Mossbauer Spectroscopy, Dilatometry and Neutron

Diffraction Detection of the "-Phase Fraction

in Fe–Mn Shape Memory Alloys

J. MARTINEZ1, G. AURELIO2, G. CUELLO3, S. M. COTES1,

A. FERNANDEZ GUILLERMET2 and J. DESIMONI1,*1Departamento de Fısica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata,

IFLP-CONICET, C.C.6 7, 1900 La Plata, Argentina; e-mail: desimoni@fisica.unlp.edu.ar2Instituto Balseiro, Centro Atomico Bariloche, CNEA and CONICET, Avda. Bustillo 10000,

8400 S. C. de Bariloche, Argentina3Diffraction Group, Institut Laue-Langevin, 6 Rue Jules Horowitz, B.P. 15638042 Grenoble

Cedex 9, France

Abstract. The results of a Mossbauer spectroscopy (MS) and a neutron diffraction (ND) study

carried out on a set of Fe–Mn alloys quenched from high temperatures are reported. Upon

quenching the high temperature stable phase FCC (�) together with metastably retained HCP-(")phase are formed. The lattice parameters obtained using ND are in excellent agreement with

previous results of the literature. The phase fractions obtained from MS are coincident, within

experimental errors, with those extracted from ND, with exception of the case of an alloy with the

lowest Mn content (16.8 at.% Mn). That situation can be associated with the occurrence of a high

density of stacking faults (SF) in the � phase or to the presence of a paramagnetic � phase. " phase

fractions obtained from MS and ND decrease smoothly with Mn content but are always larger than

the values reported in literature resulting from dilatometry.

1. Introduction

The technological applications of shape-memory alloys based in the Fe–Mn

systems have motivated extensive basic and applied research. In the Fe–Mn

system, the shape-memory effect is governed by a reversible martensitic trans-

formation (MT) between the � (FCC) and " (HCP) phases. The design of im-

proved shape-memory alloys requires a deep understanding of this MT [1]. In

this way, a new thermodynamic description of the relative stabilities between the

�- and "-phases in the Fe–Mn system have been reported [2]. This description

requires a critical test such as the comparison of the predicted enthalpies of

transformation (DHm) with experimental data. In order to correctly evaluate the

experimental DHm, the fraction of " phase f " induced by the MT after quenching

* Author for correspondence.

Hyperfine Interactions (2005) 161:221–227

DOI 10.1007/s10751-005-9194-0 # Springer 2005

must be determined independently, since it varies with the content of Mn [3].

Recently, new measurements of DHm have been obtained [4] by combining

differential scanning calorimetry (DSC) with MS and dilatometry measurements.

In that work [4], MS yielded a "-phase fraction about 30% larger than that of

dilatometry. This is a serious discrepancy since dilatometry is traditionally used

to derive phase fractions from measured volume changes. Anyhow, the model

predictions about DHm better represented the experimental results obtained using

the f " determined using MS.

The present work intends to clarify the discrepancy between dilatometry and

MS results, by applying a third method, namely, a neutron diffraction study

of the quenched Fe–Mn alloys in the [10–30 wt.%] range of Mn content. More

specifically, we established the phase fractions of "- and �-phases by a full-

pattern Rietveld analysis of neutron diffraction spectra taken in the D1B

diffractometer at ILL (Grenoble, France), using a rotating sample holder to

satisfactorily approach the conditions of a powder diffraction experiment. We

present a detailed comparison and discussion of the results obtained by Mossbauer

spectroscopy, dilatometry and neutron diffraction.

2. Experimental

Alloys in the concentration range from 15 to 30 wt.% Mn were prepared using Fe

and Mn, both of 99.98% purity, and melted in an arc furnace, on a water-cooled

copper crucible, in a 350 Torr Ar atmosphere. Polycrystalline samples used for

ND experiments consisted of about 2–3 g of small cubes of about 2 mm side, cut

from the original ingots. Samples for MS had approximately 2.5 cm � 2.5 cm �0.08 cm size. To obtain the maximum amount of " phase, all specimens were

annealed for 1 h at 950-C under Ar atmosphere, quenched at room temperature

(RT) without breaking the capsule and then cooled down to liquid nitrogen

temperature.

Mossbauer spectra were recorded using a 5 mCi 57CoRh source and a

standard 512 channels constant acceleration spectrometer. The samples were

analyzed using Conversion Electron Mossbauer Spectroscopy with a constant

flux helium–methane detector. Velocity calibration was performed against a 12

mm-thick �-Fe foil. All isomer shifts are referred to this standard at 298 K. The

spectra were fitted with Lorentzian line shapes with a non-linear least-squares

program with constraints. The relative fraction of phases were determined as-

suming the same Lamb–Mossbauer factor for the different phases.

