Mechanosynthesis of Fe1-YAuYNX nitride (X = 0.25): Influence of Au on process, microstructure and aging

Nano!%~ctured Materials. Vol. 8, No. 5. pp. 561-518. 1991 Elsevier Science Ltd 0 1997 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773197 $17.00 + .OO

Pergamon

PII SO9659773(97)00199-2

MECHANOSYNTHESIS OF Fel-yAuyNx NITRIDE (X=0.25): INFLUENCE OF Au ON PROCESS, MICROSTRUCTURE AND AGING R.S. de Figueiredo’** and J. Fact’

‘Laboratoire de Metallurgic Physique, URA CNRS 234, Bat C6 2Ctage USTL 59655 Villeneuve d’ Ascq Cedex France *Grupo de Magnetism0 e de Materiais Magneticos, Dep. de Fisica UFC 60.45 l-970 Fortaleza CE Brazil (Accepted June 24,1997) Abstract-Grinding ofFezN, Fe andAupowder mixture gives rise to bee-like single phase Fel_yAuyNo2ssolidsolution, O_qS.25. Agingofthisproductleads toa y-typenitride. Compounds are studied by Miissbauer spectroscopy, X-ray diffraction, scanning electron microscopy and thermal analysis. The role of Au atoms on the micromechanisms taking place during mechanosynthesis, magnetic properties, and nitrogen distribution and redistribution is discussed. 0 1997Acta Metallurgica Inc.

INTRODUCTION

Many studies have been devoted to the synthesis of iron nitrides and to their properties. Synthesis has been performed either by surface treatment (chemical vapor deposition, plasma and ion nitriding, molecular beam epitaxy...) or by mechanical alloying M.A. (1). The reasons for this interest mainly arose from the attractive magnetic properties of FeN, iron nitrides and solid solutions, especially y ‘-Fefi and a”-Fel& (2). Because of its good mechanical properties, high 4xM, saturation magnetization and low coercivity a”-Fe&;! appeared to be a good candidate for making a new generation of magnetic recording heads (2). It is often observed that the increase of Fe-Fe distance increases the hype&e magnetic field; therefore, alloying nitrides with gold of which the atomic radius is about 20% larger than that of iron should noticeably modify the magnetic properties. In addition, the absence of reactivity of Au with N avoids M.A. process to be reactive and therefore reduces the risks of demixing of the product. A former interpretation (3) of the results obtained on binary Fe-N alloys has shown that phases produced by M.A. are greatly dependent on the stress-strain field produced by the impacts applied on the powders during the process. For FeNo.125, which corresponds to the composition of ct”-Fel$rTz, neither a’ martensite nor 01” nitride have been observed, but a new phase a”‘, which appeared to be cubic rather than tetragonal and for which the interstitial distribution is equally shared on the three families of octahedral sites (3). 567

566

RS DE FIGUEIREDOAND J FCCT

The aim of this study is therefore (i) to obtain new nitrogen compounds, (ii) to improve magnetic properties of already existing phases, (iii) to analyze the influence of Au on the nitrogen distribution and therefore to get a better understanding of the M.A. process, and (iv) to understand the role of alloying element on thermal aging of the M.A. product. EXPERIMENTAL

METHODS

In order to distinguish the role of the interstitial element from that of the substitutional one, the following notation is used: Fel-yAuyNx. Whereas Y is the atomic concentration of the binary substitutional solid solution and X refers to the interstitial site occupation rate of a lattice of which the amount of metallic sites and interstitial sites are equal, such as octahedral sites in fee or hcp lattices. The M.A. synthesis performed in this study is based on former results showing that in the present conditions the kinetics of solid state nihiding reaction occurring by “forced” diffusion is much faster than loss or uptake solid-gas reactions (3). This can be summarized as follows: aFeNx + PFe + yMe a

