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The effect of ageing on the microstructure and mechanical properties of Ni53Mn23.5Ga18.5Ti5 ferromagnetic shape memory alloy
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Scripta Materialia 59 (2008) 268–271 www.elsevier.com/locate/scriptamat
The effect of ageing on the microstructure and mechanical properties of Ni53Mn23.5Ga18.5Ti5 ferromagnetic shape memory alloy G.F. Dong,a C.L. Tan,b Z.Y. Gao,a Y. Feng,a J.H. Suia and W. Caia,* a
National Key Laboratory Precision Hot Processing of Metals, School of Materials Science and Engineering, P.O. Box 405, Harbin Institute of Technology, Harbin 150001, China b Department of Applied Physics, P.O. Box 125, Harbin University of Science and Technology, Harbin 150080, China Received 16 January 2008; revised 13 February 2008; accepted 14 February 2008 Available online 23 April 2008
The effect of ageing on the microstructure and mechanical properties of five-layered tetragonal Ni53Mn23.5Ga18.5Ti5 alloy has been studied. The results show that ageing increases the amount and decreases the size of Ni3Ti precipitates. In addition, the compressive strength of alloys is clearly enhanced and compressive strain is slightly decreased by ageing, which can be attributed to the strengthening of the Ni3Ti precipitates and dislocation. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ni53Mn23.5Ga18.5Ti5 alloy; Ageing; Dislocation strengthening; Mechanical properties; Ferromagnetic shape memory alloys
Ni–Mn–Ga ferromagnetic shape memory alloy (FSMA) has attracted much attention due to its large magnetic-field-induced strain (MFIS) and its high response frequency [1,2]. The martensitic and magnetic transformations of this alloy system have been extensively studied in recent years [3–10]. The maximum strain of FSMAs is limited by the crystal structure of the material, which is known to be composition dependent [11]. Ni–Mn–Ga compositions exhibiting tetragonal and orthorhombic martensite have shown up to 6% and 10% strain, respectively [3–6,11,12]. Unfortunately, the brittleness, low strength and poor processability of Ni–Mn–Ga alloys greatly limit their application. In order to improve the mechanical properties without sacrificing the magnetic and thermoelastic properties, the modification of Ni–Mn–Ga FSMAs by adding a fourth element has become a new field of research. Several rare earth elements, such as Tb, Sm, Dy and Nd, have been added to ternary Ni–Mn–Ga alloys. Their effects on the phase transformation behavior and magnetic and mechanical properties have been studied [13–16]. Recently, we reported that adding Ti to a polycrystalline Ni53Mn23.5Ga23.5 alloy yielded a sig* Corresponding author. Tel.: +86 451 86418649; fax: +86 451 86415083; e-mail: [email protected]
nificant improvement in the compressive strength and ductility of the alloy with appropriate ageing treatments [17,18]. On the other hand, a significant improvement in the shape memory effect of Fe13.5Mn4.86Si3.82Ni0.16C alloy was achieved by deformation ageing [19]. Therefore, it is important to further investigate ageing-free and aged Ni–Mn–Ga–Ti alloys. The purpose of our study was thus to investigate for the first time the effect of ageing on the microstructure and mechanical properties of polycrystalline Ni53Mn23.5Ga18.5Ti5 alloy. The results show that the addition of Ti can significantly enhance the compressive strength and improve the ductility of Ni–Mn–Ga alloy by appropriate ageing. The nominal composition of the alloy Ni53Mn23.5Ga18.5Ti5 was prepared with high-purity elements melted four times in an arc-melting furnace under an argon atmosphere. The master rod was sealed in a quartz tube under a vacuum, then annealed at 1273 K for 5 h and quenched in iced water for homogeneity. Subsequently, aged-free samples were heated at 573, 673, 773, 873, 973, 1073 for 3 h at each temperature, and quenched into water to introduce second phases. Samples were aged at the same temperatures. X-ray diffraction (XRD) measurements were performed to determine the crystal structure using a Rigaku D/max-rB with Cu Ka radiation. Microstructures of the alloys were studied by a MX2600FE scanning electron microscope
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.02.058
Figure 1. Microstructure of the Ni53Mn23.5Ga18.5Ti5 alloy were agedfree at different temperatures for 3 h (a) 823 K; (b) 873 K; (c) 973 K; (d) 1073 K; (e) 1173 K.