The neutron diffraction experiments were performed with the D1B two-axis

powder diffractometer at the Institut Laue–Langevin, using a Ge monochromator

(311 reflection) to obtain a wavelength of 1.28 A. The flux on the sample was

0.4 � 106 n cmj2sj1. This diffractometer is equipped with a 3He multidetector

containing 400 cells, with an angular span of 80-, providing high quality spectra.

The measurements were performed at room temperature using a vanadium

222 J. MARTINEZ ET AL.

cylinder filled with about 2 g of sample and mounted on a rotator device. The

rotation of the sample-holder around its axis allows attaining a very good ap-

proximation to a powder spectrum [5]. The neutron wavelength was calibrated

using an Al2O3 standard. To establish the fractions of the present phases, a full-

pattern Rietveld analysis [6] was performed using the Fullprof 2K software [7].

3. Results and discussion

The relative fraction of " phase was determined from Mossbauer data analysis

f "MS

� �, and from full-pattern Rietveld analysis of neutron diffraction f "ND

� �.

Figure 1 presents the recorded experimental diffraction patterns. These dif-

fractograms show reflections associated with the � phase and to the " phase, with

exception of that corresponding to the alloy with 30.95 wt.% Mn where only

the � phase is observed. In several samples, there are also additional lines that

can be attributed to the antiferromagnetic ordered � phase. Two fitting pro-

cedures were carried out. The first one considered the paramagnetic " phase

40 60 80 1002 θ (degrees)

16.8 at % Mn

21.5 at % Mn

25.0 at % Mn

26.8 at % Mn

27.6 at % Mn

30.9 at % Mn

Inte

nsity

(a.

u.)

Figure 1. Neutron diffractograms taken with D1B instrument at ILL. Circles: antiferromagnetic

�-phase, squares: paramagnetic �-phase and triangles: "-phase.

MS AND ND DETECTION OF THE "-PHASE FRACTION IN Fe–Mn ALLOYS 223

contribution in addition to two � phase contributions, one paramagnetic and the

other non-collinear antiferromagnetic using the Q3 magnetic model [8–10]. The

magnetic moments for Fe and Mn were taken from ref. [10] and were maintained

fixed during the fit. The second one was performed taking into account only two

contributions to the diffractograms, one corresponding to the paramagnetic "phase and the other to the antiferromagnetic � phase using again the Q3 model

[8–10], but in this case the parameters related to texture and preferred orien-

tations were permitted to vary during the fit. Both methods satisfactorily re-

produced the experimental data within the experimental error and the resulting

f " differs in less than 3%.

Figure 2 shows the agreement between the lattice parameters for � and "phases obtained from the ND results and those taken from literature [11] and

[12].

Typical MS spectra for the various alloys are shown in Figure 3. The fact that

the ordering Neel temperatures, T�N , are higher than RT [13] and the MS spectra

have not a definite structure rules out the possibility to analyze them taking into

account the two magnetic states for the � phase, as it could be suggested by the

15 20 25 30

3.59

3.60

3.61

4.09

4.10

2.530

2.535

2.540

aγ (Å

)cε

(Å)

aε (Å

)

at % Mn

Figure 2. Composition dependence of the present a� , a" and c" lattice parameters. Dashed lines

correspond to ref. [11] and solid line to ref. [12].

224 J. MARTINEZ ET AL.

ND results. Then, the spectra were analyzed considering a non-resolved magnetic

interaction representing antiferromagnetic � phase (I1) and a single line repre-

senting paramagnetic " (I2) phase. The resulting hyperfine parameters and rel-

ative fractions are quoted in Table I together with the corresponding T�N taken

from literature [13].

-2 -1 0 1 2

a)

-2 -1 0 1 2

-2 -1 0 1 2 -2 -1 0 1 2

-2 -1 0 1 2 -2 -1 0 1 2

b)

c) d)

e) f)

v (mm/s) v (mm/s)

Inte

nsity

(a.

u.)

Figure 3. Conversion electron Mossbauer spectra recorded on the Fe–Mn alloys. a) 16.8 at.%,

b) 19.6 at.%, c) 19.7 at.%, d) 21.5 at.%, e) 25.4 at.%, and f) 27.6 at.%.

Table I. Mossbauer hyperfine parameters and relative fractions for the various alloys

Mn at % T�N (K) d(mm/s) G(mm/s) f " (%) B(T) d(mm/s) G(mm/s) f � (%)

16.8 318 j0.141 0.301 1002 – – – –

19.6 345 j0.131 0.261 881 2.8 j0.091 0.291 111

19.7 347 j0.141 0.331 783 2.41 j0.091 0.251 221

21.5 366 j0.141 0.331 664 2.71 j0.091 0.251 341

25.4 398 j0.131 0.261 442 2.91 j0.091 0.291 561

27.6 412 j0.131 0.261 353 2.81 j0.091 0.261 651

The ordering temperatures, T�N are also included.

