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Materials Science and Engineering A 437 (2006) 323–327

Structural properties of FCC and HCP phases in the Fe–Mn–Sisystem: A neutron diffraction experiment

J. Martınez a, G. Aurelio b, G.J. Cuello c, S.M. Cotes a,∗, J. Desimoni a

a Departamento de Fısica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata (UNLP), IFLP-CONICET, C.C. 67, 1900 La Plata, Argentinab Instituto Balseiro, Centro Atomico Bariloche, CNEA and CONICET, Avda. Bustillo 9500, 8400 S.C. de Bariloche, Argentina

c Institut Laue-Langevin, 6 rue Jules Horowitz, B.P. 156, F-38042 Grenoble Cedex 9, France

Received 10 May 2006; received in revised form 20 July 2006; accepted 8 August 2006

bstract

A neutron diffraction study was carried out on metastable FCC and HCP phases in the Fe–Mn–Si system using small pieces of materialnside a rotating device to simulate a polycrystalline randomly oriented sample. The experiments were performed on a set of Fe–Mn–Si alloys

16 at.% ≤ CMn ≤ 32 at.% and 0 at.% ≤ CSi ≤ 12 at.%). FCC and HCP phases were retained at room temperature by a fast quench into water andsubsequent cooling down in liquid nitrogen. The effect of Mn and Si on lattice parameters, average volume per atom and phase fractions were

stablished. The cell parameters for both phases are well described by means of an extrapolation of Vegard’s law to three alloying elements.2006 Elsevier B.V. All rights reserved.

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eywords: Fe–Mn–Si system; FCC/HCP martensitic transformation; Metastab

. Introduction

Materials with the so-called shape memory effect (SME) haveeld the attention of researches in the last years due to theiriversity of technological applications. The SME phenomenons present in many materials, which exhibit a martensitic trans-ormation. Typical shape memory alloys such as NiTi, CoTi,nd Cu-based have been studied for decades and many appli-nces based on them have been developed [1,2]. Since Murakamit al. [3] in the 1980s developed polycrystalline Fe–Mn–Silloys showing a commercially applicable SME feature, muchffort has been devoted to study and improve the properties ofhese ternary shape memory alloys [4]. In the Fe–Mn–Si sys-em, like in the binary Fe–Mn system, the SME is governedy the reversible FCC (� or austenite) ↔ HCP (� or martensite)artensitic transformation. The addition of Si to the Fe–Mn

ystem improves the SME because, among other effects, the Siiminishes the Neel temperature of the FCC-phase, significantly

ncreases the yield stress, and lowers the stacking fault energyf the austenite favouring the formation of martensite. Recently,sing electron spin resonance, Gavriljuk et al. [5] have found that

∗ Corresponding author. Tel.: +54 221 4246062; fax: +54 221 4252006.E-mail address: cotes@fisica.unlp.edu.ar (S.M. Cotes).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.08.014

ses; Lattice parameters; Neutron diffraction

i increases the concentration of free electrons tending towardhort range atomic ordering. This phenomenon is considered athe origin of the increased recovery strain in Fe–Mn–Si shape

emory alloys respect to the binary system [5].In a series of previous works [6–9] the thermodynamic and

tructural properties of the FCC/HCP martensitic transforma-ion in Fe–Mn, Fe–Mn–Si and Fe–Mn–Co alloys have beentudied. Recently, this information was used [10] to analysehe energetics of the martensitic transformation in the binarye–Mn system. The predictions of the thermodynamic model fornthalpy of the FCC/HCP transformation was tested [11] againstalues extracted from calorimetric measurements of the transfor-ation in Fe–Mn alloys. Those comparisons required an inde-

endent evaluation of the relative phase fractions. In Ref. [11],discrepancy between the phase fractions obtained from two

ifferent techniques, dilatometry and Mossbauer spectroscopy,as detected. However, the predictions of the model were satis-

actorily reproduced only when Mossbauer phase fractions werepplied to evaluate the molar enthalpy of transformation.

In order to extend this analysis [10,11] to higher order alloys,dditional information on the structural data of both phases

nvolved in the transformation are needed as a key strategy inhe formulation of a theory able to deeply describe the naturef the SME in the Fe–Mn–Si alloys. With this aim, we haveystematically investigated by means of neutron diffraction, the

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24 J. Martınez et al. / Materials Science

ependence with the content of Mn (16 at.% < CMn < 32 at.%)nd Si (0 at.% < CSi < 12 at.%) of the structural parameters andhase fractions. We have then obtained a complete set ofell parameters and phase fractions in the concentration inter-al where the FCC ↔ HCP transformation is present in thee–Mn–Si system. An empirical dependence of these FCC andCP cell parameters with Mn and Si concentrations is proposedy fitting the experimental data with an extrapolation of Vegard’saw to three components [12]. HCP-phase fractions obtainedrom the present neutron diffraction experiments are comparedith the previous results obtained from Mossbauer spectrometry

n the binary Fe–Mn system [11].

