key: cord-0066248-pqqz3u3c
authors: Hosmani, Santosh S.; Schacherl, Ralf E.; Mittemeijer, Eric J.
title: Morphology and constitution of the compound layer formed on nitrided Fe–4wt.%V alloy
date: 2009-01-01
journal: J Mater Sci
DOI: 10.1007/s10853-008-3090-3
sha: 659ac536c32f6eaff590304938e1670ebcfa9d26
doc_id: 66248
cord_uid: pqqz3u3c

The microstructure of the compound (“white”) layer formed on the surface of Fe–4wt.%V alloy, by nitriding in a gas mixture of ammonia and hydrogen at 580 °C, has been investigated by employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. The compound layer is dominantly composed of γ(|)-Fe(4)N nitride. Quantitative analysis of the composition data demonstrated that V is present in the compound layer as VN precipitates, i.e. V is not taken up significantly in (Fe, V) nitrides. A mechanism for compound-layer formation has been proposed.

Nitriding of Fe-V alloys was the subject of several, also recent investigations [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] . In these investigations, attention was solely paid to the ferritic zone, close to the surface, where vanadium-nitride precipitates develop in the ferritic matrix (internal nitriding), i.e. the formation of the diffusion zone. In the case of nitrided Fe-V alloys, no research has focused on the formation of the surface layer, usually indicated as compound layer and also as ''white layer'', which is (dominantly) composed of iron nitrides (external nitriding). Such a compound layer can bring about distinct improvement of the tribological and anti-corrosion properties [11] [12] [13] . The present study involves an analysis of the microstructure of the compound layer formed on nitrided Fe-4wt.%V alloy. In particular, it was investigated whether or not the alloying element V is incorporated (dissolved) in the iron nitride formed.

Fe-4wt.%V alloy was prepared from pure Fe (99.98 wt.%) and pure V (99.80 wt.%) by melting under argon atmosphere (99.999 vol.%) in an Al 2 O 3 crucible using an inductive furnace. The composition of the ingot, as determined by chemical analysis (inductive-coupled plasmaoptic emission spectroscopy), is shown in Table 1 .

The cast of the Fe-4wt.%V alloy was cold rolled to about 1.1-mm-thick sheets. These sheets were cut into pieces with lateral dimensions 1.5 9 1.5 cm 2 . The sheet pieces were annealed at 700°C for 2 h to get a recrystallized grain structure. Next, before nitriding, the specimens were ground, polished (last step: 1 lm diamond paste) and cleaned in an ultrasonic bath filled with ethanol.

The specimens were (gaseously) nitrided at 580°C for 12 and 48 h at the nitriding potential of 0.54 atm -1/2 (345 mL/min hydrogen and 155 mL/min ammonia; for definition of nitriding potential, see Ref. [14] ). Under these nitriding conditions iron nitrides can be formed at the surface [15] .

Two pieces were cut from each nitrided specimen. One piece of each specimen was prepared to a cross section by embedding (PolyFast, Buehler GmbH) and polishing (last step: 0.25 lm diamond paste). Such prepared cross sections were subjected to electron probe microanalysis (EPMA) of the elements N, V and Fe. The procedure for EPMA has been described in detail in Ref. [16] . After EPMA analysis the cross sections were etched using 2.5% nital (2.5 vol.% HNO 3 in ethanol) for about 5 s and investigated by light optical microscopy.

Non-embedded pieces of the nitrided specimens were used for X-ray diffraction phase analysis. A Philips X'Pert diffractometer was used applying Cu K a and Co K a radiations and employing the Bragg-Brentano geometry with a graphite monochromator in the diffracted beam. The diffraction angle (2O -) range scanned was 10-150°, with a step size of 0.05°and a counting time of 8 s per step. The X-ray diffractograms were recorded from specimen surfaces as obtained after nitriding. To identify the phases from the positions of the diffraction peaks, the data from the JCPDS database [17] were used.

