Effect of Nitriding Potential KN on the Formation and Growth of a … · 2020. 12. 1. ·...

ORIGINAL RESEARCH ARTICLE

Effect of Nitriding Potential KN on the Formationand Growth of a ‘‘White Layer’’ on Iron AluminideAlloy

NGOC MINH LE , CHRISTIAN SCHIMPF , HORST BIERMANN ,and ANKE DALKE

This paper investigates the effect of nitriding potential under well-defined gas nitridingconditions on the formation and growth of a compound layer called ‘‘white layer’’ on a FeAl40(with the composition of 40 at. pct Al) iron aluminide alloy. The nitriding potential wassystematically varied in the range of 0.1 to 1.75 bar�1/2 at 590 �C for 5 hour nitriding time withan ammonia-hydrogen-nitrogen atmosphere. Characterization of the microstructure and phasesformed within the white layer was performed using optical and scanning electron microscopy,X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and glow discharge opticalemission spectroscopy (GDOES). Experimental results indicated that the nitriding potentialstrongly influences morphology and crystal structure of the white layer. The nitride compoundlayer consists of the phases c¢-Fe4N, e-Fe2-3N, and AlN. A mechanism is proposed for theformation and growth of the white layer, depending on the effect of the nitriding potential.

https://doi.org/10.1007/s11663-020-02029-x� The Author(s) 2020

I. INTRODUCTION

ALUMINIDE intermetallic alloys, based on phasesof TiAl, FeAl, and Fe3Al, have been of great scientific aswell as industrial interest due to their beneficial prop-erties at elevated temperatures such as excellent oxida-tion and creep resistance, and low density compared toNi-based superalloys. These intermetallic alloys havegreat potential for substituting superalloy materials forapplications in the aerospace and automotive indus-tries.[1,2] However, they also have disadvantages such aslow ductility at room temperature, and poor tribologicalproperties due to their insufficient surface hardness. Toovercome these limitations, there is a lot of research toimprove the surface properties of aluminides.[3–5]

Iron aluminides based on the B2-FeAl intermetallicphase exhibit an aluminum content of up to 53 at. pctand can be used for high-temperature applications inwhich the working temperature can reach up to800 �C.[6,7] However, for mechanical parts which workunder abrasive conditions, the hardness of 300 HV up to400 HV of FeAl-based alloys is too low.[8–10] Therefore,surface modification methods such as thermochemicalsurface treatments are applied to enhance the surfacehardness and to improve the friction properties andwear resistance.[8,9,11] Iron aluminides show a goodresponse to nitriding treatment, including fine nitrideprecipitations, increasing the surface hardness.However, there is a limited number of studies apply-

ing the nitriding process, which is known as a commonsurface hardening method, on iron aluminide alloysbased on B2-FeAl. These studies focused mainly on theinfluence of temperature and incubation time on thenitride layer formation during gas[9] and plasma nitrid-ing processes[11] in the range between 450 �C and850 �C. Abnormal behavior in nitride layer growthwas observed at temperatures above 600 �C, as both thethickness and the hardness of the nitride layers weresignificantly reduced. Besides, there was rarely a sys-tematic study about the effect of nitriding atmosphere ornitriding potential KN on B2-FeAl alloys. The nitridingpotential describes nitride phases formation ability ofthe nitriding atmosphere, which is defined by KN =

NGOCMINH LE is with the Technische Universität BergakademieFreiberg, Institute of Materials Engineering, Gustav-Zeuner-Straße 5,09599 Freiberg, Germany and also with the School of MaterialsScience and Engineering, Hanoi University of Science and Technology,Dai Co Viet Street No 1, Hanoi, 10000, Vietnam. Contact e-mail:[email protected] CHRISTIAN SCHIMPF is withthe Technische Universität Bergakademie Freiberg, Institute ofMaterials Science, Gustav-Zeuner-Straße 5, 09599 Freiberg,Germany. HORST BIERMANN and ANKE DALKE are with theTechnische Universität Bergakademie Freiberg, Institute of MaterialsEngineering.

Manuscript submitted June 8, 2020; accepted October 31, 2020.Article published online December 1, 2020.

414—VOLUME 52B, FEBRUARY 2021 METALLURGICAL AND MATERIALS TRANSACTIONS B

http://orcid.org/0000-0002-6989-3201http://orcid.org/0000-0003-4130-1400http://orcid.org/0000-0002-6036-0687http://orcid.org/0000-0001-9477-299Xhttp://crossmark.crossref.org/dialog/?doi=10.1007/s11663-020-02029-x&domain=pdf

pNH3/pH21.5 [atm�1/2 or bar�1/2]; where pNH3 and pH2

are the partial pressures of the ammonia and hydrogengases, respectively.

