Microstructure and Martensitic Transformation of Ni50Mn37. 5Sn12. 5 ...

6 downloads 925 Views 1MB Size Report
Available online at www.sciencedirect.com ... aInstitute of Metallurgy and Materials Science, Polish Academy of Sciences, ... bAGH University Science and Technology, Faculty of Physics and Applied Computer Science, ... slightly changes Ms temperature in alloys featuring magnetostructural coupling at room temperature.
Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 2S (2015) S523 – S528

International Conference on Martensitic Transformations, ICOMAT-2014

Microstructure and martensitic transformation of Ni50Mn37.5Sn12.5xGex (X=0, 1, 2, 3) Heusler alloys produced by various technologies W. Maziarza,*, P. Czajaa, A. Wójcika, J. Dutkiewicza, J. Przewoźnikb, E. Cesaric a

Institute of Metallurgy and Materials Science, Polish Academy of Sciences, 25 Reymonta Str, 30-059 Krakow, Poland AGH University Science and Technology, Faculty of Physics and Applied Computer Science, Department of Solid State Physics, Al. Mickiewicza 30, 30-059 Krakow, Poland c Department de Fisica, Universitat de les Illes Balears, Ctra. de Valldemossa, km 7.5, Palma de Mallorca E-07122, Spain

b

Abstract This paper presents the effects of isoelectronic substitution of Sn by Ge and subsequent microstructure refinement on microstructure and martensitic transformation in Ni50Mn37.5Sn12.5 Heusler alloys. It is observed that the increase of Ge content slightly changes Ms temperature in alloys featuring magnetostructural coupling at room temperature. The extent of chemical micro-segregation resulting from solidification conditions upon the employed melt spinning technique has significant influence on microstructure and martensitic transformation in the studied alloys. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by responsibility Elsevier Ltd. This is an open of access article under the CC BY-NC-ND license Transformations Selection and Peer-review under of the chairs the International Conference on Martensitic (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2014. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014.

Keywords: Ni-Mn-Sn Heusler alloys; melt spinning; micro-segregation; martensitic transformation; LM; SEM; TEM

1. Introduction Recently Ni-Mn-Sn Heusler alloys have gained attention due to reports of direct and an inverse magnetocaloric effect [1]. In general the former effect is associated with the second-order magnetic transition of austenite at the respective . The latter on the other hand is based on an abrupt magnetization change (ΔM) accompanying the reversible first order martensitic transformation from high symmetry ferromagnetic austenite to low symmetry, weak

* Corresponding author. Tel.: +48 12 295 2857; fax: +48 12 295 2804. E-mail address: [email protected]