Phase identification and morphology of the nitrided zone X-ray diffractograms recorded from the surface of Fe-4wt.%V specimen nitrided at 580°C for 48 h at nitriding potential of 0.54 atm -1/2 by employing Co K a (k = 0.178897 nm) and Cu K a (k = 0.154056 nm) radiations are shown in Fig. 1 . The X-ray diffractograms show reflections of c | -Fe 4 N, 1 a-Fe and VN. The diffractogram recorded with Co K a shows a relatively higher intensity for the a-Fe reflexes than the diffractogram recorded with Cu K a , which is due to the higher penetration depth of the Co K a radiation (a-Fe is present underneath the compound layer). Similar results were obtained for the specimen nitrided for 12 h.

Optical micrographs of the cross sections (at three different locations) of the Fe-4wt.%V specimens nitrided for 12 and 48 h are shown in Figs. 2 and 3, respectively. The nitrided zone and the unnitrided core can be distinguished in the optical micrographs recorded from the specimen nitrided for 12 h, by the distinct visibility of the grain boundaries in the nitrided zone ( Fig. 2 ; the specimen 3. Bright regions at larger depth in the diffusion zone, which have experienced (only) the ''continuous'' precipitation of very fine (submicroscopical) vanadium-nitride particles (see also Refs. [6, 9] ).

Consider again Fig. 1 . For iron-based materials, the penetration/information depth is much less for Cu K a than for Co K a radiation (for discussion and quantification of the penetrative power of the X-rays, see Ref. [18] ). The presence of VN reflections in the diffractogram recorded with Co K a radiation with, with respect to the intensity of the c 0 -Fe 4 N reflections, more or less the same intensity as in the diffractogram recorded with Cu K a radiation, whereas the a-Fe reflections are relatively much weaker if Cu K a radiation is used, already hints at the presence of VN precipitates in the white layer.

Quantitative phase analysis of the surface layer (compound/''white'' layer)

The elemental concentration-depth profiles (EPMA results) of the nitrided layer of the Fe-4wt.%V specimen, nitrided at 580°C for 48 h at 0.540 atm -1/2 , are shown in Fig. 4a together with the optical micrograph (see Fig. 4b ) of the analyzed part of the cross section of the sample (which was etched after EPMA analysis; compound-layer thickness at the location of measurement is about 200 lm). In the optical micrograph, the line, along which the EPMA measurements were performed, has been indicated with a black dotted line. The compound layer characterized by a high level of nitrogen uptake (at the location chosen the compound layer was particularly thick) can be clearly distinguished from the diffusion zone underneath. Table 2 . If it is assumed that vanadium in Fe-4wt.%V alloy would not form any nitrides within the compound layer and would be present as a dissolved component in the c | phase, then the nitrogen content (under the given nitriding conditions: 580°C and 0.540 atm -1/2 nitriding potential) should be around 19.85 at.% [15] . However, the observed average nitrogen content within the compound layer is about 23.1 (maximum deviation ± 0.7) at.% (see Figs. 4 and 5). If we assume that all vanadium in the compound layer is present as VN, then the total nitrogen uptake (i.e. 23.1 at.%) minus the vanadium content of the layer (i.e. 3.4 (maximum deviation ± 0.4) at.%) gives the amount of nitrogen (i.e. 23.1 -3.4 = 19.7 at.%) associated with Fe (73.5 (maximum deviation ± 1.1) at.%) as c | -Fe 4 N 1-x phase (X-ray diffractograms show the presence of c | phase: see Fig. 1 ). Taking the above-mentioned given experimental values for the N and Fe contents ascribed to the c | phase (19.7 and 73.5 at.%), the nitrogen content in the c | phase equals 19 At this place it is appropriate to remark that it has been suggested that CrN precipitates in nitrided Fe-Cr would in reality be (Cr, Fe)N precipitates [19] . It may then also be suggested that in the present case of nitrided Fe-V the VN precipitates might be (Fe, V)N precipitates. However, dedicated nitriding and denitriding experiments (reported in Refs. [10, 20] ) have demonstrated that the precipitates, as remaining after denitriding, have the stoichiometric composition, i.e. CrN and VN, respectively. If the excess nitrogen uptake, upon nitriding iron-based Fe-Cr and Fe-V alloys without compound-layer formation, would be due to the development of (Cr, Fe)N or (V, Fe)N precipitates, rather than CrN and VN precipitates, the denitriding experiments would have shown a higher nitrogen content (in at.%) than the chromium content (in at.%) or the vanadium content (in at.%), which clearly is not the case [10, 20] .