Fe,+gMeyNa

where FeN, is the nitrogen source, here [-Fem, and Me the metallic alloying element, here Au. Commercial iron powder (99.9% purity and 50 pm particle size) was nitrided underNH3 gas at 720 K in order to obtain stoichiometric C-FezN nitride. Fefi nitride was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission Mossbauer speclroscopy (TMS), which revealed it to be single phased, c ordered and composed of spherical 15 pm particles. c-Fe* nitride was mixed with commercial iron and powders (99.95% 50 pm) milled in SPEX 8000 under inert during 64 in order obtainFe1 -yAu~Nx (X = and 0 Y I0.25). steel vials balls have usedanda ball powder ratio kept near (2 g metallic powder 2 balls -10 g In order minimize contamination the alloy steel of device, each has been by a experiment under conditions. The was studied TMS using 25 mCi of 57Co a Rh all isomer (IS) have reported to and by in a geometry with Co anticathode a 0.02 as 28 The differential calorimeter (DSC) been employed a heat of 40 and using mg of RESULTS Figure presents the patterns of powders during h with gold concentrations: (A); Feo.9sAuo.osNo.u Feo.875Auo.t~No.~ (C)andFeo.75Auo.2sNo.u It has demonstrated that of y leads to congruent transformation ’ + E (fee + hcp) (4). As can be seen in Figure 1A, the crystallographic structure obtained for Y = 0 is unambiguously indexed as a hcp structure. In contrast with binary FeNo.25, Y = 0, when gold is present, 0.05 5 Y I 0.25, the X-ray diffraction pattern revealed a clear crystallographic change (Figure 1B to 1D). Despite the poor definition of the diffraction peaks, as shown below these diffractograms are consistent with a nanocrystalline bee-like structure.

MECHANOSYNTHESIS OF

Fe,_yA~yN, NITRIDE

569

0.80

bcx-like

llOa

F

W 211 a

0.00 0.80 1

0.00 0.80

.z? i! s ,r

0.40

0.00 101 E

hw

4

A

3.00

2.50

2.00

1.50

1.00

d (A) Figure 1. XRD patterns of 64 h milled FeNa. (4); Feo.95Auo.osNo.z (B); Feo.sxAuo.n~No.zs (C) and Feo.~sA~o.zsNo.zs(D) show@ a hcp a-mm (A) and a not well defined bee-like structure (B to D).

RS DE FIGUEIREDOANDJ FOCT

570

0.98

0.98 1.oo 0.98

0.96

0.98 0.96

0.98

0.96

0.94

I

I -10

I

I -5

I

I

0

I 5

I

I 10

Velocity (mm/s) Figure 2. Mijssbauer spectra at room temperature of FeNo.25 (A); Feo.9sAuo.osNo.z 03); Feo.mho.mNcm 0 and Feo.xAuo.zNo.25 03 after 64 h milling.

MECHANOSYNTHESIS OF Fe,_,AuyNx NITRIDE

571

Figure 2 presents the TMS results of as-milled powders: FeNo.25(A); Feo.9sAuo.osNo.u (B); Feo.875Auo.lzsNo.25 (C) and Feo.7sAuo.2sNo.2s(D). Mijssbauer data are summarized in Table 1. TMS results clearly confii the importance of the changes resulting from gold addition: a significant abundance reduction of 21 T environment (attributed to Fe with two N nearest neighbors, II) and an increasing abundance of 33 T and 29 T environments (attributed to Fe with zero and one N nearest neighbor respectively, 0 and I). In order to study transformations occurring during thermal treatments of Feo.s75Auo.lzNo.25 and Feo.75Auo.25No.25,DSC measurements where performed. DSC curve of Feo.75Auo.25No.25 (solid line), Figure 3 showed two exothermic peaks at 666 K (“1”) and 747 K (“2”). The two exothermic peaks suggest to perform aging treatments at 580 K and 820 K. Figure 4 shows the XFW pattern of Feo.75Auo.25No.25nitride after 5 min at 580 K (A) and after 5 min at 820 K (B). As proven by XRD, Fel-yAuyNo.25 transforms into a y ‘-Fel-YAuYNo.25Perovskite-like nitride. M&sbauer spectra of aged Feo.75Auo.25No.25nitride after 5 min aging at 580 K (A) and 5 min at 820 K (B) are presented in Figure 5. TABLE 1

Data Corresponding to Figure 2 IS isomer shift is quoted relative to a-Fe, QS quadrupole splicing. Hi hyperfine magnetic field and S relative abundance. * Refers to splitting related with different Au atom neighborhoods. Fig.

