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Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment
Journal of Rare Earths xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment A. Łaszcz*, M. Hasiak, J. Kaleta Wrocław University of Science and Technology, Department of Mechanics, Materials Science and Engineering, Smoluchowskiego 25, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2018 Received in revised form 16 January 2019 Accepted 23 January 2019 Available online xxx
Polycrystalline Ni50Mn25Ga20Gd5 (at%) magnetic shape memory alloy was investigated in the asprepared state and after annealing at 1430 K for 3 h. Microstructural analysis reveals dual-phase nature of the material with substantial distinction between Gd-rich and Gd-poor phases. Magnetic measurements performed in wide range of temperatures confirm reversible martensitic transformation in the annealed sample undergoing close to the room temperature. When it comes to the magnetic transition, the Curie temperature of the investigated alloy remains approximately unchanged at 370 K. Topography investigations conducted on the atomic force microscope in contact mode allow to measure 8 mm difference between minimum and maximum point of the martensite profile. The results from a series of nanoindentation tests show that hardness of the Gd-rich phase is 23%e35% higher than hardness of the Gd-poor phase, depending on the annealing state. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: NiMnGa Magnetic shape memory alloys Reversible martensitic transition Gd-doped alloys Rare earths
1. Introduction NiMnGa-based magnetic shape memory alloys (MSMAs) are becoming one of the most promising group of modern multifunctional smart materials. As a result of complex interactions between structural, magnetic and mechanical properties in these materials MSMAs exhibit numerous interesting magnetically, thermally and mechanically induced effects, such as superelasticity,1 magnetic field induced strains,2e5 large magnetoresistance,6 exchange bias7 or magneto and mechanocaloric effects.8e11 All abovementioned properties stem from reversible martensiticeaustenitic transition and twin boundary mobility in martensite phase.3 What is also interesting, phase transformation in MSMAs does not depend on magnetic transition and is far more sensitive to chemical composition. Considering this behaviour, the idea of an introduction of the fourth alloying element to the NiMnGa-based materials draws attention of many researchers. It was recently reported in several papers that some alloying metallic elements (e.g. Co, Cu, Fe or Al) may significantly change the temperature of phase transformations and improve mechanical strength as well as ductility of
* Corresponding author. E-mail address: [email protected] (A. Łaszcz).
MSMAs.12e16 Furthermore, rare earth elements, such as Nd, Sm, Tb or Gd are also considered as the potential alloying elements due to their complex magnetic behaviour.17,18 Gadolinium is one of the most promising rare earth candidates for beneficial alloying of NiMnGa-based alloys. It was reported in Refs. 18e20 that Gd addition to Ni-Mn-Ga composition improves mechanical properties of the material by promotion of a dual-phase microstructure and refinement strengthening. Moreover, the addition of Gd also has significant influence on reversible martensitic transformation temperatures in (NiMnGa)-Gd materials.20e22 Nevertheless, due to the many factors like ratio between concentrations of Ni/Mn/Ga/Gd atoms or annealing conditions the influence of Gd on physical properties of MSMAs is still not yet adequately examined. Thus, the aim of this study is to investigate the influence of post-preparation heat treatment on microstructure, magnetic and nanomechanical properties of the Gd-doped Ni50Mn25Ga20Gd5 (at%) alloy.