Upon gaseous nitriding, nitrogen diffuses into and through the ferrite matrix containing dissolved vanadium, which leads to the formation of VN precipitates. As soon as the amount of nitrogen dissolved (at octahedral interstitial sites of the ferrite matrix) exceeds the solubility limit, c | ironnitride can develop. The highest amount of nitrogen in the ferrite matrix occurs at the surface and there the ironnitride compound layer then develops. Evidently, ''open'' grain boundaries 4 in direct contact with the outer gas atmosphere also provide sites for iron-nitride development. Further, ''closed'' grain boundaries directly connected with the surface may provide a path for relatively fast nitrogen diffusion and thus an enhanced (as compared to the adjacent bulk material) nitrogen concentration can occur as well along these grain boundaries (however, it is noted that no proof exists for a faster grain-boundary diffusion, as compared to bulk diffusion, for an interstitial as N in a-Fe) and iron-nitride could develop along these ''closed'' grain boundaries too. On this basis, the irregular ''penetration'' of the substrate by the compound layer (cf. Figs. 2d and 3d and see Fig. 5 ) can be understood. After the iron-nitride phase has developed at grain boundaries, iron-nitride growth can progress toward the center of the grains more or less enclosed by iron-nitride bands along their boundaries. When a grain has fully transformed to iron-nitride (containing vanadium-nitride phase), it has become an integral part of the compound layer (see Fig. 6 ). Before a grain transforms to iron-nitride and becomes part of the compound layer, VN has already precipitated in the ferrite matrix. This VN precipitation process is associated with the development of pronounced misfit strain in the ferrite matrix which leads to enhanced nitrogen solubility of the ferrite matrix, i.e. excess nitrogen uptake [23] .

During the phase transformation of (strained) ferrite (surrounding the precipitates) to the c | phase, relaxation of the internal misfit strain fields in the ferrite matrix occurs, and thereby the capacity for excess nitrogen in dissolved fashion is lost. Some excess nitrogen may then segregate at grain boundaries and form N 2 gas, which leads to the development of pores at the grain boundaries (see Fig. 5b,  d) . As a result, the presence of some excess of nitrogen in the compound layer (a result deduced in section ''Quantitative phase analysis of the surface layer (compound/ ''white'' layer)''), developing by the transformation of ferrite grains as described earlier, may be understood.

1. Upon nitriding Fe-4wt.%V specimens at 580°C and at the nitriding potential of 0.54 atm 1/2 a ''white'' layer (compound layer), dominantly composed of c | ironnitride, forms adjacent to the surface. The thickness of the compound layer is not uniform: at some places it penetrates deeply and irregularly into the diffusion zone, especially along the grain boundaries of the matrix. 2. The compound layer is composed of iron nitride and vanadium nitride. Vanadium is not taken up significantly in the iron nitride. 3. Iron-nitride bands formed along the grain boundaries of the matrix grow (thicken) toward the center of the ferrite grains. When a grain has fully transformed to iron nitride (containing vanadium-nitride phase), it has become an integral part of the compound layer. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

Source book on nitriding

Heat treating

JCPDS-International Center for Diffraction Data, PCPDFWIN, Version

Acknowledgements The authors are grateful to Mr. P. Kress for assistance with the nitriding experiments and especially to Mrs. S. Haug for carrying out the electron probe microanalysis measurements.