. Experimental

The alloys were prepared with Fe, Mn and Si of 99.98%urity, melted in an arc furnace under Ar atmosphere. The melt-ng procedure has been described in detail elsewhere [13]. Theesulting samples weighed about 20 g each. The alloys werencapsulated in a quartz tube with Ar atmosphere, kept 48 h at273 K, and then water-quenched breaking the tube. The chem-cal composition of the alloys was determined by wavelengthispersion spectrometry.

The samples for neutron diffraction were obtained by cut-ing small pieces of about 2 mm × 2 mm × 2 mm size using a

ow-speed diamond-blade sawing machine. After cleaning theurface of the pieces, an annealing at 1173 K was performed dur-ng 1 h in a quartz tube under Ar atmosphere with a subsequentater-quenching without breaking the tube to avoid oxidation.

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able 1amples used in this work grouped by their content of Mn and Si. The label of the cond the cHCP/aHCP relationship are shown. The fitting errors are quoted as subscripts

lloy Mn group Si group Mn (at.%) Si (at.%) fFCC (%)

2 M17 S0 16.81 – 161

M20 S0 19.63 – 362

3 M20 S0 19.61 – 251

4 M22 S0 21.51 – 391

M22 S0 22.23 – 434

M25 S0 25.06 – 481

0 M25 S0 25.45 – 552

1 M27 S0 26.87 – 623

2 M27 S0 27.62 – 853

M27 S0 28.05 – 793

M29 S0 29.69 – 805

M31 S0 30.93 – 1005

2 M20 S2 19.93 2.11 431

0 M22 S2 22.89 2.01 481

M25 S2 25.76 2.01 411

2 M32 S2 32.63 3.91 1009

3 M20 S4 19.47 3.94 451

2 M25 S4 25.87 3.52 552

5 M22 S5 22.53 5.41 491

4 M20 S8 19.52 7.92 462

5 M22 S8 21.62 7.62 481

8 M24 S8 23.83 8.01 522

M31 S8 31.75 8.02 953

8 M31 S9 31.15 9.43 842

8 M22 S12 21.93 11.45 472

9 M24 S12 24.13 11.93 583

Engineering A 437 (2006) 323–327

hen, a final cleaning with Nital (5%) was done. Before per-orming the experiments, all the samples were cooled down to7 K to obtain the maximum amount of HCP martensite phase.or a better description of the influence of the Mn and Si con-

ent, all samples were classified in groups with similar contentf these elements (see Table 1).

Neutron diffraction measurements were performed using the1B instrument, a two-axis powder diffractometer at the Institutaue-Langevin (Grenoble, France), with a Ge monochromatorλ = 1.28 A), and a flux of 0.4 n cm−2 s−1 over the sample. Aultidetector with 400 cells covering a range of 80◦ was used to

ecord the diffractograms. The measurements were performedt RT using about 2 g of sample inside vanadium cylinders andounted on a rotating device. The rotation of the container

llows obtaining a good powder diffraction approximation [14].he calibration of the neutrons wavelength was made usingl2O3. The analyses of the diffractograms were performed using

he Rietveld refinement method [15] with the FullProf software16]. In the cases of those alloys with a Neel temperature ofCC-phase, TN

FCC, higher than room temperature [17], an anti-erromagnetic component was added, allowing the magneticoments to vary.

. Results and discussion

Typical diffractograms obtained from some of the alloys arehown in Fig. 1. The lines associated to HCP and FCC-phasesre observed. The diffractograms of alloys where the TN