2. Experimental The material with the nominal composition of Ni50Mn25Ga20Gd5 (at%) was prepared from high purity elements in a form of buttonshape sample by arc-melting method under a protective argon
https://doi.org/10.1016/j.jre.2019.01.004 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
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atmosphere. To ensure a good homogeneity, the ingot was remelted several times during preparation process. The fabricated alloy was subsequently sealed in a vacuumed quartz tube and annealed at 1430 K for 3 h. This annealing temperature was determined from differential scanning calorimetry (DSC) investigation to increase the volume fraction of Gd-poor phase in the Ni50Mn25Ga20Gd5 material. Microstructure and chemical composition of the produced alloy were analysed by a scanning electron microscope (SEM, JOEL JSM-5800LV) equipped with an energy-dispersive detector (EDS). Microstructural investigations were supplemented by an
atomic force microscope (AFM, Park System XE-100) measurements performed in contact mode. Temperature dependences of magnetization in zero-field cooled (ZFC) and field cooled (FC) modes in a temperature range from 50 to 400 K and external magnetic field of 10 mT were recorded by VersaLab System (Quantum Design). Thermomagnetic curves were used to determine both structural and/or magnetic transformation temperatures. In order to investigate the nanomechanical properties of the Ni50Mn25Ga20Gd5 alloy, prepared samples were subjected to a series of nanoindentation tests conducted on a nanoindentation
Fig. 1. SEM BSE images with corresponding EDX spectra for the Gd-poor and Gd-rich phases of the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy.
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
A. Łaszcz et al. / Journal of Rare Earths xxx (xxxx) xxx
tester (NHT2, CSM Instruments) equipped with three-sided pyramid Berkovich tip. Maximum applied load was set to 20 mN with 40 mN/min loading and unloading rate and 10 s dwell time at
3
maximum load. Recorded load-displacement characteristics were analysed following the Oliver-Pharr protocol.23,24 Above procedure
Table 1 Chemical compositions of Gd-poor and Gd-rich phases for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy obtained from EDX analysis. Condition
Phase
Ni content (at%)
Mn content (at%)
Ga content (at%)
Gd content (at%)
As-prepared
Dark “Gd-poor” phase Bright “Gd-rich” phase Dark “Gd-poor” phase Bright “Gd-rich” phase
51.3 50.8 51.5 57.3
27.6 20.5 26.2 9.7
19.3 19.7 20.3 19.2
1.8 9.0 2.0 13.8
Annealed at 1430 K/3 h
Fig. 2. 2D and 3D AFM images of the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy. The close-up presents martensitic structure morphology with corresponding profile across the line AB.
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
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allows to estimate hardness of each individual phase of the investigated material. 3. Results and discussion 3.1. Microstructure and topography Fig. 1 depicts backscattered electron SEM images and corresponding EDX spectra for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy. It is well seen that annealing process has significant influence on material microstructure. The as-prepared sample was characterized by uneven dendritic dual-phase microstructure, whereas in the annealed sample dark matrix grains are fully surrounded by continuous bright second phase net. The chemical compositions of each observable phase are summarized in Table 1. According to this analysis, dark and bright phases seen in Fig. 1 correspond to Gd-poor and Gd-rich phase, respectively. In both as-prepared and annealed samples concentration of Gd in Gdpoor phase (dark region) is about 2.0 at%. For the Gd-rich phase (bright region) the increase in the Gd concentration from 9.0 at% for the as-prepared material to 13.8 at% for annealed material was clearly observed. Moreover, the image analysis shows that the volume fraction of dark Gd-poor phase increases significantly after the annealing process from 55% for the as-prepared sample to 76% for the annealed sample. The increasing of the Gd concentration in Gd-rich phase after the heat treatment is connected with the decreasing of a volume fraction of the bright phase (Fig. 1). Complementary 2D and 3D AFM images of the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy are presented in Fig. 2. When it comes to the as-prepared material only a slight outline of existing phases on the depicted surface morphology are visible. On the contrary, the annealed alloy presents a clear dual phase microstructure, which corresponds with the SEM images. Moreover, AFM studies for the annealed sample (Fig. 2(b)) reveal some regions similar to the thin needles, which are characteristic for martensitic