Site

IS (mm/s)

2.A

0 I II III

0.070 0.185 0.267 0.58

2.B

0 I I” II II* III

2.c

2.D

QS (mm/s>

Hi (T)

S (%)

-0.03 -0.02 -0.02 -0.59

33.04 27.98 21.09 12.28

19 35 42 4

0.025 0.132 0.120 0.206 0.237 0.157

-0.009 -0.024 -0.008 0.058 0.013 0.061

34.07 31.29 28.60 25.13 21.19 16.17

26.4 15.7 25.5 10.5 14.6 7.3

0 0* I 1* II IIl

0.046 0.030 0.127 0.116 0.153 0.058

-0.082 0.010 -0.064 -0.005 -0.040 -0.225

34.67 33.05 30.34 27.57 23.98 18.97

21.8 16.0 22.7 18.1 10.9 10.5

0 I II III

0.078 0.124 0.179 0.072

-0.048 -0.040 0.008 -0.262

34.10 30.08 26.59 19.63

33.6 34.2 23.8 10.4

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DISCUSSION In the as-milled conditions, mechanosynthesis of binary iron nitrides gave rise to two different phases depending on N concentration. For X less than 0.20, the ground powder is essentially bee (a”‘FeNx) (5), for X greater than 0.25 (FeN, 0.25 < X < 0.5) the ground powder is hcp (&-FeN& (4), as confirmed here by milled FeNe.25 for which lattice parameters measurement led to a = 0.267 nm and c = 0.438 nm (Figure 1A). As shown by XRD and TMS (Figures 1 and 2), gold alloying hinders the hcp structure. XRD results of Fe-Au-N nitrides presented in Figure 1 are quite similar to those presented by iron nitrides with N concentration smaller than 20%, and consistent with abcc structure of which two diffraction peaks 110 and 211 are identified (Figure 1B to lD). Meanwhile, it is worth pointing out that the lattice parameter aa”‘obtained for a”-Feo.75Auo.25No.25 is much less (-15%) than that calculated from a linear additions of the contributions (6) corresponding to Au and N alloying in a bee iron lattice. The bee indexation of o” leads to a lattice parameter a = 0.295,0.296 and 0.299 nm for a”‘-Fel_yAuyNu.25 with Y = 0.05,0.125 and 0.25 respectively, which corresponds to a volume change from 13.52 10m3(c-FeNu.25) to 12.84 - 13.37 10e3nm3 per metal atom (Y = 0.05...0.25) in Cc”‘,suggesting that N atom induces a smaller lattice distortion in bee-Fe-Au-N phase than in hcp-Fe-N phase and that point defects may result from M.A. XRD reveals that Au atoms preferentially occupy (O,O,O)position in Perovskite lattice, as remarked by the high intensity of peaks with h, k, and 1having different parity (e.g. 110,210). The lattice parameter of y ‘-Fet-yAuyNu.25 varies from a = 0.3784 nm (Y = 0) to 0.3804 (Y = 0.125) and 0.3834 nm (Y = 0.25), which corresponds to a volume per metal atom from 13.55 10” nm3 (Y = 0) to 14.09 10” nm3 (Y = 0.25). It is important to observe that during E + y ’ transformation (4) the volume change of binary alloy is very small, nevertheless, during cl”’ + y ‘transformation these change is very important and corresponds to an anomalous reduced volume of as-milled Fe-Au-N compound. As pointed out below, this small volume of as-milled a”‘-Fet_yAuyNu.25 nitride is consistent with Mossbauer spectroscopy through the reduced isomer shift IS0 = 0.078 mms’ (Figure 2D) exhibited by virgin milled product compared with IS0 = 0.134 mms-l (Figure 5B) in aged y’-Fe-Au-N (pressure effect). TABLE 2 Data corresponding to Figure 2. IS isomer shift is quoted relative to o-Fe, QS quadrupole splitting. Hi hyperfiie magnetic field and S relative abundance. *Refers to splitting related with different Au atom neighborhoods. Fig.