MS AND ND DETECTION OF THE "-PHASE FRACTION IN Fe–Mn ALLOYS 225

Finally, Figure 4 shows the present f "MS and f "ND and those extracted from

literature [14] corresponding to dilatometric measurements. It is observed that

the MS and ND determinations present similar trends, and noticeably differ at the

low concentration range. This difference could be related to a higher density of

stacking faults (SFs) present in the �-phase, since at the 16 at.% Mn composition

the SF energy has a minimum [15]. Due to the local atomic order, these SFs

generate "-like environments. Then, MS could be overestimating the " relative

fraction in these cases. The other possibility is the presence of � paramagnetic

phase that also could be shared in the " MS signal. Figure 4 also shows that f "MS

and f "ND are systematically higher than those obtained from dilatometry [14]. This

discrepancy could be associated with the self-accommodation of variants [16], or

may be related to the polycrystallinity of the samples, giving then a macroscopic

volume change, which does not necessarily reflects the real " phase fraction

determined by dilatometry.

4. Conclusions

The evolution with Mn concentration of the " relative fractions obtained by MS

and ND is similar, decreasing with the Mn content. The resulting MS and ND

fractions are always larger than those obtained from dilatometry suggesting that

the last technique becomes inappropriate to deduce phase fractions in the present

situation. However, the use of MS for phase fraction detection in the Fe–Mn

system where both phases " and paramagnetic � have very similar hyperfine

parameters can be difficult, even more when a high concentration of SFs is ex-

pected. An additional technique like ND was necessary to use to clarify such a

situation.

16 20 24 28 32

0

20

40

60

80

100

[14]

fε (%

)

at % Mn

fεND

fεMS

Figure 4. Variation with Mn content of the " phase fraction obtained from Mossbauer

spectroscopy f "MS

� �and neutron diffraction f "ND

� �experiments. Results of dilatometry extracted

from ref. [14] are included for comparison.

226 J. MARTINEZ ET AL.

Acknowledgements

We wish to thank the support of the Spanish Cooperation Research Group at the

Institut Laue–Langevin, which allowed us to use the D1B neutron diffractometer.

Research grants from CONICET (Consejo Nacional de Investigaciones Cientıf-

icas y Tecnicas, Argentina), Fundacion Rocca and Fundacion Antorchas from

Argentina are gratefully acknowledged.

References

1. Kopitsa G. P., Runov V. V., Grigoriev S. V., Bliznuk V. V., Gavriljuk V. G. and Glavatska

N. I., Physica B 335 (2003), 134.

2. Cotes S., Fernandez Guillermet A. and Sade M., Mater. Sci.Eng., A 273–275 (1999), 503.

3. Cotes S., Sade y M. and Fernandez Guillermet A., Met. and Mat. Trans. A, 26A (1995), 1957.

4. Martınez J., Cotes S. M., Cabrera A. F., Desimoni J. and Fernandez Guillermet A., Met.

Trans., A, send to publication in march 2004.

5. Benitez G. M., Aurelio G., Fernandez Guillermet A., Cuello G. J. and Bermejo F. J., J. Alloys

Comp. 284 (1999), 251.

6. Young R. A. (ed.), The Rietveld Method, Oxford University Press, New York, 1995.

7. Rodrıguez-Carvajal J., Fullprof version 35d, LLB-JRC, France, 1998.

8. Umebashashi H. and Ishikawa Y., J. Phys. Soc. Jpn. 21(7) (1966), 1281.

9. Hirai K. and Jo T., J. Phys. Soc. Jpn. 54(9) (1985), 3567.

10. Schulthess T. C., Butler W. H., Stocks G. M., Maat S. and Mankey G. J., J. Appl. Phys. 85(8)

(1999), 4842 – 4844.

11. Marinelli P., Baruj A., Sade M. and Fernandez Guillermet A., Z. Met. Kd, 91(11) (2000),

957–962.

12. Marinelli P., Baruj A., Sade M. and Fernandez Guillermet A., Z. Met. Kd. 92(5) (2001),

489 – 493.

13. Huang W., CALPHAD 13 (1989), 243–252.

14. Marinelli P., Baruj A., Pons J., Sade M., Fernandez Guillermet A. and Cesari E., Mater. Sci.

Eng., A. 335 (2002), 137–146.

15. Cotes S. M., Fernandez Guillermet A. and Sade M., Met. Trans., A 35A (2004), 83.

16. Yang J. H. and Wayman C. M., Mat. Charact. 28 (1992), 28–37.

MS AND ND DETECTION OF THE "-PHASE FRACTION IN Fe–Mn ALLOYS 227