structures. The close-up of the exemplary martensitic region is presented in Fig. 2(b). Line A-B shows the profile across five martensite laths, where the maximum height of one lath reaches about 8 nm. 3.2. Magnetic properties Thermal dependence of magnetization for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy is depicted in Fig. 3. Thermomagnetic measurements were conducted under the low value of external magnetic field (m0H ¼ 10 mT) to ensure the significant changes in magnetization of the samples during the phase transformations.25 This behaviour describing the reversible martensitic transition is well seen for the Ni50Mn25Ga20Gd5 sample annealed at 1430 K for 3 h (Fig. 3(b)). In this case, sudden increase of magnetization from 1.1 to 3.1 A m2/kg on the ZFC branch during heating (starting at As ¼ 276 K and ending at Af ¼ 300 K) corresponds to the martensite to austenite transition, as in this case austenite phase is characterised by higher magnetization than martensite one. Consequently, abrupt drop of magnetization from 4.2 to 2.5 A m2/kg on the FC branch during cooling (starting at Ms ¼ 282 K and ending at Mf ¼ 258 K) characterises martensite to austenite transformation. It is clearly observable that these abrupt changes of magnetization related to structural transition (visible at the temperature range 225e325 K) are not apparent in the as-prepared sample (Fig. 3(a)). This behaviour is connected with the previously described microstructure evolution. The significant amount of Gd-rich phase is responsible for the disappearance of structural transformation in the as-prepared alloy. The decrease of magnetization in the vicinity of 370 K for the both as-prepared and
Fig. 3. Thermal dependence of magnetization for the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy measured in zero field (ZFC) and field cooled (FC) modes at external magnetic field of 10 mT.
annealed samples is connected with magnetic transition characterized by the Curie point. Thermomagnetic curves recorded in ZFC and FC modes show that the annealing process leads to the increase of the overall magnetic transformation temperature by approximately 10 K. It is worth noticing that the dual-phase nature (described by two different Curie temperatures) of the investigated materials is not observable on M(T) characteristics (Fig. 3) due to the relatively close Curie points of the Gd-rich and Gd-poor phases (the Curie points of the as-prepared and annealed Gd-free Ni50Mn25Ga25 alloy are about 380 K). All the temperatures of structural and magnetic transitions, including austenitic/martensitic transformation start temperatures (As/Ms), austenitic/ martensitic transformation finish temperatures (Af/Mf) and the C Curie temperature during heating/cooling (TH C ,TC) are summarised in Table 2. As a last point, it should also be mentioned that reversible structural transformation in the studied alloy undergoes
Table 2 Temperatures of structural and magnetic transformations in as-prepared and annealed Ni50Mn25Ga20Gd5 alloy (As/Ms e austenitic/martensitic transformation start temperatures, Af/Mf e austenitic/martensitic transformation finish temperaC tures, TH C /TC e Curie temperature during heating/cooling). Condition
Ms (K)
Mf (K)
As (K)
Af (K)
TH C (K)
TCC (K)
As-prepared Annealed at 1430 K/3 h
e 282
e 258
e 276
e 300
370 379
352 368
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
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Fig. 5. Examples of load-displacement curves for the Gd-rich (filled markers) and Gdpoor phases (unfilled markers) of the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy.
hardness of the Gd-poor phase decreases slightly from 4981 to 4772 MPa, whereas the average hardness of the Gd-rich phase drops substantially from 7678 to 6263 MPa. Observed decrease in hardness is connected with previously described evolution of the chemical composition of each phase created in the Ni50Mn25Ga20Gd5 alloy.
4. Conclusions
Fig. 4. DC hysteresis loops for the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy measured at 100, 250 and 400 K in a field up to 2 T.
close to the room temperature, which is especially important from the potential applications point of view. Fig. 4 presents the DC hysteresis loops for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy measured at indicated temperatures. In both as-prepared and annealed states of the investigated material the loops recorded at 100 and 250 K relate to the magnetization process of the ferromagnetic martensite phase. The hysteresis loop recorded at 400 K (above the Curie temperature e according to M(T) curves) characterises austenite phase occurring in paramagnetic state. What is worth noting, magnetization of the annealed alloy is almost two times higher than magnetization of the as-prepared material.