Spies et al.[9] carried out gas nitriding on an ironaluminide FeAl40 alloy at the above-mentioned range oftemperature in ammonia atmosphere controlled nitrid-ing potential KN. They reported two disadvantagesregarding the diffusion of nitrogen atoms at nitridingtemperatures above 650 �C: (i) the nitrogen potentialKN was strongly reduced related to the thermal decom-position of NH3 since the growth of the nitride layer wasreduced, and (ii) an ‘‘external nitriding’’ process tookplace more strongly. Thereby, the iron atoms diffusedoutwards to the surface and combined with the nitrogenatoms of the gas atmosphere to form a compound layerof iron nitride phases, which prevented the furtherabsorption of nitrogen atoms into the substrate. Theauthors[9] stated that the optimum nitriding temperaturewas in the range between 450 �C and 550 �C. For theinfluence of nitriding potential on the microstructureand the properties of the nitride layer, the authors alsogave preliminary evaluation results at 550 �C with twonitriding potential values of below 0.5 bar�1/2 and above2 bar�1/2. The results showed that the structure of thenitride case comprised two zones; the outer zone was a‘‘white layer’’ with a thickness of 1 to 3 lm and the zonebeneath was a hard layer in which hexagonal AlN wasprecipitated.

In a plasma nitriding process, Zhang et al.[11] inves-tigated the effects of different nitrogen-hydrogen andnitrogen-argon gas mixtures at temperatures in therange of 450 �C to 700 �C with various times of 1 to 20hour on the resulting nitride layer structure of a FeAlalloy. By using two different mixtures of gas composi-tions, 25 pct N2 + 75 pct Ar and 75 pctN2 + 25 pct Ar,the effect of the nitrogen content in the plasma atmo-sphere was investigated; these values correspond to thelow and high nitriding potentials in the conventional gasnitriding. Again, the results showed abnormal growthkinetics of the nitride layer similar to that reported forthe gas nitriding process. According to Zhang, nitridelayers generated in a temperature range of 450 �C to600 �C showed both high hardness and thickness.However, the authors also not have made any specificassessment of the influence of nitrogen content on themicrostructure of the nitride layers.

Martin et al.[10] carried out plasma-assisted nitridingat 600 �C for 15 minutes on ODS FeAl40 and foundthat the hardness of the nitride layer was due to thepresence of an outer sublayer made of c¢-Fe4N exhibit-ing moderate hardness values between 550 and 850 HV,and a much harder inner sublayer of AlN with 1400 HV.

Çelikyurek[8] studied the nitriding effect on the Fe3Alalloy by a salt bath nitriding process at 580 �C. Theirresults revealed that the hardness of the nitride layer hada value of about 1200 HV, while the matrix Fe3Al hadonly 300 HV. Moreover, an increase in the nitridingduration did not affect the layer hardness but influencedthe wear resistance to decrease. Celikyurek found thatthe layers formed on nitrided Fe3Al mainly composed ofAlN and Fe3N nitride.

The above-mentioned studies have shown that appro-priate nitriding temperatures are lower than 600 �C andthat the microstructure of the generated nitride layersshowed a twofold-layer structure consisting of a ‘‘whitelayer’’ and a ‘‘diffusion layer’’ depending on the appliednitriding parameters. However, up to now, no system-atic study investigating the influence of the compositionof the nitriding atmosphere on the nitriding effect ofiron aluminide alloys has been conducted. At moderngas nitriding processes with ammonia, the nitridingpotential KN is used to set the atmosphere’s potential toenable a controlled diffusion of nitrogen into a material.The structure and properties of the nitride layer can beachieved as desired through the precise control of thecomposition of the atmosphere expressed as the nitrid-ing potential KN.In this study, the main objective is to evaluate the

effect of the nitriding potential on the formation andgrowth of the compound layer, also called ‘‘whitelayer’’, on B2-FeAl alloys during the gas nitridingprocess using ammonia. Based on the obtained results,the effect of nitriding potential on the formed phases isthoroughly discussed, and an optimal KN value is givenfor this alloy.