FCC isigher than room temperature, present additional Bragg peaks

rresponding group for each alloy, lattice parameters, HCP- and FCC-fractions

fHCP (%) aFCC (A) aHCP (A) cHCP (A) cHCP/aHCP

841 3.59153 2.53171 4.08852 1.61491

642 3.59397 2.53395 4.0891 1.61371

761 3.59462 2.53331 4.09113 1.61491

621 3.59622 2.53522 4.09204 1.6142

515 3.59867 2.53747 4.0892 1.61153

522 3.59842 2.53722 4.09295 1.61321

462 3.60263 2.53864 4.09259 1.61211

382 3.60233 2.53944 4.0941 1.61221

161 3.60602 2.54226 4.0952 1.61084

211 3.60403 2.54055 4.0931 1.61111

202 3.60364 2.54067 4.0932 1.61103

– 3.60923 – – –571 3.59272 2.53402 4.09373 1.6161

522 3.59582 2.53642 4.09615 1.61491

592 3.59962 2.53862 4.09775 1.61421

– 3.60896 – – –551 3.59172 2.53422 4.09714 1.61671

451 3.59722 2.53802 4.10045 1.6161

511 3.59232 2.53502 4.10104 1.6181

542 3.59103 2.53462 4.10555 1.61981

521 3.59063 2.53412 4.10515 1.61991

482 3.59443 2.53683 4.10856 1.61961

51 3.60152 2.5421 4.1254 1.62279

171 3.60102 2.54134 4.1191 1.6212

532 3.59245 2.53583 4.11509 1.6233

421 3.59356 2.53658 4.1242 1.62593

J. Martınez et al. / Materials Science and Engineering A 437 (2006) 323–327 325

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Fig. 2. HCP-phase fraction, fHCP, as a function of: (a) Mn content [(�) S0, (�)S2, (�) S4, (�) S5, (�) S8, (�) S9, and (©) S12; dotted line represent Eq. (1)] and(MM

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ig. 1. Diffractograms obtained from some of the alloys studied in this work.he inset shows a low angle reflection corresponding to the antiferromagneticontribution of FCC phase.

hat were ascribed to the antiferromagnetic contribution of FCChase (inset Fig. 1). The results of the fit procedure are reportedn Table 1, where the lattice parameters for the HCP and FCChases and the phase fractions are shown, labeled with the cor-esponding group’s identifications.

Fig. 2(a) and (b) shows HCP-phase fraction given in percent-ge, fHCP, as a function of Mn and Si contents, respectively. Inhe alloys with CMn < 25 at.%, the increase of Mn concentrationroduces a linear decrease of fHCP (Fig. 2(a)), but it remainsearly constant, within experimental errors for all Si concentra-ions, when Mn is kept sufficiently constant (Fig. 2(b)). A lineart to the experimental data gives rise to the following depen-ence of fHCP with the Mn content:

HCP = (159 ± 2) − (4.75 ± 0.06) at.% Mn, (1)

hich is valid in the interval 20–25 at.% Mn and 0–12 at.% Si.In the binary Fe–Mn system when CMn > 25 at.%, the anti-

erromagnetism of FCC phase inhibit the FCC/HCP martensitic

ransformation [6]. On the other hand, Si additions to Fe–Mnlloys push down the Neel temperature of FCC favouring theartensitic transformation [6]. This situation is observed in the31 group of samples (white triangle up in Fig. 2(b)), where

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b) Si content [(�) M17, (�) M20, (�) M22, (�) M24, (�) M25, (�) M27, (©)29, (�) M31, and (�) M32]. (c) fHCP in the Fe–Mn system, obtained from (�)ossbauer [11] and (�) neutron diffraction. Dashed lines are guides to the eyes.

HCP vanishes for 0 at.% Mn but increases when Si is added tohe alloy.

HCP-phase fractions obtained here for the Fe–Mn systemsing neutron diffraction are shown in Fig. 2(c) along with thealues obtained in Ref. [11] from Mossbauer spectroscopy. Ineneral, diffraction fractions are about 16–18% lower than thosextracted from Mossbauer spectra. This discrepancy is consid-red to arise from the fact that the Mossbauer technique is aanoscopic one and detects changes in the surroundings of a Fetom. Then, it could be detecting stacking faults in FCC matrixs an HCP region, consequently enhancing the HCP fractionespect to the real value.

Fig. 3 reports the evolution of the lattice parameters aFCC,HCP and cHCP with Mn and Si contents, respectively. The dot-ed lines result from a linear fit to the data. The increase in

n content produces a linear increase in aFCC, aHCP and cHCP

attice parameters. On the other hand, the increase of the Si con-ent decreases aFCC and aHCP parameters, and strongly increasesHCP parameter. According to the analysis of data reported inig. 3 the dependence of the lattice parameters with both, Mnnd Si, can be written in the following way:

FCC = 3.569 + 0.00122 at.% Mn

−2.393 × 10−5(at.% Mn)(at.% Si) (2)

HCP = 2.520 + 0.00073 at.% Mn

−3.614 × 10−6 (at.% Mn)(at.% Si) (3)

HCP = 4.081 + 0.00047 at.% Mn

+9.810 × 10−5 (at.% Mn)(at.% Si) (4)

A good agreement between the aFCC lattice parameter of S0amples with those of the literature for Fe–Mn alloys [8] (solidine in Fig. 3(a)) is found. However, the effect of Mn contentn both lattice parameters of the HCP-phase, aHCP and cHCP, as

isplayed in Fig. 3(c) and (e), is weaker than that reported in theiterature (solid line) [9].