3.3. Mechanical properties Mechanical properties of the dual-phase Ni50Mn25Ga20Gd5 alloy in the as-prepared and annealed states were studied by the means of nanoindentation tests, according to the protocol described in experimental procedure. Fig. 5 presents the examples of loaddisplacement curves for the Gd-rich and Gd-poor phases of the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy. It is clearly seen that hardness of Gd-rich phase is significantly higher than hardness of Gd-poor phase. It indicates that Gd addition to the Ni-Mn-Ga composition has significant influence on mechanical properties of the particular phases. In regard to the post-preparation heat treatment, the annealed alloy exposes noticeably lower hardness than the as-prepared material. After annealing the average
This work presents the influence of the heat treatment on microstructure, magnetic and nanomechanical properties of the Ni50Mn25Ga20Gd5 magnetic shape memory alloy. Microstructural investigation shows characteristic dendritic dual phase microstructure in the as-prepared sample. Subsequent annealing of the alloy at 1430 K for 3 h leads to the aggregation of Gd-rich phase along matrix grain boundaries. Microstructure evolution has substantial influence on magnetic properties in the investigated alloy and only annealed sample undergoes reversible martensitic transition. Moreover, the structural transition temperatures were estimated close to the room temperature, which is important for the potential future applications of this material. The Curie temperatures of the Ni50Mn25Ga20Gd5 alloy in both states stay close to 370 K. Detailed nanoindentation tests show that Gd-rich phase is significantly harder than Gd-poor phase by about 35% and 23% for the as-prepared and annealed sample, respectively. All of the presented studies show that addition of Gd and subsequent heat treatment have significant influence on microstructure as well as magnetic and mechanical properties of the polycrystalline Ni50Mn25Ga20Gd5 multifunctional material.
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5. Chmielus M, Zhang XX, Witherspoon C, Dunand DC, Müllner P. Giant magneticfield-induced strains in polycrystalline NieMneGa foams. Nat Mater. 2009;8(11):863. 6. Pal D, Mandal K, Gutfleisch O. Large negative magnetoresistance in nickel-rich NieMneGa Heusler alloys. J Appl Phys. 2010;107(9), 09B103. 7. Han ZD, Qian B, Wang DH, Zhang P, Jiang XF, Zhang CL, et al. Magnetic phase separation and exchange bias in off-stoichiometric Ni-Mn-Ga alloys. Appl Phys Lett. 2013;103(17):172403. ~ osa L, Acet M. Magnetocaloric effect and its relation to shape8. Planes A, Man memory properties in ferromagnetic Heusler alloys. J Phys Condens Matter. 2009;21(23):233201. n T, Man ~ osa L, Planes A, Mathur ND, Moya X. Multicaloric 9. Stern-Taulats E, Casta materials and effects. MRS Bull. 2018;43(4):295. 10. McLeod MV, Giri AK, Paterson BA, Dennis CL, Zhou L, Vogel SC, et al. Magnetocaloric response of non-stoichiometric Ni2MnGa alloys and the influence of crystallographic texture. Acta Mater. 2015;97:245. 11. Gschneidner KA, Pecharsky VK. Rare earths and magnetic refrigeration. J Rare Earths. 2006;24(6):641. 12. Xin Y, Li Y, Chai L, Xu HB. Shape memory characteristics of dual-phase NieMneGa based high temperature shape memory alloys. Scr Mater. 2007;57(7):599. 13. Yang SY, Ma YQ, Jiang HF, Liu XJ. Microstructure and shape-memory characteristics of Ni56Mn25xCoxGa19 (x ¼ 4, 8) high-temperature shape-memory alloys. Intermetallics. 2011;19(2):225. 14. Ma YQ, Yang SK, Jin WJ, Liu XJ. Ni56Mn25xCuxGa19 (x ¼ 0, 1, 2, 4, 8) hightemperature shape-memory alloys. J. Alloys Compd. 2009;471(1e2):570. 15. Yang SY, Liu Y, Wang CP, Shi Z, Liu XJ. The mechanism clarification of NieMneFeeGa alloys with excellent and stable functional properties. J Alloys Compd. 2013;560:84.