II. EXPERIMENTAL DETAILS

The investigated material was a B2-FeAl compoundbased intermetallic alloy with the composition of 40 at.pct Al. The ordered FeAl phase forms on thebody-centered cubic (bcc) lattice, with B2 structureover a wide range of compositions. The ideally orderedB2 structure has a composition of 50 at. pct Fe-50 at.pct Al.[1,12,13]

The specimens were machined with a diameter of 12mm and a thickness of 5 mm. Before nitriding, they werewet-ground with SiC sandpapers up to 1200 grit andcleaned in ethanol to remove contaminants on thesurface.The nitriding experiments were carried out at 590 �C

for the nitriding time of 5 hours in an atmosphere thatcontains NH3, H2, and N2 gases. During the nitridingprocess, the nitriding potential KN was controlled byhydrogen sensors used to measure the partial pressure ofH2 in the outlet exhaust gas. In order to investigate theeffect of the composition of the gas atmosphere on thenitride layer formation, the nitriding potential KN wasvaried in the range between 0.1 and 1.75 bar�1/2.The surface topography of the treated samples was

investigated using a MIRA3 TESCAN scanning elec-tron microscope (SEM). SEM analysis was done inorder to characterize the morphology of the compoundlayer on the substrate for different KN values. Besides, atactile surface roughness measuring device MarSurfPS10 was used to determine the roughness average Ra ofthe nitrided samples’ surfaces.Cross-sectional characterization of the nitrided sam-

ples was performed to investigate the microstructure andto assess the thickness of the nitride layer. Therefore, thesamples were sectioned and metallographically polished

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 52B, FEBRUARY 2021—415

up to surface quality of 1 lm, and finished with oxidepolishing suspension. Due to the clear contrast betweenthe nitride layer and the substrate, the samples wereinvestigated in a non-etched condition. The microstruc-ture and the thickness of the nitride layers werecharacterized by light optical microscopy on a Neophot32 (LOM) equipped with an OLYMPUS UC30-cameraand the OLYMPUS Stream Motion analysis software,and by scanning electron microscopy (SEM) at aMIRA3 TESCAN.

Glow discharge optical emission spectroscopy(GDOES) was performed on a Leco SDP-750 to analysethe element-concentration depth profiles of the nitridedsamples.

X-ray diffraction (XRD) analysis was performed witha Seifert FPM URD 6 diffractometer operating inBragg–Brentano configuration. XRD patterns ofnitrided samples were determined with Cu-Ka radiation(k1 = 1.54056 Å, k2 = 1.54439 Å) in order to limit thetotal penetration depth (approx. 4 lm for Cu-radiation).Patterns were recorded from 2h = 20 to 150 deg at ascan rate of approximate 0.01�/min with a step size of0.02 deg. The phases present in the surface compoundlayers were determined by a Search-Match routine(Panalytical HighScore+) involving the ICDDPDF-4+ database. The quantitative phase fractionswere then determined by the Rietveld refinementmethod, as implemented in the Maud software.[14]

Electron backscatter diffraction (EBSD) was con-ducted by using a scanning electron microscope (SEM)of JEOL JSM-7800f equipped with an EBSD analysissystem of EDAX Hikari Super. An acceleration voltageof 20 kV and a step-width of 50 nm were employed forEBSD measurements. EBSD data were analyzed usingthe free Matlab toolbox MTEX.[15] The data werecleaned by removing data points with a confidence index

et al. also have referred to Reference 11, and finally thesubstrate. In Figure 4, the effect of the nitriding poten-tial on the white layer’s structure is displayed at highermagnification. Figure 5 shows the thickness of the whitelayer with values in the range of 0.5 to 2 lm as afunction of the nitriding potential KN. It reveals that thecharacteristics of the white layer are different from eachother with the change of KN value. The influence of thenitriding potential on the behavior of the diffusion layerwill be addressed in another paper.

The images of the structure of the white layer shownin Figure 4 are entirely consistent with the results of theroughness and topography of the samples’ surfaces,respectively, given in Figure 2. As seen in Figure 4(a), inthe case of KN = 0.1 bar

�1/2, the white layer was thinand discontinuous. The SEM image for KN = 0.1 bar�1/2 (Figure 2(a)) proves that the white layer consistsonly of scattered discrete particles that do not cover theentire sample surface. With increasing KN, these parti-cles were more precipitated and thicker (Figures 2(b)and 4(b)). When increasing the nitriding potential up to1.0 bar�1/2, these particles covered the whole surface toform the uniformly developed white layer, whichreduced the roughness (Figures 2(c) and 4(c)). Withincreasing thickness of the white layer, more and morecolumn-like particles were embedded with further increas-ing KN values, corresponding to KN = 1.5 bar

�1/2

(Figures 2(d) and 4(d)), so the surface roughness Ravalue again increased.

Figure 5 shows that the thickness of the white layer issteadily increasing with increasing values of KN. How-ever, with KN < 1.0 bar

�1/2, the white layer is notcontinuous in the range of 0.5 to 1.5 lmas easily visible bythe LOMmethod. As shown in Figures 2 and 4; it can beseen that the white layer thickness increased slowly, whileits clusters of nitride phases developed spreadly to coverthe surface that increased the roughness. For higher KNvalues, particularly KN above 0.75 to 1.0 bar

�1/2, thewhite layer covered the entire sample and developedmoreevenly, compared to KN below 0.75 to 1.0 bar

�1/2, so thesurface roughness tends to decrease (Figure 1).

