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Materials Letters 61 (20

Thermally induced martensite properties in Fe–29%Ni–2%Mn alloy

E. Güler ⁎, H. AktaşDepartment of Physics, Kirikkale University, Kirikkale, Turkey

Received 18 June 2006; accepted 5 November 2006Available online 27 November 2006

Abstract

Thermally induced martensite properties in Fe–29%Ni–2%Mn alloy were investigated according to martensitic transformation kinetics,morphology, magnetism of both austenite and martensite phases and also in terms of martensitic transformation start temperatures (Ms) fordifferent austenite grain sizes of alloy. Kinetics of the transformation was found to be athermal. Also only lenticular martensite morphology wasobserved during investigations. On the other hand, Mössbauer spectra revealed a paramagnetic character for austenite phases and a ferromagneticcharacter for thermally induced martensitic phases. Determined Ms temperatures were found to be at −128 °C for large grained samples and−135 °C for small grained samples.© 2006 Elsevier B.V. All rights reserved.

Keywords: FeNiMn; Athermal; Lenticular; Martensite; Mössbauer spectroscopy

1. Introduction

As well reported by Shimizu and Kakeshita [1] martensitictransformation has been generally understood as a phenomenonwhich starts at theMs temperature when a specimen is quenchedor cooled from the austenitic region in Fe based alloys andsteels. It is also well known that the plastic deformation ofaustenite can induce martensitic transformation in Fe basedalloys [2–4].

The various above mentioned external forces for inducing amartensitic transformation in the austenite phase of relatedspecimen influence some transformation behaviours especiallytransformation kinetics and existing martensite morphology [2].

The kinetics of martensitic transformations has been broadlyclassified into two groups as isothermal and athermal kineticbehaviour. Although the isothermal kinetics displays explicitlytime-dependent kinetics with temperature, athermal kinetics de-pends only on the temperature of transformation and is in-dependent of holding time at that temperature. Early studies onFe–Ni–Mn ternary alloys which exhibit athermal, isothermal orboth athermal and isothermal kinetic behaviours explained thatthese kinetic variety depends not only on applied external forces

⁎ Corresponding author. Tel.: +90 3183572478; fax: +90 3183572461.E-mail addresses: emre71@email.com (E. Güler), haktas@kku.edu.tr

(H. Aktaş).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.11.034

but also on the slight changes of Mn content in Fe–Ni–Mn alloys[5–7].

Moreover, reported results on ferrous martensite, have shownthat a variety of morphologies i.e. lath, butterfly, lenticular andthin plate [2,8–12] can be seen within thermally induced trans-formations for different alloy compositions.

From a magnetical point of view, martensitic phase exhibits adistinct magnetic diversity relative to the magnetism of austenitephase [12–15]. In many ferrous alloys and steels undergoingmartensitic transformation, despite the paramagnetic characterof austenite phase, martensitic phase shows a tendency toferromagnetism [1].

Grain size of prior austenite phase is another major factorwhich affects the behaviour of martensitic transformations. Thatis, as the austenite grain size becomes smaller, the amount of theexisting martensite is decreased and also the starting temper-ature of transformation (Ms) is slightly lowered [2].

In addition, a considerable volume of work was made onmartensitic transformations in Fe–Ni–Mn alloys with Nicontent ranging between 20% and 31% and Mn content ranging0.2%–6% alloys [2,16–20] because of their technologicalimportance, wide industrial applications and interesting mar-tensitic transformation characteristics.

The purpose of the present work was to clarify thermallyinduced martensite properties in a Fe–Ni–Mn alloy with 29%Niand 2%Mn content according to transformation kinetics, existing

Fig. 1. SEM micrographs of samples A1, B1 and A2.

Fig. 2. Obtained Mössbauer spectra for samples A1 and A2.

Table 1Some Mössbauer parameters of the studied samples

Mössbauer parameters A1 A2 B1 B2

Austenite (%) 100 35 100 55Martensite (%) – 65 – 45Bhf (T) – 33.3 – 34.6δAustenite (±0.01 mm/s) 0.17 0.11 0.16 0.49δMartensite (±0.01 mm/s) – 0.07 – 0.16

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martensite amount and morphology, magnetism of austenite andmartensite phases and also Ms values by using some effectivetechniques about which no work has yet been reported.

2. Experimental details

In order to observe the effect of austenite grain size onmartensite formation in this alloy two heat treatments weredesignated. One of them was homogenization of samples at1100 °C for 12 h followed by quenching into water at room

temperature (group A), while the other was the homogenizationof samples at 1100 °C for 6 h followed by quenching into waterat room temperature (group B). Austenite phases of both groupswere denoted as A1 (with an average grain size ∼275 μm) andB1 (with an average grain size ∼145 μm). After microstructuralobservations of A1 and B1, these samples were immersed inliquid nitrogen (−196 °C) for 3 s in order to obtain thermallyinduced athermal martensite in these samples. Samples kept at−196 °C were also denoted as A2 and B2.

Microstructural observations were carried out by a Jeol JSM-5600 type scanning electron microscope (SEM) which wasoperated at 30 kV. Samples from the two groups were cutmechanically to ∼100 μm thickness and mechanically polishedwith diamond pastes through a conventional procedure.Samples for microstructural observations were etched in a

Fig. 3. DSC cooling curves of samples A1 and B1.

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chemical solution containing 5 ml HF+20 ml H2O2+25 mlH2O.

Samples investigated by means of SEM were again thinned∼50 μm thickness for Mössbauer spectrometer. Mössbauersamples were examined with a spectrometer at room temperatureby using 50 mCi 57Co source diffused in Rh. The Mössbauerspectra were calibrated with respect to α-Fe and isomer shiftswere given relative to the center of the α-Fe spectrum.