16. Xin Y, Li Y. Microstructure and transformation behaviour of Ni54Mn25Ga15Al6 high-temperature shape memory alloy. Trans Nonferrous Met Soc China. 2014;24(1):126. 17. Tsuchiya K, Tsutsumi A, Ohtsuka H, Umemoto M. Modification of NieMneGa ferromagnetic shape memory alloy by addition of rare earth elements. Mater Sci Eng A. 2004;378(1e2):370. 18. Zhang X, Sui JH, Zheng XH, Yang ZY, Cai W. Effects of Gd doping on Ni54Mn25Ga21 high-temperature shape memory alloy. Mater Sci Eng A. 2014;597:178. 19. Gao L, Cai W, Liu AL, Zhao LC. Martensitic transformation and mechanical properties of polycrystalline Ni50Mn29Ga21xGdx ferromagnetic shape memory alloys. J. Alloys Compd. 2006;425(1e2):314. 20. Zhang X, Sui JH, Yu ZL, Cai W. Structure and shape memory effect in a Ni54Mn25Ga20Gd1 alloy with a high transformation temperature. J Alloys Compd. 2011;509(31):8032. 21. Cai W, Gao L, Gao ZY. Microstructure and phase transformations in a Ni50Mn29Ga16Gd5 alloy with a high transformation temperature. J Mater Sci. 2007;42(22):9216. 22. Bao B, Long Y, Duan JF, Wu GH, Ye RC, Wan FR. Phase transition and magnetocaloric effect of Ni55.2Mn18.6Ga26.2exGdx (x ¼ 0, 0.05, 0.15) alloys. J Rare Earths. 2009;27(5):875. 23. Oliver WC, Pharr GM. Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology. J Mater Res. 2004;19(1):3. 24. Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res. 1992;7(6):1564. 25. Chernenko VA, L'vov VA, Cesari E. Martensitic transformation in ferromagnets: experiment and theory. J Magn Magn Mater. 1999;196-197:859.
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
Contents lists available at ScienceDirect
Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment A. Łaszcz*, M. Hasiak, J. Kaleta Wrocław University of Science and Technology, Department of Mechanics, Materials Science and Engineering, Smoluchowskiego 25, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2018 Received in revised form 16 January 2019 Accepted 23 January 2019 Available online xxx
Polycrystalline Ni50Mn25Ga20Gd5 (at%) magnetic shape memory alloy was investigated in the asprepared state and after annealing at 1430 K for 3 h. Microstructural analysis reveals dual-phase nature of the material with substantial distinction between Gd-rich and Gd-poor phases. Magnetic measurements performed in wide range of temperatures confirm reversible martensitic transformation in the annealed sample undergoing close to the room temperature. When it comes to the magnetic transition, the Curie temperature of the investigated alloy remains approximately unchanged at 370 K. Topography investigations conducted on the atomic force microscope in contact mode allow to measure 8 mm difference between minimum and maximum point of the martensite profile. The results from a series of nanoindentation tests show that hardness of the Gd-rich phase is 23%e35% higher than hardness of the Gd-poor phase, depending on the annealing state. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: NiMnGa Magnetic shape memory alloys Reversible martensitic transition Gd-doped alloys Rare earths
1. Introduction NiMnGa-based magnetic shape memory alloys (MSMAs) are becoming one of the most promising group of modern multifunctional smart materials. As a result of complex interactions between structural, magnetic and mechanical properties in these materials MSMAs exhibit numerous interesting magnetically, thermally and mechanically induced effects, such as superelasticity,1 magnetic field induced strains,2e5 large magnetoresistance,6 exchange bias7 or magneto and mechanocaloric effects.8e11 All abovementioned properties stem from reversible martensiticeaustenitic transition and twin boundary mobility in martensite phase.3 What is also interesting, phase transformation in MSMAs does not depend on magnetic transition and is far more sensitive to chemical composition. Considering this behaviour, the idea of an introduction of the fourth alloying element to the NiMnGa-based materials draws attention of many researchers. It was recently reported in several papers that some alloying metallic elements (e.g. Co, Cu, Fe or Al) may significantly change the temperature of phase transformations and improve mechanical strength as well as ductility of
* Corresponding author. E-mail address: [email protected] (A. Łaszcz).