Fig. 2—SEM images captured from the surface of treated samples processed under different nitriding potential values: (a) KN = 0.1 bar�1/2, (b)

KN = 0.5 bar�1/2, (c) KN = 1.0 bar

�1/2, and (d) KN = 1.5 bar�1/2.

Fig. 3—Typical structure of the nitride layer after nitriding at590 �C for 5 h with KN = 1.5 bar�1/2, SEM.


As discussed in the previous section, in case the valueof KN exceeds 1 bar

�1/2, the external nitriding processtakes place more intensely. As a result the thickness ofthe white layer increases, and it becomes more and moreporous with the development of iron nitride phases andother compounds, which are the cause of increasingroughness with high KN, cf. Figures 4 and 5 at KN =1.5 bar�1/2.

The formation and growth of the white layer reflectedsignificantly the influence of the nitriding potential atconstant nitriding temperature and time.

C. Elemental Depth Distribution

Figure 6 displays a GDOES analysis of a sampletreated with KN = 1 bar

�1/2 that demonstrates a typicalconcentration—depth distribution of the elements nitro-gen, iron, aluminum, and oxygen. The nitrogen concen-tration exhibits a peak at a depth of 0.5 to 1 lm beneaththe surface, and afterward, the content decreasedgradually. This area of maximum nitrogen concentra-tion corresponds to the white layer thickness, which hadan average thickness of about 1.5 lm in the exampleshown. At the position A near the surface, the ratios ofN/Al and N/Fe were 1.7 and 1.19, respectively. Up toposition B, the ratios changed continuously to N/Al ~1.4 and N/Fe ~ 1.35, respectively. This change inconcentration ratios gives evidence that the white layercontains nitride phases of iron and aluminum. Theproportion of iron nitride phases in the white layer nearthe surface is higher than that of the aluminum nitridephase. At deeper positions in the white layer, the N/Feratio decreases vice versa of N/Al. Besides, a smalloxygen content was detected in the near-surface regionsindicating that the outer surface has a porous structure.

D. Phase Composition and Structure of the White Layer

XRD patterns of the nitrided samples and theuntreated base material are shown in Figure 7. Thetypical peaks of the B2-FeAl phase (PDF # 00-033-0020)are identified as denoted by number ‘‘1’’ in the XRDpatterns. As seen in Figure 7, the B2-FeAl phase is theonly phase detected for the untreated sample condition.

Fig. 4—SEM cross-section images of the samples nitrided at 590 �C for 5 h with different KN values: (a) KN = 0.1 bar�1/2, (b) KN = 0.75bar�1/2, (c) KN = 1.0 bar

�1/2, and (d) KN = 1.5 bar�1/2.

Fig. 5—Effect of nitriding potential on the thickness of the whitelayer at 590 �C for 5 h.


For nitrided samples with increasing nitriding potential,the characteristic peaks of the FeAl phase in the XRDpatterns decreased and disappeared at KN ‡ 1 bar�1/2.These results indicate that when the white layer formedand covered the entire surface no information from thesubstrate can be received, as shown in Figure 7(b).

The characteristic peaks of B2-FeAl are similar to bcca-Fe (PDF # 00-006-0696, denoted as ‘‘2’’), except forthe superstructure reflections and small differences in thelattice parameters. Peaks of both phases overlap in theXRD patterns, which makes it difficult to distinguishbetween them. However, the lattice parameter of FeAl(a= 0.291 nm) is a little larger than that of bcc ferrite (a= 0.286 nm) as Al possesses a larger atoms size (0.143nm) than Fe (0.128 nm). Therefore, the main peaks ofbcc ferrite are shifted to the right of the B2 phase of theuntreated sample.

Besides, the iron nitride phase of c¢-Fe4N (PDF #00-064-0134, denoted as ‘‘3’’) is detected in allXRD-patterns of the nitrided samples. Additionally,another iron nitride phase e-Fe2-3N (PDF #00-049-1663, denoted as ‘‘4’’) is identified at KN ‡ 1bar�1/2. These results prove that a certain amount ofiron on the surface combines with nitrogen to form ironnitride phases creating the porous white layer.

Finally, an aluminum nitride phase of AlN (PDF #00-025-1133, hexagonal structure, denoted as ‘‘5’’) isalso marked in XRD patterns of the samples treated, asshown in Figure 7.

These data imply that the structure of the white layerincludes mainly iron nitride phases. The aluminumnitride phase is observed only at KN < 1 bar

�1/2, asshown in Figure 7(b).