Differential scanning calorimetry (DSC) technique wasem-ployed in the study in order to clarify the effect of austenitegrain size on martensitic transformation temperatures. Newsamples of A1 and B1 were mechanically prepared forDSC in the form of discs of 3 mm radius. DSC measurementsof these alloys were performed by using a Perkin-ElmerSapphire model thermal analyser. DSC measurementswere taken at a cooling rate of 5 °C/min between 25 °C and−150 °C.

3. Results and discussion

Fig. 1 shows SEM micrographs of large grained austenite phase ofsample A1, small grained austenite phase of sample B2 and thermallyinduced martensites in sample A2, respectively. Thermally induced

martensites in Fe–Ni–Mn alloys have been observed with packed lathor separated lenticular morphologies [12,19–21]. Separated lenticularmorphology of thermally induced martensites can be clearly seen inFig. 1. Some certain evidences such as new nucleation sites or growthof existed martensite crystals achieved by further isothermal holdingtimes during isothermal martensitic transformations are well elucidatedby Kajiwara [6]. Although samples A2 and B2 were kept at −196 °Cfor 5 min, 5 h and 5 days, these isothermal holding times did not causethe formation of new nucleation sites or growth of existing martensitecrystals.

Fig. 2 shows an example of the obtained Mössbauer spectra forsamples A1 and A2 respectively. Previous Mössbauer spectroscopystudies on the austenite–martensite phase transformations in ferrousalloys emphasized that a singlet corresponds to a paramagneticmagnetic character, while a sextet represents a ferromagnetic behaviour[12–15]. A singlet for sample A1 and a sextet with prior retainedaustenite singlet for sample A2 can be seen in Fig. 2. Other Mössbauerspectrometer parameters such as % volume fractions and also isomershift values (δ) of both austenite and martensite phases with internalmagnetic fields (Bhf) of martensite phases are given in Table 1.

DSC measurements of sample A1 and sample B1 from theiraustenite phases to a subzero temperature can be seen in Fig. 3,respectively. Fig. 3 indicates that a sharp peak of transformationappeared in sample A1 at −128 °C which deals with the maximummartensite nucleation. On the other hand, the Ms temperature forsample B1 appeared at −135 °C with a little martensite nucleationduring cooling from austenite phase.

4. Conclusions

(i) The kinetics of thermally induced martensite for thestudied alloy was athermal.

(ii) Thermally induced martensites were formed with alenticular morphology for all studied samples in Fe–29%Ni–2%Mn alloy.

(iii) Increasing of homogenization time at a constant temper-ature increased the austenite grain size. As the austenitegrain size increases, the amount of the transformedmartensite is also increased. This case proved byMössbauer spectroscopy shows a fair agreement withearly reports on ferrous alloys.

(iv) As the austenite grain size increases, Ms is also increased.This case revealed by DSC measurements exhibited agood correspondence for various ferrous alloys inliterature.

References

[1] K. Shimizu, T. Kakeshita, ISIJ Int. 29 (1989) 97.[2] Z. Nishiyama, Martensitic Transformation, Academic Press, London,

1978, p. 246.[3] T.N. Durlu, J. Mater. Sci. Lett. 11 (1992) 702.[4] T.N. Durlu, J. Mater. Sci. 34 (1999) 2887.[5] S.J. Kim, C.M. Wayman, Mater. Sci. Eng., A Struct. Mater.: Prop.

Microstruct. Process. 136 (1991) 121.[6] S. Kajiwara, Mater. Trans., JIM 33 (1992) 1027.[7] A. Chanda, H. Pal, M. De, S. Kajiwara, T. Kikuchi, Mater. Sci. Eng.,

A Struct. Mater.: Prop. Microstruct. Process. 265 (1999) 110.[8] C.M. Wayman, Mat. Sci. Forum 1 (1990) 56.[9] Y. Himuro, O. Ikeda, R. Kainuma, K. Ishida, J. Phys. IV 11 (2001) 205.[10] D.F. Li, X.M. Zhang, E. Gautier, J.S. Zhang, Acta Mater. 46 (1998) 4827.

3318 E. Güler, H. Aktaş / Materials Letters 61 (2007) 3315–3318

[11] S. Morita, H. Tanaka, R. Konishi, T. Fruhara, Acta Mater. 51 (2003) 1789.[12] A. Aydin, E. Guler, H. Aktas, H. Gungunes, Bull. Mater. Sci. 25 (2002)

359.[13] T.N. Durlu, Scr. Metall. 15 (1981) 689.[14] A. Zararsız, A. Gedikoğlu, T.N. Durlu, Scr. Metall. 15 (1981) 999.[15] A. Zararsız, A. Gedikoğlu, T.N. Durlu, Scr. Metall. 15 (1981) 595.[16] S. Kajiwara, Acta Metall. 32 (1984) 407.

[17] E.X. Sun, D.Z. Yang, T.Y. Hsu, F.M. Yang, Y.W. Zhao, ISIJ Int. 29 (1989)154.

[18] T. Kakeshita, K. Kuroiwa, K. Shimizu, T. Ikeda, A. Yamagishi, M. Date,Mater. Trans., JIM 4 (1993) 423.

[19] J. Singh, C.M. Wayman, Mater. Sci. Eng. 93 (1987) 227.[20] J. Singh, C.M. Wayman, Mater. Sci. Eng. 93 (1987) 233.[21] K. Wakasa, C.M. Wayman, Metallography 14 (1981) 37.