MSMAs.12e16 Furthermore, rare earth elements, such as Nd, Sm, Tb or Gd are also considered as the potential alloying elements due to their complex magnetic behaviour.17,18 Gadolinium is one of the most promising rare earth candidates for beneficial alloying of NiMnGa-based alloys. It was reported in Refs. 18e20 that Gd addition to Ni-Mn-Ga composition improves mechanical properties of the material by promotion of a dual-phase microstructure and refinement strengthening. Moreover, the addition of Gd also has significant influence on reversible martensitic transformation temperatures in (NiMnGa)-Gd materials.20e22 Nevertheless, due to the many factors like ratio between concentrations of Ni/Mn/Ga/Gd atoms or annealing conditions the influence of Gd on physical properties of MSMAs is still not yet adequately examined. Thus, the aim of this study is to investigate the influence of post-preparation heat treatment on microstructure, magnetic and nanomechanical properties of the Gd-doped Ni50Mn25Ga20Gd5 (at%) alloy.
2. Experimental The material with the nominal composition of Ni50Mn25Ga20Gd5 (at%) was prepared from high purity elements in a form of buttonshape sample by arc-melting method under a protective argon
https://doi.org/10.1016/j.jre.2019.01.004 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
2
A. Łaszcz et al. / Journal of Rare Earths xxx (xxxx) xxx
atmosphere. To ensure a good homogeneity, the ingot was remelted several times during preparation process. The fabricated alloy was subsequently sealed in a vacuumed quartz tube and annealed at 1430 K for 3 h. This annealing temperature was determined from differential scanning calorimetry (DSC) investigation to increase the volume fraction of Gd-poor phase in the Ni50Mn25Ga20Gd5 material. Microstructure and chemical composition of the produced alloy were analysed by a scanning electron microscope (SEM, JOEL JSM-5800LV) equipped with an energy-dispersive detector (EDS). Microstructural investigations were supplemented by an
atomic force microscope (AFM, Park System XE-100) measurements performed in contact mode. Temperature dependences of magnetization in zero-field cooled (ZFC) and field cooled (FC) modes in a temperature range from 50 to 400 K and external magnetic field of 10 mT were recorded by VersaLab System (Quantum Design). Thermomagnetic curves were used to determine both structural and/or magnetic transformation temperatures. In order to investigate the nanomechanical properties of the Ni50Mn25Ga20Gd5 alloy, prepared samples were subjected to a series of nanoindentation tests conducted on a nanoindentation
Fig. 1. SEM BSE images with corresponding EDX spectra for the Gd-poor and Gd-rich phases of the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy.
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
A. Łaszcz et al. / Journal of Rare Earths xxx (xxxx) xxx
tester (NHT2, CSM Instruments) equipped with three-sided pyramid Berkovich tip. Maximum applied load was set to 20 mN with 40 mN/min loading and unloading rate and 10 s dwell time at
3
maximum load. Recorded load-displacement characteristics were analysed following the Oliver-Pharr protocol.23,24 Above procedure
Table 1 Chemical compositions of Gd-poor and Gd-rich phases for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy obtained from EDX analysis. Condition
Phase
Ni content (at%)
Mn content (at%)
Ga content (at%)
Gd content (at%)
As-prepared
Dark “Gd-poor” phase Bright “Gd-rich” phase Dark “Gd-poor” phase Bright “Gd-rich” phase
51.3 50.8 51.5 57.3
27.6 20.5 26.2 9.7
19.3 19.7 20.3 19.2
1.8 9.0 2.0 13.8
Annealed at 1430 K/3 h
Fig. 2. 2D and 3D AFM images of the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy. The close-up presents martensitic structure morphology with corresponding profile across the line AB.