Table I presents the results of the identification ofphases and their volume fractions within the white layer,indicating the effect of the nitriding potential on thestructure and composition of the white layer. It must bementioned, that the XRD quantitative measurementswere conducted by means of Cu-Ka radiation, whichpenetrated the surface up to 3 lm, and thus surpassed

the white layer’s thickness. Therefore, the resultsobtained include the composition of the white layerand its adjacent subsurface region.In the case of the nitriding treatment applying KN =

0.1 bar�1/2, the layer was composed mainly of the phasesAlN, a-Fe, and FeAl. Surprisingly in view of the smallvalue of KN, a very small amount of c¢-Fe4N was alsodetected with a value of 2 vol pct. However, theinvestigations carried out in this study could notconclusively clarify the formation of this phase at sucha low nitriding potential.With further increasing nitriding potential KN> 0.1

bar�1/2, the phase composition of the white layerchanged. The data in Table I show that the volumefraction of c¢-Fe4N phase increased with KN up to 0.5bar�1/2. Subsequently, with a further increase of thenitriding potential to values KN ‡ 0.75 bar�1/2, thee-Fe2-3N phase formed and reached a value of 75 vol pctat KN = 1.5 bar

�1/2. The fraction of the remainingphases (AlN, FeAl, and/or a-Fe) decreased with increas-ing nitriding potential. This implies that with increasingKN, iron atoms are available at the surface, e.g., due toan outward diffusion, for forming nitrides, yielding theexternal nitriding mechanism.In gas-nitriding technology, the presence of the white

layer is always associated with porosity as c¢-Fe4N ande-Fe2-3N develop at increasing KN values. The porousstructure is an undesired consequence of nitrided sam-ples since the presence of this type of defect will increasethe roughness and degrade the surface properties inmost cases.The results of qualitative (Figure 7) and quantitative

(Table I) XRD analysis systematically described thedevelopment of the phase composition of the white layeror at least the surface area within 3 lm, respectively, as adirect effect of the nitriding potential applied duringcontrolled gas nitriding of the FeAl alloy. The resultsalso demonstrated that the white layer formation wasdue to the development of iron nitride phases by anexternal nitriding mechanism. However, the presentedXRD results were influenced by the white layer’sadjacent region and thus do not present the specificphase distribution within the nitride layer. Therefore,complementary EBSD measurements were conducted toobtain additional information on the crystal structureand the phase distribution in the selected area of thesample. Based on the XRD results presented in Figure 7and Table I, two samples nitrided with the conditions ofKN = 0.5 and 1 bar

�1/2 were analyzed by the EBSDmethod. They represented the low and high nitridingpotential corresponding to two primary phases (c¢ and e)of the white layer.Figure 8 presents the results of EBSD measurements

that were conducted on samples nitrided at 590 �C for5 hour with values of KN = 0.5 bar

�1/2 and KN = 1bar�1/2, respectively. When analyzing the EBSD data,only the iron nitride phases were indexed to evaluate theeffect of KN on the structure of the white layer. Alreadypublished studies indicated that there was hardly Al inthe white layer, meanwhile, the nano-sized AlN lamellaeprecipitated in the diffusion zone.[10] Since these

Fig. 6—Typical GDOES depth profile of chemical composition of awhite layer formed during nitriding with KN = 1.0 bar

�1/2 at 590 �Cfor 5 h.


nano-sized particles are beyond the resolution limit ofconventional EBSD only the signal of the surroundingregions such as a bcc matrix was detected.

At the low value of KN = 0.5 bar�1/2 (Figure 8(a)),

the white layer, which was very thin and not dense, wasformed on the surface. EBSD analysis showed that itscomposition was mainly c¢-Fe4N phase formed abovethe a-Fe substrate (in which nano-sized AlN lamellaewere embedded, based on the literature[10]). The wave-like features, indexed with green color in the EBSDphase map in Figure 8(a), were c¢-Fe4N close to thesurface and became a-Fe with increasing distance fromthe surface. The existence of these structures was alsoconsistent with a formation mechanism proposed by

Zhang et al.[11] This result not only proved that the whitelayer structure was c¢-Fe4N, but also clarified the reasonfor the presence of a-Fe and AlN phases in the XRDresults of KN = 0.5 bar