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.339

524

W. Maziarz et al. / Materials Today: Proceedings 2S (2015) S523 – S528

magnetic martensite [2]. For the design and processing of high quality Ni-Mn-Sn alloys, two key points should be addressed as was proposed by Zheng et al. [3]. The first is the complex crystal structure of the low-temperature martensite phase and the second is the control of the martensitic transformation temperature. This second point is strongly correlated with the method of fabrication and application of heat treatment and also with composition, linked to the valence electron concentration (e/a) and atom size effects [4]. In recent literature concerning this alloy system four methods of fabrication can be distinguished as most popular that is arc melting, induction melting, melt spinning and single crystal growth. There are few papers concerning the influence of chemical micro-segregation on microstructure and magnetic properties observed in as melt-spun ribbons [5,6] as well as in chill cast sample [7]. The atom size effect on martensitic transformation and magnetocaloric effect in Ge substituted Ni-Mn-Sn alloys was investigated by Han et al. [8]. These authors found that substituting smaller Ge (atomic radius, r = 0.140 nm) for larger isoelectronic Sn (r = 0.163 nm) atoms in Ni43Mn46Sn11-xGex results in an increase of the phase transition temperatures. The effects of Ge addition on the martensitic transition and magnetic properties in the Mn-rich Heusler alloys of Mn2Ni1.6Sn0.4-xGex composition was reported by Xuan et al. [9]. It was found that an increase of Ge content leads to a rapid increase of the martensitic transformation temperature but a gradual decrease of the Curie temperature of austenitic phase. These two above cited papers were focused on the scenario, where the magnetostructural transitions take place below room temperature. In the present work the authors report results of microstructural and thermal analysis of Ni50Mn37.5Sn12.5-xGex system, which features the magnetic and structural transitions in the vicinity of room temperature, and which is prepared by two different technologies: induction casting combined with subsequent heat treatment and rapid solidification by melt spinning. 2. Experimental procedure Four polycrystalline ingots with Ni50Mn37.5Sn12.5-xGex (x = 0, 1, 2, 3) nominal compositions, hereafter referred to as 0Ge, 1Ge, 2Ge, 3Ge respectively, were prepared by means of induction melting from high purity metals (>99.9%) in an argon atmosphere. For homogenization the ingots were sealed in quartz tubes under vacuum and annealed for 23 h at 1220 K followed by water quenching. Some of the homogenized ingots’ pieces were used for production of ribbons by melt spinning method. Ribbon flakes were subsequently produced by ejecting the liquid alloys with argon overpressure (0.25 MPa) onto a surface of a copper wheel rotating at a linear speed of 25 m s -1. The process was carried out under an argon atmosphere. Thermal effects were investigated by differential scanning calorimetry (DSC) using a Mettler DSC 823 instrument in the temperature range 173-423 K and with heating-cooling rates of 10 K/min. X-ray diffraction (XRD) patterns were collected with a Philips (PW1830) diffractometer using CoKα radiation. The data obtained were analyzed using the profile fitting program FullProf based on the Rietveld method. Microstructure and chemical composition of the samples were examined with Leica DM IRM light microscope (LM), scanning electron microscope (SEM) FEI ESEM XL30 equipped with X-ray energy dispersive spectrometer EDAX GEMINI 4000 and a transmission electron microscope (TEM) Tecnai G2 operating at 200 kV equipped with an Energy Dispersive X-ray (EDX) microanalyser and High Angle Annular Dark Field Detector (HAADF). Thin foils for TEM were prepared with TenuPol-5 double jet electropolisher using an electrolyte of phosphoric acid (20%) and ethanol (80%) at 243 K. 3. Results and Discussion The characteristic microstructures, taken with SEM in the Back Scattered (BSE) and Secondary (SE) Electron modes, of investigated alloys in bulk form as well as melt spun ribbons are presented in Fig. 1. The bulk alloys feature mainly single phase martensite microstructure consisting of several variants of martensite plates (Fig. 1a) with coarse grains in the size of hundreds of microns. The cross section of ribbons shows typical columnar grains with their longer axis perpendicular to the ribbon plane (Fig. 1b). Such microstructure is very often reported for Heusler alloy melt spun ribbons [10,11] and generally it is accepted that it is formed due to heat gradient during rapid solidification. The free side surface of ribbons (Figs. 1c and d) is characterized by a more complex morphology consisting of two types of regions. The first one is formed by regular dendritic grains of size of about 40 m with characteristic conical shape (Fig. 1c), and the second is represented by fine cellular grains of size of about 1.5 m (Fig. 1d), which surround dendritic regions. Existence of such kind of grains’ morphology at the free ribbon surface

525

W. Maziarz et al. / Materials Today: Proceedings 2S (2015) S523 – S528

indicates that two types of crystal growth occurred during rapid solidification: dendritic and cellular. In the case of dendritic growth the micro-segregation is obvious due to two phase structure of dendrites and inter-dendritic areas respectively, whereas cellular growth is characterized by homogeneous chemical composition of grains. The chemical composition of ribbons measured at free and wheel sides as well as of bulk samples is presented in Table 1. Generally the composition of the studied alloys agrees with the nominal composition. Any deviations, especially with respect to Mn content may be ascribed to the accuracy of EDS and relative ease of Mn evaporation during casting and subsequent heat treatment.

Fig. 1. SEM microstructures of: a) 3Ge bulk sample polished surface (BSE) b) 0Ge ribbon fracture cross section (SE), c) 1Ge and d) 2Ge ribbons free side surface (SE). Table 1. EDS chemical composition of bulk and ribbon samples measured for the latter at both free and wheel side surfaces and the corresponding e/a ratio. Alloy

0Ge

1Ge

2Ge

3Ge

at. % Ni

Mn

Sn

Ge

e/a

Bulk

52.0

36.2

11.8

-

8.21

Free side

53.5

34.4

12.1

-

8.24

Wheel side

53.5

34.6

12.0

-

8.24

Bulk

51.5

36.0

11.0

1.6

8.17

Free side

52.9

33.8

11.3

2.0

8.19

Wheel side

52.7

34.3

11.1

1.9

8.19

Bulk

51.0

36.0

9.9

3.0

8.14

Free side

51.0

36.3

9.9

2.9

8.15

Wheel side

51.3

36.2

9.8

2.7

8.16

Bulk

51.1

36.1

9.3

3.5

8.15

Free side

51.3

36.0

8.9

3.8

8.16

Wheel side

51.2

35.9

8.9

4.0

8.15

Figure 2 presents DSC curves after second cooling-heating cycle recorded for bulk solution treated alloys (Fig. 2a) and melt-spun ribbons (Fig. 2b). Two different phenomena can be observed from these results. In the case of bulk alloys with increase of Ge content an increase of martensite start temperature (Ms) by about 3 to 6K is observed,

526

W. Maziarz et al. / Materials Today: Proceedings 2S (2015) S523 – S528

whereas in the case of ribbons a decrease of Ms temperature by about 1 to 10K is noted. The graphic representation of changes of characteristic temperatures of martensitic and reverse transformations in bulk samples and ribbons is presented in Fig. 3.