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
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allows to estimate hardness of each individual phase of the investigated material. 3. Results and discussion 3.1. Microstructure and topography Fig. 1 depicts backscattered electron SEM images and corresponding EDX spectra for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy. It is well seen that annealing process has significant influence on material microstructure. The as-prepared sample was characterized by uneven dendritic dual-phase microstructure, whereas in the annealed sample dark matrix grains are fully surrounded by continuous bright second phase net. The chemical compositions of each observable phase are summarized in Table 1. According to this analysis, dark and bright phases seen in Fig. 1 correspond to Gd-poor and Gd-rich phase, respectively. In both as-prepared and annealed samples concentration of Gd in Gdpoor phase (dark region) is about 2.0 at%. For the Gd-rich phase (bright region) the increase in the Gd concentration from 9.0 at% for the as-prepared material to 13.8 at% for annealed material was clearly observed. Moreover, the image analysis shows that the volume fraction of dark Gd-poor phase increases significantly after the annealing process from 55% for the as-prepared sample to 76% for the annealed sample. The increasing of the Gd concentration in Gd-rich phase after the heat treatment is connected with the decreasing of a volume fraction of the bright phase (Fig. 1). Complementary 2D and 3D AFM images of the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy are presented in Fig. 2. When it comes to the as-prepared material only a slight outline of existing phases on the depicted surface morphology are visible. On the contrary, the annealed alloy presents a clear dual phase microstructure, which corresponds with the SEM images. Moreover, AFM studies for the annealed sample (Fig. 2(b)) reveal some regions similar to the thin needles, which are characteristic for martensitic structures. The close-up of the exemplary martensitic region is presented in Fig. 2(b). Line A-B shows the profile across five martensite laths, where the maximum height of one lath reaches about 8 nm. 3.2. Magnetic properties Thermal dependence of magnetization for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy is depicted in Fig. 3. Thermomagnetic measurements were conducted under the low value of external magnetic field (m0H ¼ 10 mT) to ensure the significant changes in magnetization of the samples during the phase transformations.25 This behaviour describing the reversible martensitic transition is well seen for the Ni50Mn25Ga20Gd5 sample annealed at 1430 K for 3 h (Fig. 3(b)). In this case, sudden increase of magnetization from 1.1 to 3.1 A m2/kg on the ZFC branch during heating (starting at As ¼ 276 K and ending at Af ¼ 300 K) corresponds to the martensite to austenite transition, as in this case austenite phase is characterised by higher magnetization than martensite one. Consequently, abrupt drop of magnetization from 4.2 to 2.5 A m2/kg on the FC branch during cooling (starting at Ms ¼ 282 K and ending at Mf ¼ 258 K) characterises martensite to austenite transformation. It is clearly observable that these abrupt changes of magnetization related to structural transition (visible at the temperature range 225e325 K) are not apparent in the as-prepared sample (Fig. 3(a)). This behaviour is connected with the previously described microstructure evolution. The significant amount of Gd-rich phase is responsible for the disappearance of structural transformation in the as-prepared alloy. The decrease of magnetization in the vicinity of 370 K for the both as-prepared and
Fig. 3. Thermal dependence of magnetization for the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy measured in zero field (ZFC) and field cooled (FC) modes at external magnetic field of 10 mT.