�1/2.Meanwhile, with the increasing value of KN = 1.0

bar�1/2, the white layer became thicker and denser incomparison to the previous case, and largely composedof a compact e-Fe2-3N layer (Figure 8(b)). Specifically, itseemed that there was a local double layer of e/c¢, inwhich a compact layer of e-Fe2-3N formed on a sublayerof c¢-Fe4N. Nevertheless, the EBSD results cannotclearly prove whether this ‘‘c¢ sublayer’’ should beconsidered as a ‘‘true’’ sublayer of the white-layer forseveral reasons: At positions, where the presumably ‘‘c¢sublayer’’ appears thicker, the layer was blown up bypores (marked by grey arrows in Figure 8(b)). Theorigin of the pores was considered to relate to the‘precipitation’ of molecular nitrogen (N2) in the nitridelayer, which had a high dissolved N content under thecondition of high nitriding potential. Furthermore, itseems that wave-like features have occasionally formedadjacent to the surface (marked with white arrows inFigure 8(b)), and were subsequently overgrown bye-Fe2-3N surface layers. These wave-like features ofc¢-Fe4N adjacent to the surface were not detected in caseof low nitriding potential KN = 0.5 bar

�1/2. Theiroccurence can be explained by the fact that withincreasing nitriding potential the amount of N thatpenetrates the nitride layer also increases. Especially in

Fig. 7—XRD patterns (plotted on a square root scale) for untreated and treated samples: (a) low KN £ 0.5 bar�1/2; (b) high KN> 0.5 bar�1/2,Cu-Ka radiation.

Table I. The Phase Composition of the White Layers

KN (bar�1/2)

Phase Composition (Vol Pct)a

FeAl ± Fea (bcc)b Fe4N Fe2-3N AlN

Untreated 100 — — —0.10 39 ± 3 2 ± 1 — 59 ± 50.25 14 ± 2 45 ± 4 — 41 ± 40.50 14 ± 2 50 ± 4 — 36 ± 30.75 12 ± 2 45 ± 4 12 ± 2 31 ± 31.00 — 21 ± 2 63 ± 5 16 ± 21.50 — 8 ± 1 75 ± 6 17 ± 2

aCu-Ka, information depth 0 to 3 lm.bXRD pattern of FeAl overlaps with that of a-Fe.


the adjacent region to the surface and the white layerwith a very high dissolved nitrogen content inside,precipitation of iron nitride phases occurred stronglyand resulted in an increase in residual stress by phasetransformations.[17] This leads to an assumption thatbecause of the effect of the very high residual stress, theiron atoms were displaced and rearranged in a wave-likeform in the adjacent region to the surface and combinewith dissolved N atoms to precipitate c¢-Fe4N. There-fore, it is assumed that they belong to the diffusion layerand are not really related to the white layer. This findingrequires further investigations and will be investigated infuture research which deals in particular with theprocesses within the diffusion layer of nitrided B2-FeAlalloy.

Generally visible in Figure 8(b), the wave-like featureswere also observed in the diffusion layer near the surfacesimilar to the ones already observed in case of KN = 0.5bar�1/2. The e-Fe2-3N was also found, while the a-Fe wasnot present in the region. Therefore, with the structureas described, the obtained XRD analysis results of thewhite layer showed mainly the iron nitride phases ofc¢-Fe4N and e-Fe2-3N with an amount of 16 vol pct AlN(KN = 1 bar

�1/2) caused by the influence of itsneighborhood layer.

The EBSD results have shown the effect of thenitriding potential on the phase composition of thewhite layer with and the formation of iron nitride phasescorresponding to the nitriding conditions.

E. Mechanism of the Development of White Layerson B2-FeAl

The aim of the modern controlled nitriding process isto measure and control the nitriding potential in thenitriding atmosphere to obtain the desired nitrogenconcentration and phase composition in the nitridelayers on iron-based alloys. The experimental Lehrerdiagram for pure iron[18] is widely used by researchersand engineers to determine the reference nitridingpotential for the formation of nitride layers, specifically

for the prediction of the phase composition of com-pound layers. In this study, the Fe-N Lehrer diagram iscalculated by using Commercialized Thermo-Calc soft-ware and shown in Figure 9.According to the phase analysis results presented in

Table I, at KN = 0.1 bar�1/2 the ‘‘white layer’’ consisted

of about 2 vol pct of c¢-Fe4N phase. Comparing thisvalue with the boundary nitriding potential of a/c¢ forpure iron in the Lehrer diagram shows that the value ofKN = 0.1 bar