The martensitic transformation involves the coordinateotion of atoms along small distances if compared with the

326 J. Martınez et al. / Materials Science and Engineering A 437 (2006) 323–327

Fig. 3. (a) and (b) aFCC. Dotted lines were obtained from Eq. (2): (a) for 0, 2,4 and 8 at.% Si and (b) for 20, 22, 25 and 31 at.% Mn. (c) and (d) aHCP. Dottedlines were obtained from Eq. (3): (c) for 0, 2, 4 and 8 at.% Si and (d) for 20, 22,25 and 31 at.% Mn. (e) and (f): cHCP. Dotted lines were obtained from Eq. (4):(e) for 0, 2, 4 and 8 at.% Si and (f) for 20, 22, 25 and 31 at.% Mn. In (a), (c) and(e): (�) S0, (�) S2, (�) S4, (�) S5, (�) S8, (�) S9, and (©) S12. In (b), (d) and(Md

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twhiitstacking fault increases by a factor between 3 and 5, dependingon the Mn content, when Si is added to the matrix up to a 12 at.%.However, it is interesting to note that the fHCP fraction (Fig. 2(b))remains nearly constant, independently of the Si content. The

f): (�) M17, (�) M20, (�) M22, (�) M24, (�) M25, (�) M27, (©) M29, (�)31, and (�) M32. The solid lines in (a), (c) and (e) corresponding to Fe–Mn

ata were taken from Refs. [8,9].

attice parameters, consequently strength and distortions coulde built up in the crystal. In the particular case of the Fe–Mn–Silloys, the HCP-phase grows with its basal plane parallel to the1 1 1}plane of the FCC-phase, i.e. the{1 1 1}� is the habit planeetween austenite and martensite. Then, it is useful to comparehe cHCP and aHCP lattice parameters with their equivalent dis-ances in the FCC matrix, i.e. the 2 × dFCC

1 1 1 and dFCC1 1 0

istances between atoms in the {1 1 1}� and {1 1 0}� planes,espectively, as defined in Ref. [18]. 2 × dFCC

1 1 1 and cHCP dis-ances involve the smaller distances between each two closedacked planes in FCC and HCP, respectively, and dFCC

1 1 0 andHCP are the shorter interatomic distances inside each closedacked plane in FCC and HCP, respectively. Hence, to quan-ify the lattice distortion introduced by the martensitic trans-ormation, the ratios aHCP/dFCC

1 1 0 and cHCP/2dFCC1 1 1 were

alculated and are shown in Fig. 4. Both ratios result smallerhan the unit and are nearly constant with Mn content, beinghe aHCP/dFCC

1 1 0 closer to the unit, in the studied composi-ion range. In addition, as the Si content increases, both ratios,HCP/dFCC

1 1 0 and cHCP/2dFCC1 1 1, also increase, tending to the

nit. These facts indicate that as Si is incorporated to the alloys

he lattice distortion becomes less important, favouring the trans-ormation.

Fig. 5(a) shows the FCC and HCP average volumes per atom18], VFCC and VHCP, versus the Mn content, as evaluated using

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ig. 4. (a) aHCP/dFCC1 1 0 and (b) cHCP/2dFCC

1 1 1 ratios as a function of Mnontent. (�) S0, (�) S2, (�) S4, (�) S5, (�) S8, (�) S9, and (©) S12. Dottedines are guides to the eyes.

he lattice parameters from Table 1. These results are groupedy the Si content of the samples. This figure indicates that theverage volume per atom for both close packed phases, FCCnd HCP, varies with the Mn content essentially in a similarrend, increasing with Mn content. On other hand, a tendency toncrease the HCP volume and to decrease the FCC volume ashe Si content increases is also observed. The relative volumehange, �V/V = (VFCC − VHCP)/VFCC, due to the FCC → HCPartensitic transformation is plotted in Fig. 5(b). The �V/V val-

es present a slight variation with the Mn concentration but theytrongly decrease with Si additions. In fact, the �V/V valuesecrease about a 50% respect to the Fe–Mn system when up to2 at.% Si is added to the alloys.