Site

IS (mm/s)

QS (mm/s)

Hi (T)

S (%)

5A

0 I II III

0.025 0.149 0.020 0.120

-0.072 -0.227 -0.343 -0.293

34.93 30.16 24.49 19.44

45.3 24.4 4.3 25.9

5B

0 II II*

0.134 0.217 0309

-0.010 -0.458 -0.08 1

34.69 20.76 13.77

9.7 59.2 31.1

MECHAN~SYNTHESISOF Fe,_YAuyN,

0.50

Fe

Q.76*“6

673

NITRIDE

.26 N6 .26

w +

0.25

Exothermic I

0.00 -

-_FeN026hc‘ . ..+&

I

__‘_

___ - -.-.. _^

I

I

I

500

400

N loss I

600

I

I

I

I

I

900

600

700

Temperature (K) Figure 3. DSC curves of FeNo.25 (dotted) and Feo.75Au0.25No.25(solid) showing two exothermic peaks, heat rate of 40 Kmin-‘and a sample mass of 20 mg. 4.00 3.00

.*

z

B

2@3Y

2.00 1.00 0.00

0.40 1 0.30

-

Perovskite bee - llke

+ IlOa

0.20 -

A

0.10 0.00

III1 3.00

I

2.50

I

I

I

2.00

I

I

I

I

I

1.50

I

I

I

1.00

d(A) Figure 4. XRD patterns of as milled Fec1.75Auo.25No.25 aged during 5 min. at580 K (A) and aged during 5 min. at 820 K (B) showing a + y ’ transformation.

574

RS DE FIGUEIREDOAND J FCCT

Mossbauer spectra of Figure 2, Table 1, show that milled FeN0.v nitride presents four sub-spectra characterized by 33.04, 27.98, 21.09 and 12.28 T hyperfine fields. These environments are attributed to iron atoms with 0,1,2 and 3 N nearest neighbors (here noted by 0, I, II and III), since the magnetic hyperfine field of iron nearly linearly decreases with the N contents of the first interstitial coordination sphere (7). Moreover, this interpretation, based on the variation of H with nitrogen neighborhood, is consistent with the balance equation established when nitrogen occupies octahedral sites (8): X =i,ii.Si

I-O which gives X as a function of i and Si, where i is the number of nitrogen nearest neighbors and Si the corresponding abundance measured by Mossbauer spectroscopy. Attribution of iron sub-spectrum of Fet-yAuyNo.25 nitrides has been made by analogy between measured magnetic hyperfine fields and values determined in binary iron nitrides. These data are summarized in Tables 1 and 2. In the as-milled state, the average value of the hyperfine magnetic field in Fet_yAuyNu.25 increases from 25.49 T to 28.11,29.32 and 30.11 T when Y varies from 0 to 0.05,0.125 and 0.25 respectively. This result is consistent with crystallographic E+ cc”’change induced by Au, as well as with lattice distortion resulting from Au alloying. Mijssbauer spectra proved that aging of Feu.75Auo.zsNo.25led to qualitatively different changes at 580 K and 820 K, Figures 5A and 5B. In both cases, the density of environment I (H = 30 T) has diminished or even disappeared and the appearance of peaks corresponding to 0 environment (H = 34 T) has revealed the formation of a y-like structure Figure 5B. Meanwhile, complete ordering in y ‘ with Y = 0.25 which would completely replace Fe in 0 environment by Au did not occur. This partial y ‘ordering is due to the low mobility of Au atoms which led to long aging time at high temperature incompatible with the stability of the y ’ nitride. In y ‘-Fed nitride N acts as a p-electron donor, yielding d-down electrons at the II iron site (9,lO). Additions of gold to Perovskite-like nitride did not change dramatically isomer shift of II sites, suggesting that electronic interaction between iron II and N remains nearly constant as explained by electronic calculations (11). Since the isomer shift 6 is very sensible to the local environment, i.e., number of nitrogen nearest neighbors (i) and to mean distance between resonant d6 atom and its neighbors, it is expected that due to the donor role of N, d increases with i - > 0 and dt with the mean Fe-Fe distance g > 0 or $

< 0, where Pis the hydrostatic pressure. This is clearly

verified for @,&I, and 611in 01”’, Table 1, and for 60 and 61 in y ‘, Table II. Mossbauer spectra at 115 K of agedy’-Feo.75Auo.2sNo.25andy’-Feo.s7sAuo.t25No.25 nitrides present a very large magnetic hypefime field for iron atoms placed at 0 site (-40 T), which is comparable to those observed in a”-Fet&I;! nitride. They are the largest magnetic hyperfine fields observed in iron alloys or intermetallic compounds. This enhancement of magnetic hyperfine field is attributed to the lattice expansion created by gold. Electronic structure determination by “Linear Muffin Tin Orbitals” (LMTO) calculations and magnetic measurements revealed that Au substitutions do not substantially change magnetic iron moments at II sites (center of faces) (ll), as