Fig. 2. DSC curves after second cooling-heating cycle of (a) bulk samples after solution treatment and (b) as cast ribbons.

Fig. 3. Characteristic temperatures martensite start (Ms), martensite finish (Mf), austenite start (As), austenite finish (Af) versus alloy compositions of (a) bulk solution treated samples and b) ribbons.

The Ms temperature in both bulk and ribbon alloys lies in the temperature range between 310 and 330K. It is seen that for bulk alloys the martensite transformation temperature increases initially for 1Ge when compared to 0Ge sample. However it is observed that further increasing of Ge content to 2 at. % and 3 at. % is less significant in terms of further characteristic transformation temperatures increase. In relation to isoelectronic substitution of Sn by Ge the initially observed Ms temperature increase may be primarily attributed to the difference in atomic radii between these elements. Because Ge has lower atomic radius than Sn the decreasing of unit cell volume of austenite is likely to occur with increasing Ge for Sn substitution influencing in result martensite transition temperature [8]. These assumptions are supported by X-ray diffraction studies (Fig. 4) combined with the Rietveld refinement performed for bulk samples (Table 2). It was observed that all the bulk samples feature the two phase structure consisting of the austenite cubic L21 phase with Fm3m symmetry and orthorhombic 4O modulated martensite with Pmma symmetry. The volume fractions of these phases increase with increasing content of Ge along with the reduction in unit cell volumes of austenite and martensite. The decrease of unit cell volume of austenite and a simultaneous increase of martensite fraction in the two phase structure agrees well with the current view on the impact of the transformational volume change on the martensite transition temperature in Ni-Mn based alloys [12]. The crystalline structure and grain size of ribbons and bulk samples were verified by microstructure observations performed by both TEM and LM. Fig. 5 presents typical microstructures for investigated alloys. LM picture of microstructure of 0Ge alloy in bulk form is presented in Fig. 5a.

527

W. Maziarz et al. / Materials Today: Proceedings 2S (2015) S523 – S528

Fig. 4. Set of X-Ray diffraction patterns performed for solution treated bulk samples. Table 2. Lattice parameters (ac) and (am, bm,cm), unit cell volumes (Va) and (Vm) and relative mass contributions (pa) and (pm) of austenite and martensite phase, respectively. Alloy

L21 Fm3m Va

4O Pmma

ac [nm]

[nm ]

pa [%]

am [nm]

bm [nm]

cm [nm]

[nm ]

pm [%]

0Ge

0.60067(8)

0.216738(46)

13.65(0.62)

0.86122(6)

0.56273(5)

0.43743(4)

0.211989(28)

86.35(2.4)

1Ge

0.60010(12)

0.21611(8)

5.56 ( 0.60)

0.86042(4)

0.56282(3)

0.43674(3)

0.211494(19)

94.4( 3.7)

2Ge

0.59975

0.215730

3.6(0.72)

0.85895(4)

0.56257(4)

0.43570(4)

0.210539(26)

96.4( 5.5)

3Ge

0.59975(22)

0.21573(14)

1.94(0.47)

0.85746(4)

0.56135(4)

0.43498(3)

0.209372(22)

98.1( 4.4)

3

Vm

3

Coarse martensitic grains of size scattered from 50 up to 200 m with martensite variants and network of cracks can be observed. Although the grain size of bulk alloys with Ge addition is larger as compared to the ribbons by about two orders of magnitude the difference in Ms temperature is only about 10 K. Wang et al. [12] reported that the internal stress is induced into Ni50Mn41Sn9 melt spun ribbons due to the highly-oriented microstructure, which leads to the decrease of the transition temperature because of the refined martensite plate and the formation of dense martensitic variants with different orientations. However in this case in ternary alloys (0Ge) Ms temperature of bulk alloy is slightly lower than in the case of ribbon by about 3 K. So theory concerning the increase of internal stress is no always obvious and global structure should be taken into account. Scanning transmission electron microscopy (STEM-HAADF) micrograph of 0Ge ribbon sample (Fig. 5b) shows area of ribbon (from dendritic grains presented in Fig. 1c) with two phase microstructure consisting of equi-axed grains (dendrites) of size of about 1.5 m of austenite and martensite phase of width of about 200 nm located at grain boundaries (inter-dendritic areas) of austenite. The crystal structures of these phases were confirmed by selected area diffraction patterns (SADP) as the L21 austenite and the 4O martensite structures with [001] and [010] zone axes, respectively. The ribbons with Ge additions contain a significant greater fraction of martensite and also are characterized by a more uniformed microstructure what is presented in Figs. 5c and d. Transmission electron microscopy bright field (TEM-BF) micrographs show single phase microstructure consisting of uniform equi-axed grains of martensite of size of about 1.5 m. The structure of martensite was confirmed as four-layered 4O modulated martensite (the same type as in the two phase structure presented in Fig. 5b) by use of SADP showing two variants of 4O martensite with [012] zone axis (Fig. 5d inset). The morphology of grains indicates that in the case of ribbons with Ge additions the cellular crystal growth is prevalent, what leads to the chemically homogeneous microstructure. It can be stated that the main reason for the change in the type of crystal growth during solidification as well as homogeneity of microstructure is the difference in melting point of Sn and Ge. Substitution of Sn (low melting point element - 504K) by Ge (high melting point element - 1211K) leads to more homogenous melt, narrowing the liquids and solidus region and finally prevented the micro-segregation during solidification.

