of the Fe-N phase diagram. y is the concentration of the initial e-phase after nitriding, and yeq is the concentration of the e-phase in equilibrium with the precipitated g’ at the applied annealing temperature.	Figure 2. 113 reflection of e-FeNy as obtained by powder diffraction using synchrotron radiation: original sample (after nitriding) and sample annealed at 673 K for 1 hour, 1 and 3 days, respectively. The shifts of the reflections are mainly caused by an increase in nitrogen content.

In this paper high resolution powder diffraction data obtained using synchrotron radiation (B2 at HASYLAB, Hamburg, l = 1.13985 Å) on original and heat treated (up to 3 d at 360°C and 400°C, respectively) FeN_0.33 powder (particle size 2-5 mm) are presented. The data reveal with increasing annealing time the formation of g’-Fe₄N and a gradual increase of the N content in e-FeN_y, as evidenced by the emergence of reflections of g’-Fe₄N and shifts of the reflections of the present e-FeN_y (Figure 2). Furthermore, anisotropic and asymmetric diffraction-line broadening of the e reflections was observed.

The narrowest e reflections are observed for the sample annealed for 3 d at 400°C, in which apparently equilibrium between e and g’ has been reached already.

An analysis of the dependence on the directions of the diffraction vector of the broadening and the asymmetry of the reflections of the different samples shows an important contribution due to inhomogeneities (local variations in the N content [4]). These inhomogeneities can be reduced by further annealing leading to their virtual disappearance after 3 d at 400°C.

Transmission electron microscopy performed on the powder particles (Figure 3) reveals that g’-Fe₄N grains are formed only in relatively few powder particles. These observations together with the finding the narrow reflections in the powder diffraction patterns taken after long-term annealing exclude the presence of large local composition variations (e.g. from particle to particle), it can be concluded that the nitrogen atoms can move from powder particle to powder particle. This can only occur via direct contact of the mainly spherical particles, since N transport via the gas phase can be excluded: loss of N₂ to the atmosphere is well known to be fully irreversible [5].

Figure 3. (a) Bright field image of some particles in an e-FeN_y + g’-Fe₄N sample. Inset is an g’-Fe₄N diffraction pattern ([001] zone) of the top right area of the central Fe_2-3N (FeN_y) particle. (b) The Fe₄N grain outlined in (a).

1. T. Liapina, A. Leineweber, E. J. Mittemeijer, W. Kockelmann, Acta Mater. 52 (2004) 173-180.

2. A. Leineweber, E.J. Mittemeijer, J. Appl. Crystallogr. 37 (2004) 123-135.

3. A. Leineweber, PhD thesis, University of Dortmund (1999).

4. T. Liapina, A. Leineweber, E.J. Mittemeijer, Scr. Mater. 48 (2003) 1643-1648.

5. E. Lehrer, Z. Elektrochem. 36 (1930) 383-392.