MECHANOSYNTHESIS OF Fe,_yAuyN, NITRIDE

575

0.98 0.96

g

0.99

‘5

1

2

0.98

0.97 0.96

.I1 -10

-5

0

5

10

Velocity (mm/s) Figure 5. Mossbauer spectra of as-milled Fee.7sAu0.25No.25 aged during 5 min. at 580 K (A) and aged during 5 min. at 820 K (B).

measured by Mossbauer spectroscopy-neither did the corresponding hyperfine field change much. In case the usual correlation between the magnetic moment and H holds, the increase of Ho resulting from gold alloying opens a possibility to enhance magnetic properties of these nitrides. In fact, magnetic measurements (11) revealed that in the y ‘-like structure the magnetic moment ofFe3AuN is not very high, in contrast with the a”’ structure, where the magnetic moment equals 7.93 jig at room temperature per unit formula (FesAuN). Since the Value of magnetic moment of Au is negligible, the magnetic moment per iron atom reaches 2.64 )1~,about 20% larger than in a iron. Although the impact resulting from ball milling mainly creates compressive stress field likely to stabilize the most compact structure either fee or hcp, theexperimental results showed that M.A. of Fel-yAuyNo.25 creates a bee-like structure. As already pointed out, this structure is very similar to that observed in FeNx with X IO.20, which led to a”‘. The 01”’label was chosen in order to distinguish this phase: (i) from a ferrite for which the solubility limit is about 100 times less than in Or”‘; (ii) from a’ martensite characterized by interstitial occupation of only one type of

RS DE FIGUEIREDOANDJ FOCT

0.0

0.2

0.8

1.0

Figure 6. Experimental abundances of iron atoms having i N nearest neighbors (solid lines) and calculated densities of iron with j Au nearest neighbors (dotted lines) as a function of Au contents, showing changes in nitrogen interstitial ordering due to gold.

octahedral sites; and (iii) from CL”which corresponds to an ordering of N distribution in a’. Interstitial distribution in 01”’has been interpreted as resulting from the influence of stress strain field, which by analogy with Snoek effect favors some interstitial sites which are not automatically those of 01’or CC’. The present result confirms former interpretation of the role of stress-strain field: Au atoms of which the atomic radius is (0.144 nm) much larger than that of the Fe (0.122 nm) solvent create strain heterogeneities in the matrix which stabilize some interstitial sites at the expense of others. According to this interpretation, nitrogen neighborhoods of Fe atoms: 0, I, II and III are influenced by Au neighborhood, OAu, I*“... In order to test this possible correlation, densities of Fe atoms having “j” (0, l... 8) Au nearest neighbor have been calculated. The reasonable assumption is that Au distribution resulting from random distribution of impacts during mechanosynthesis leads to a random distribution of Au atoms and therefore to the following environment densities:

Dj=c~.(l_y)*-j.yj

(OljS8)