annealed samples is connected with magnetic transition characterized by the Curie point. Thermomagnetic curves recorded in ZFC and FC modes show that the annealing process leads to the increase of the overall magnetic transformation temperature by approximately 10 K. It is worth noticing that the dual-phase nature (described by two different Curie temperatures) of the investigated materials is not observable on M(T) characteristics (Fig. 3) due to the relatively close Curie points of the Gd-rich and Gd-poor phases (the Curie points of the as-prepared and annealed Gd-free Ni50Mn25Ga25 alloy are about 380 K). All the temperatures of structural and magnetic transitions, including austenitic/martensitic transformation start temperatures (As/Ms), austenitic/ martensitic transformation finish temperatures (Af/Mf) and the C Curie temperature during heating/cooling (TH C ,TC) are summarised in Table 2. As a last point, it should also be mentioned that reversible structural transformation in the studied alloy undergoes
Table 2 Temperatures of structural and magnetic transformations in as-prepared and annealed Ni50Mn25Ga20Gd5 alloy (As/Ms e austenitic/martensitic transformation start temperatures, Af/Mf e austenitic/martensitic transformation finish temperaC tures, TH C /TC e Curie temperature during heating/cooling). Condition
Ms (K)
Mf (K)
As (K)
Af (K)
TH C (K)
TCC (K)
As-prepared Annealed at 1430 K/3 h
e 282
e 258
e 276
e 300
370 379
352 368
Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004
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Fig. 5. Examples of load-displacement curves for the Gd-rich (filled markers) and Gdpoor phases (unfilled markers) of the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy.
hardness of the Gd-poor phase decreases slightly from 4981 to 4772 MPa, whereas the average hardness of the Gd-rich phase drops substantially from 7678 to 6263 MPa. Observed decrease in hardness is connected with previously described evolution of the chemical composition of each phase created in the Ni50Mn25Ga20Gd5 alloy.
4. Conclusions
Fig. 4. DC hysteresis loops for the as-prepared (a) and annealed (b) Ni50Mn25Ga20Gd5 alloy measured at 100, 250 and 400 K in a field up to 2 T.
close to the room temperature, which is especially important from the potential applications point of view. Fig. 4 presents the DC hysteresis loops for the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy measured at indicated temperatures. In both as-prepared and annealed states of the investigated material the loops recorded at 100 and 250 K relate to the magnetization process of the ferromagnetic martensite phase. The hysteresis loop recorded at 400 K (above the Curie temperature e according to M(T) curves) characterises austenite phase occurring in paramagnetic state. What is worth noting, magnetization of the annealed alloy is almost two times higher than magnetization of the as-prepared material.
3.3. Mechanical properties Mechanical properties of the dual-phase Ni50Mn25Ga20Gd5 alloy in the as-prepared and annealed states were studied by the means of nanoindentation tests, according to the protocol described in experimental procedure. Fig. 5 presents the examples of loaddisplacement curves for the Gd-rich and Gd-poor phases of the as-prepared and annealed Ni50Mn25Ga20Gd5 alloy. It is clearly seen that hardness of Gd-rich phase is significantly higher than hardness of Gd-poor phase. It indicates that Gd addition to the Ni-Mn-Ga composition has significant influence on mechanical properties of the particular phases. In regard to the post-preparation heat treatment, the annealed alloy exposes noticeably lower hardness than the as-prepared material. After annealing the average
This work presents the influence of the heat treatment on microstructure, magnetic and nanomechanical properties of the Ni50Mn25Ga20Gd5 magnetic shape memory alloy. Microstructural investigation shows characteristic dendritic dual phase microstructure in the as-prepared sample. Subsequent annealing of the alloy at 1430 K for 3 h leads to the aggregation of Gd-rich phase along matrix grain boundaries. Microstructure evolution has substantial influence on magnetic properties in the investigated alloy and only annealed sample undergoes reversible martensitic transition. Moreover, the structural transition temperatures were estimated close to the room temperature, which is important for the potential future applications of this material. The Curie temperatures of the Ni50Mn25Ga20Gd5 alloy in both states stay close to 370 K. Detailed nanoindentation tests show that Gd-rich phase is significantly harder than Gd-poor phase by about 35% and 23% for the as-prepared and annealed sample, respectively. All of the presented studies show that addition of Gd and subsequent heat treatment have significant influence on microstructure as well as magnetic and mechanical properties of the polycrystalline Ni50Mn25Ga20Gd5 multifunctional material.
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Please cite this article as: Łaszcz A et al., Microstructure, magnetism and nanomechanical properties of Ni50Mn25Ga20Gd5 magnetic shape memory alloy before and after heat treatment, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.01.004