528

W. Maziarz et al. / Materials Today: Proceedings 2S (2015) S523 – S528

Fig. 5. Typical microstructures of investigated materials (a) LM image of 0Ge bulk sample, (b) STEM-HAADF image of 0Ge ribbon, (c) and (d) TEM-BF images of 2Ge and 3Ge ribbons with corresponding selected area diffraction pattern.

4. Summary In summary, the effect of Ge substitution for Sn on microstructure and martensitic transformation in Ni50Mn37.5Sn12.5-xGex (X=0, 1, 2, 3) Heusler alloys produced by two technologies was investigated. It was found that the atom size effect of substitution by isoelectronic elements is insignificant in the case of Ni-Mn-Sn alloys both bulk and melt spun ribbons having the structural transitions in the vicinity of room temperature. Based on microstructural investigations it was found that both in the case of ribbons as well as heat treated bulk samples the chemical micro-segregation can be expected due to solidification conditions. Therefore in investigations of Heusler alloys obtained by mass production technologies (induction melting or melt spinning) one should consider the influence of possible chemical micro-segregation effects. Acknowledgements This work was financially supported by Polish National Science Centre in frame of Project DEC2012/06/M/ST8/00451. References [1] Y. Sutou, Y. Imano, N. Koeda, T. Omori, R. Kainum, K. Ishida, K. Oikawa, Appl. Phys. Lett. 88 (2006) 4358– 4360. [2] X. Moya, Ll. Masa, A. Planes, T. Krenke, E. Duman, M. Acet, E.F. Wassermann, J. Magn. Magn. Mater. 316 (2007) 572 –574. [3] H. Zheng, W. Wang, S. Xue, Q. Zhai, J. Frenzel, Z. Luo, Acta Mater. 61 (2013) 4648–4656. [4] T. Krenke, X. Moya, S. Aksoy, M. Acet, P. Entel, Ll. Manosa, A. Planes, Y. Elerman, A. Yucel, E.F. Wassermann, J. Magn. Magn. Mater. 310 (2007) 2788–2789 [5] W. Maziarz, P. Czaja, M.J. Szczerba, L. Lityńska-Dobrzyńska, T. Czeppe, J. Dutkiewicz, J. Alloys Compd. 615 (2014) 173–177. [6] W. Maziarz, P. Czaja, J. Dutkiewicz, R. Wróblewski, M. Leonowicz, Mater. Sci. Forum 782 (2014) 23 –30 [7] D.L. Schlagel, R.W. McCalluma, T.A. Lograsso, J. Alloys Compd. 463 (2008) 38–46. [8] Z.D. Han, D.H.Wang, C.L. Zhang, H.C. Xuan, J.R. Zhang, B.X. Gu, Y.W. Du, Mater. Sci. Eng. B 157 (2009) 40-43. [9] H.C. Xuan P.D. Han, D.H. Wang, Y.W. Du, J. Alloys Compd. 582 (2014) 369–373 [10] P. Czaja, W. Maziarz, J. Przewoznik, A. Zywczak, P. Ozga, M. Bramowicz, S. Kulesza, J. Dutkiewicz, Intermetallics 55 (2014) 1 –8 [11] B. Hernando, J.L. Sanchez Llamazares, J.D. Santos, M.L. Sanchez, Ll. Escoda, J.J. Sunol, R. Varga, C. Garcia, J. Gonzalez, J. Magn. Magn. Mater. 321 (2009) 763 –768. [12] R.L. Wang, J.B. Yan, H.B. Xiao, L.S. Xu, V.V. Marchenkov, L.F. Xu, C.P. Yang, J. Alloys Compd. 509 (2011) 6834–6837. [13] W. Wang, J. Yu, Q. Zhai, Z. Luo, H. Zheng, Intermetallics 42 (2013) 126 –129.