MECHANOSYNTHESIS OF

Fe,_.,AuyNx

NITRIDE

577

Comparison of experimental densities of Mossbauer environment having 0, 1,2 and 3 N nearest neighbors with Dj proves that the expected correlation exists, as shown by the parallel variations of I density and D1 and that the presence ofAu atoms influences the nitrogen distribution and is therefore likely to change dramatically nitrogen atom solubility in a"', Figure 6. Previous description of the role of stress-strain field either resulting from impacts or from large substitutional atoms such as gold on the occupation of interstitial sites by nitrogen explains the stabilizing role of Au on the N distribution as well as the increasing solubility of N in the bee a"'phase. BecauseAu makes interstitial sites non-equivalent, thermal diffusion of N is expected to become more difficult by Au alloying. This deduction is consistent with experimental results about thermal aging and DSC. Compared with FeNa. E +y ‘and FeNo.20 a"'+ y ‘+ a transformations (45) the Fet -yAuyNo.25 a m + y ’ transformation occurs for temperatures about 200 K higher, Figure 3. That two exothermic peaks (“1” and “2”) appeared during heating of a"' -Fel-yAuyNo.25 is interpreted as follows: (i) the low temperature peak (660 K) corresponds mainly to bee -+ fee lattice transformation, as suggested by broadening and weakening of the a"'diffraction peak (Figure 4) and the emergence of peaks corresponding to fee structure: (ii) the high temperature peak (750 K) corresponds to Au and N ordering necessary to obtain a’ Perovskite-like crystallographic structure. For temperatures lower than 580 K, Mbssbauer spectra did not show any dramatic nitrogen redistribution, meanwhile N and Au atoms ordering leading to y ’ structure is only appreciable at high temperature 820 K (Figure 5B). The question whether Nordering precedes or follows Au ordering cannot be answered easily and depends on the driving force for the disorder (yor a"') + order (y ‘)transformation being either the enthalpies of I and II environment or the stress-strain energies of occupied interstitial sites. The existence of many other y ‘-like nitrides (1,ll) suggests that the driving force is rather “chemical” than stress-strain energy. Therefore, N ordering probably precedes or at least accompanies Au ordering. CONCLUSIONS In the as-milled state, the presence of gold mixed with iron nitride in the vial of the M.A. mill completely changes the structure of the product which become a"'bee-like instead of E hcp-like when X = 0.25. Thermal aging of a”‘-Fet _yAuyNu.zs leads to a new y ‘-like nitride. Although the formation of this Perovskite-like nitride is not very surprising, according to predictions of crystallochemistry, the single phased character of the a"'precursor as well as of the f product been made possible only through the M.A. process. DSCresultsrevealed twoexothermicpeaksduring the a"'-+y’phasechangeandsuggested in accordance with X-ray and Mossbauer measurements two steps would be involved for the transformation: (i) destruction of bee-lattice and reconstruction of the fee one, and (ii) ordering of N and Au on the fee-lattice to produce the Perovskite-like y ‘-Fet-yAuyNo.25 nitride. As shown above, the role ofAu on the magnetic properties appeared very promising because of the measured increase of hyperfine field. Nevertheless, further macroscopic magnetic measurements are necessary to determine the role of Au on 47rM, and possible applications.

578

RS DE FIGUEIREDO AND J FOCT

The fact that the nitrogen concentration domain of bee-like a”’ structure is clearly extended by Au alloying from X z 0.20 up to X 3 0.25 confirms former interpretation about the crucial role of stress-strain field on the interstitial distribution resulting from M.A. process. In thepresent case, the synergetic effect of stress-strain fields around the large Au atoms with that created by the impact during the process produced original interstitial distribution. REFERENCES 1.

de Figueiredo, R.S. and Fact, J., ICHME-95 Conference Proceedings, Societa Italiana di Fisica,

2.

1996,50,509. Coey, J.M.D., O'Donnell, K., Qinian, Q., Touchais, E. and Jack, K.H., Journal Physics: Condensed

3. 4 5. 6 7. 8. 9. 10. 11.

Matter, 1994, 6, L23. Feet, J. and de Figueiredo, R.S., ISIJInternational. 1996,36(7), 962. Fact, J. and de Figueiredo, R.S., Nanostructured Materials, 1994,4(6), 685. Fact, J., de Figueiredo, R.S., Richard, 0. and Morniroli, J.P., Materials Science Forum, 1996, 225-227,409. Cheng, L., van der Per-s, N.M., Bottger, A., de Keijser, ThH. and Mittemeijer, E.J., Metallurgical Transactions A, 1990,21,2857. Fact, J., Le Caer, G., Dubois, J.M. and Faivre, R.,International Conference on Carbites, Borides and Nitrides in Steels, Kolobneg Poland, 1978. Rochegude, P. and Fact, J., Physica Status Solidi, 1985,88, 137. Kubnen, CA., de Figueiredo, R.S., Drago, V. and da Silva, E.Z., Journal of Magnetism and Magnetic Materials, 1992, 111,95. Fact, J., Journal de Physique, 1974,35, C6,487. de Figueiredo, R.S., dos Santos, A.V. and Kubnen, C.A., Journal of Magnetism and Magnetic Materials, in press, 1996.