�1/2 detected in the present study is lessthan calculated for pure iron (Figure 9). Likewise, theformation of the e-Fe2-3N phase at higher KN ‡ 0.75bar�1/2 is also small than the corresponding boundarynitriding potential of c¢/e indicated in the Lehrerdiagram.During the nitriding process, the absorbed nitrogen

atoms interact with the constituents of the substrate toform nitride phases of aluminum and iron. The sequenceof the formation of phases is usually determined by therelative thermodynamic stability of the relevant nitridephases. To illustrate the nitride phases relative stability,a metastable isothermal section of the Fe-Al-N systemhas been calculated using the Thermocalc softwarebased on the TCFe7 database (see Figure 10). Thereby,the B2-FeAl phase is contained as an extended solidsolution of a-(Fe, Al) with disordered A2/bcc structure.A temperature of 585 �C was chosen in order to avoidcomplications by the eutectoid reaction in the Fe-Nsystem, which was somewhat below 590 �C, accordingto the employed database.The blue dashed line in Figure 10 illustrates the phase

evolution upon adding N to a FeAl40 alloy. As soon asN is added to the alloy, the AlN phase is expected toprecipitate, since the compositional point is locatedwithin the AlN + a two-phase field, thereby depletingthe a-(Fe, Al) phase by Al. Upon having 28 at. pct N inthe alloy, the a phase has lost virtually all Al at the costof AlN formation, and only upon a further increase ofthe N content first the c¢ and upon surpassing 36 at. pct eis formed (Figure 10). The Thermocalc results predictcorrectly that AlN recipitates first when adding N to

Fig. 8—EBSD measurements of the samples nitrided at 590 �C for 5 h with: (a) KN = 0.5 bar�1/2; and (b) KN = 1.0 bar�1/2 (grey arrows inindicate porous zones beneath e-Fe2-3N layer, white arrows indicate locally wave-like shaped c¢-Fe4N sublayer regions).


B2-FeAl alloy, which leads to the formation of AlN andpure iron (a-Fe). Furthermore, it needs a higherchemical potential of N (or high nitrogen content) toform iron nitride phases.

In the nitriding process for B2-FeAl iron aluminidealloy, different types of nitride layers develop dependingon the nitriding potential. The nitride layers can form inone of the following cases:

1. A nitride layer with a diffusion zone consisting of thetwo phases AlN and a-Fe forms. From empiricalresults, as discussed in the previous sections, the au-thors predict that this layer can be achieved undernitriding conditions with KN< 0.1 bar

�1/2.2. A nitride layer which consists of phases AlN, c¢-

Fe4N, e-Fe2-3N, and a-Fe forms at nitriding condi-tions with KN ‡ 0.1 bar�1/2. The nitride layer mayinclude the development of an outer compound layer

of iron nitride phases, called a ‘‘white layer’’, and aninner diffusive layer corresponding with the ‘‘externaland internal’’ nitriding processes

Based on schematic illustrations of the wave-line form-ing mechanism of Zhang,[11] the authors propose amodel demonstrating the formation of the white layeron FeAl40, as shown in Figure 11, which includes thefollowing steps:Step 1: Nitrogen atoms diffuse from the nitriding

atmosphere onto the surface of FeAl samples. Theyinteract with the atoms below the surface and diffuseinto the substrate along the grain boundaries(Figure 11(a)).Step 2: Due to the chemical affinity of nitrogen with

metal atoms, Al atoms will separate firstly from theFeAl matrix and form AlN. More Al diffuses to thegrain boundaries and combines with nitrogen to formAlN. The formation of AlN needles consumes alu-minum from the ordered lattice of B2-FeAl, andconsequently, a-Fe results between the AlN needles.Hence a lamellar microstructure of AlN/a-Fe is formed(Figures 11(b) and (c)).[11]

Because the limit solubility of N in a-Fe is very small,nitrogen atoms will combine with iron atoms to formiron nitride phases depending on KN. Based on theLehrer diagram for pure iron,[18] the authors suggest theexistence of the phases c¢-Fe4N and/or e-Fe2-3N, respec-tively, at each investigated KN value (Figures 11(d)through (f)). The formation of pure iron nitride on topof the surface is an indication for an external nitridingprocess that occurs involving outward diffusion of Fe.Furthermore, the diffusion of nitrogen atoms along

the grain boundaries into deeper regions of the substratecorresponds to an internal nitriding process, which leadsto the formation of AlN and iron nitride phases, thuscontributing to the formation of the ‘‘diffusion zone’’ ofthe nitride layer.

Fig. 9—Lehrer diagram is calculated by the Thermo-Calc softwarewith TCFe7 database.

Fig. 10—The isothermal section of Fe-Al-N ternary phase diagrams at 585 �C (A temperature of 585 �C was chosen in order to avoidcomplications by the eutectoid reaction in the Fe-N system, which was somewhat below 590 �C, according to the employed database) calculatedbased on the TCFe7 excluding the gas phase N2 and hence featuring also the metastable iron nitride phases e and c¢. Note that the indicatedphase equilibria with N contents higher than corresponding to the e + AlN two-phase region neither conform with experimental reality, neitherare they relevant for the current work.