The results displayed above indicate that the matrix distor-ion due to the martensitic transformation FCC ↔ HCP is lowerhen Si atoms are incorporated to the alloys. Xuejun et al. [19]ave found a relationship between the stacking fault probabil-ty in FCC Fe–Mn–Si alloys and the contents of Mn and Si,ncreasing the stacking fault probability with Si content. Usinghe reported relationship [19], it is found that the probability of

ig. 5. (a) Average volume per atom for the FCC and HCP phases, and (b)elative volume change due to the FCC/HCP martensitic transformation, forifferent Si concentrations: (�) S0, (�) S2, (�) S4, (�) S5, (�) S8, (�) S9, and©) S12.

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J. Martınez et al. / Materials Science

resent results might be indicating that in spite of a larger num-er of martensite embryos [20] are expected when adding Si tohe alloys, the growth of them resulted hampered. As pointedut by Jiang et al. [21], in alloys with low stacking fault energys Fe–Mn–Si alloys, a perfect dislocation dissociates in two par-ial dislocations separated by a stacking fault ribbon, forming anxtended dislocation. When two extended dislocations interceptach other, extended jogs and Cottrell-Lomer locks are formed,ecoming barriers for the further motion of dislocations. Jiangt al. [21] considered that this was the reason of an increase inhe strain-hardening exponent when Si is added to the alloys.n the other hand, embryos of martensite are considered to be

ormed by a sequence of a few piled-up of intrinsic stackingaults [10,20], when the temperature goes down below MS tem-erature [13] these partial dislocations move apart allowing thembryo to grow up to a full martensitic plate. Then, it is pos-ible to speculate that as the Si concentration is increased, theechanism of growth of martensite embryos is tackled by the

nhanced amount of obstacles in the FCC matrix, preventing aarger amount of HCP martensite to be formed. This explana-ion is consistent with the present results obtained from neutroniffraction analyses where fHCP does not change with Si addi-ions, when Mn is kept constant (Fig. 2(b)).

. Conclusions

An extensive dataset of cell parameters and relative phaseractions obtained after neutron diffraction experiments forustenite and martensite phases in the ternary Fe–Mn–Si sys-em has been constructed. Since the thermal treatments of theolycrystalline samples included a final cooling down to liquiditrogen temperature the maximum possible amount of HCP-hase for all compositions was achieved. A rotating sampleolder device and several small sample-pieces allowed simulat-ng samples with randomly oriented grains and thus quantifyinghe phase fractions from the neutron diffractograns. It was foundhat the martensite fraction varied with Mn, while it was almostnsensible to the Si content of the alloy, when CMn < 25 at.%.or binary Fe–Mn alloys with CMn > 25 at.%, the HCP fractionanished when Mn increased up to 31 at.% Mn. The martensiteraction increased from zero when Si was added to alloys with1 at.% Mn.

The FCC and HCP cell parameters displayed a simple varia-ion with composition and a Vegard’s law extrapolation to threelements was used to describe them as functions of composition.he ratios of equivalent distances in both HCP and FCC-phases,HCP/dFCC

1 1 0 and cHCP/2dFCC1 1 1, tended to 1 as the Si con-

ent increased, indicating a smaller distortion produced by theCC ↔ HCP transformation as the Si content is incremented.

The average volume per atom of FCC phase was found toecrease with Si incorporation to the alloy, while the average

olume per atom of HCP-phase was found to increase with theddition of this element. On the other hand, the Mn was foundo increase the volume of both phases in a similar trend. Con-equently, as the Si content of alloys was increased the volume

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Engineering A 437 (2006) 323–327 327

istortion due to the FCC ↔ HCP martensitic transformationecame less important. In spite of a less volume distortionas observed after increasing Si addition, the phase fractionf martensite resulted similar to that of the binary Fe–Mn sys-em. It was speculated that an enhanced amount of obstacleshat simultaneously appeared in the matrix due to the marten-itic transformation prevented the martensite embryos to grow.

Finally, phase fractions in the binary Fe–Mn system obtainedy the present neutron diffraction experiments were smaller by16–18% than the values previously obtained using Mossbauer

pectroscopy. Anyhow, neutron phase fraction presents theame tendencies with composition as the values obtained from

ossbauer spectroscopy. Differences could arise from the dif-erent scales that both techniques, neutron diffraction and

ossbauer spectroscopy, were sensing.

cknowledgements

Research grant PIP 5382 from Consejo Nacional de Inves-igaciones Cientıficas y Tecnicas (CONICET, Argentina) isratefully acknowledged. The authors specially acknowledgehe facility time at Institut Max von Laue-Paul Langevin, Col-aborating Research Group D1B (CSIC/CICYT-CNRS).

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