The results of GDOES (Figure 6), XRD (Figure 7),and EBSD (Figure 8) analyses confirm the predictedmechanism of the formation of the white layer onFeAl40 material. At low nitriding potential KN below0.1 bar�1/2, only the Al atoms interact with N atoms toprecipitate AlN, and at the same time to form the a-Fephase (Figure 11(c)). In this case, the obtained nitridelayers on similar alloys have the previously reportedlamellar structure.[9–11] Due to the low nitriding poten-tial, the white layer would not form on the FeAl40surface. With further increase in KN, the white layerincludes compound phases of c¢-Fe4N (KN ‡ 0.1 bar�1/2)and/or e-Fe2-3N (KN ‡ 0.75 bar�1/2) (Figures 11(d)through (f)) that would gradually cover the samplesurface. The analysis results show that the proportion ofiron nitride phases increases at the surface while thefractions of AlN and a-Fe decrease. After the whitelayer reaches a sufficient thickness, the main peaks of theAlN and a-Fe phases were not detectable in the XRDpatterns anymore.

(a) The formation of active atoms of N from thedecomposition reaction of ammonia in a nitridingatmosphere and the atoms absorb deep into thesample surface along grain boundaries,

(b) Diffusion of Al atoms from the inside of grains tothe grain boundaries,

(c) Formation of AlN and release of a-Fe,(d) Formation of c¢-Fe4N precipitates,(e) Formation of e-Fe2-3N phase on c¢-Fe4N phase

precipitates with increasing KN value, (locally, thereseems to be e/c¢ double layer)

(f) Formation of a ‘‘white’’ compound layer. Corre-sponding to the nitriding conditions, the layer’scomposition is either the c¢ phase at an appropri-ate low value of KN; or e phase at high values ofKN. Occasionally a double layer of e/c¢ is formedlocally.

IV. CONCLUSIONS

The formation and growth of the white layer onB2-FeAl alloy were studied for different nitridingpotentials of the ammonia atmosphere affecting thecomposition and thickness of the case at fixed nitridingconditions of 590 �C for 5 hour.Detailed studies using SEM, LOM, XRD, and EBSD

indicate that on all nitrided specimens, a thin white layerof varying thickness is formed. If a low nitridingpotential is used, the white layer is formed inhomoge-neously and comprises of clusters of nitride phases,which are mainly Fe4N phases. The appearance ofnitride phase clusters increases the surface roughness.When a high nitriding potential is applied, the nitride

phase clusters will cover the whole sample surface toform the white layer. The white layer composes mainlyof iron nitride phases of the type Fe4N and Fe2-3N. Itsthickness increase with further increasing nitridingpotential KN, causing a change in surface roughness.The optimal characteristics in terms of surface rough-ness of the nitrided surface correspond to nitridingpotentials KN in the range of 0.75 to 1.0 bar

�1/2.A mechanism for the formation of the white layer has

been proposed, explaining the nitriding behavior ofFeAl40 alloy occurring during the gas nitriding processas a function of different KN values.As known in the state-of-the-art nitriding process, the

structure and composition of the nitride layer can becontrolled by adjusting the nitriding potential of theatmosphere. Comparable basic knowledge is applied toFeAl materials, showing that the nitriding potentialparameter has a significant effect on the structure andcomposition of the resulting white layer. In future work,the effect of a compound layer forming as a function ofthe nitriding potential KN on the growth kinetics andthe properties of the diffusion zone will be examinedmore closely.

Fig. 11—Schematic illustrations of a mechanism of the effect of the nitriding potential KN on the ‘‘white layer’’ formation: steps a, b, c arereferred from the wave-line forming mechanism of Zhang[11] (Reprinted from [11] under the terms of the Creative Commons CC BY license (http://creativecommons.org/licenses/by/3.0/).); and steps d, e, f are expanded by authors to explain white layer formation.


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ACKNOWLEDGMENTS

The authors thank Prof. A. Leineweber (TUBergakademie Freiberg, Institute of Materials Science)for the reviews of the thermodynamic section, Mr. StefanKante (TU Bergakademie Freiberg, Institute ofMaterials Science) for EBSD analysis and Mr. E.Siegismund (TU Bergakademie Freiberg, Institute ofMaterials Engineering) for nitriding experiments.

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Effect of Nitriding Potential KN on the Formation and Growth of a ‘‘White Layer’’ on Iron Aluminide AlloyAbstractIntroductionExperimental DetailsResults and DiscussionSurface Roughness and TopographyStructure and Thickness of the White LayerElemental Depth DistributionPhase Composition and Structure of the White LayerMechanism of the Development of White Layers on B2-FeAl

ConclusionsAcknowledgments

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