20
40
M(440) M(440) M(440) M(404) M(440)
80
M(404)
M(422) 60
M(224)
A(422)
M(242) M(224)
M(224)
M(242)
M(404)
M(040) M(404)
M(400)
M(222) A(400)
M(404)
T=823K
M(222)
T=873K
(201) M(220) M(202)
T=973K
M(022)
T=1073K
M(220)
T=1173K
Intensity (a.u.)
equipped with an X-ray energy dispersive spectroscopy (EDS) analysis system. Samples for transmission electron microscopy (TEM) were electrochemically polished in a solution of 10% perchloric acid and 90% ethanol at 253 K and examined in a Philips CM-12 electron microscope operated at 120 kV. Compression testing was conducted at ambient temperature on an Instron-1186 Model machine at a strain rate of 0.05 mm s1. Figure 1 illustrates the secondary electron image of ageing-free Ni53Mn23.5Ga18.5Ti5 alloys at 823, 873, 973, 1073 and 1173 K for 3 h at each temperature. Typical unitary martensitic morphology is observed at room temperature in the Ni53Mn23.5Ga18.5Ti5 specimen aged at 823 K, as shown in Figure 1a. Straight plate twinned martensite variants are clearly observed at room temperature, whereas all the aged-free Ni53Mn23.5Ga18.5Ti5 specimens contain Ti-rich precipitates, as shown in Figure 1b–e, respectively. Moreover, it is found that both the amount of the Ti-rich precipitates and the size of the Ti-rich precipitates increases with increasing ageing temperature in the aged-free Ni53Mn23.5Ga18.5Ti5 alloy. Figure 2 illustrates the X-ray diffraction patterns of Ni53Mn23.5Ga18.5Ti5 alloy aged-free at various temperatures for 3 h at room temperature. The typical martensite peaks can be detected within the Ni53Mn23.5Ga18.5Ti5 alloys are aged at 823, 873, 1073 and 1173 K for 3 h each, respectively. In addition, the austenite peaks appear in the Ni53Mn23.5Ga18.5Ti5 alloys aged at 973 K for 3 h. According to the XRD results, the crystal structures can be indexed as five-layer tetragonal martensite with lattice parameters a = b = 0.5958 nm and c = 0.5521 nm aged at 823 K, a = b = 0.5950 nm and c = 0.5674 nm aged at 873 K, a = b = 0.5872 nm and c = 0.5638 nm aged at 1073 K, a = b = 0.5955 nm and c = 0.5480 nm aged at 1173 K. However, typical austenite peaks can clearly be seen in samples aged at 973 K with cubic lattice parameters a = b = c = 0.5844 nm. In both the martensitic and austenitic phases, some minor peaks can be detected with increasing ageing temperature, indicating that a new type of second phase precipitates in the aged-free samples. Moreover, our previous results showed that adding 5 at.% Ti to Ni53Mn23.5Ga23.5 alloy resulted in Ni3Ti phase precipitating in the aged-free samples. Therefore, the precipitates are identified as Ni3Ti type according to our previous results [17].
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G. F. Dong et al. / Scripta Materialia 59 (2008) 268–271
100
2θ (deg.)
Figure 2. XRD patterns of the Ni53Mn23.5Ga18.5Ti5 alloy aged-free at different temperature for 3 h.
Figure 3 illustrates the secondary electron image of Ni53Mn23.5Ga18.5Ti5 alloy aged at 823, 873, 973, 1073 and 1173 K for 3 h at each temperature. Typical unitary martensitic phase morphology is observed at room temperature in the Ni53Mn23.5Ga18.5Ti5 specimen aged at 823 and 1173 K. Some water-vapor-like bubbles can also be observed, which is due to the annealing or mechanical polishing process. It was found that the surface morphology of the alloy changed remarkably with increasing ageing temperature. The size of the Ti-rich precipitates clearly increases with ageing temperature. The size of aged Ni3Ti precipitates is significantly less than that of ageing-free samples, and they precipitate homogeneously in the matrix. On the other hand, ageing significantly inhibits growth and promotes uniform precipitation of Ti-rich precipitates. Accordingly, ageing increases the amount of the Ti-rich precipitates compared to that of ageing-free samples under the same conditions. However, Ti-rich precipitates can be returned to solution by ageing at 1173 K for 3 h. Figure 4 illustrates the room temperature XRD patterns of Ni53Mn23.5Ga18.5Ti5 alloys aged at various temperatures for 3 h. The typical martensite peaks can be seen clearly in the Ni53Mn23.5Ga18.5Ti5 alloy is aged at 823 and 1173 K for 3 h. The Ni53Mn23.5Ga18.5Ti5 alloys aged at 873, 973 and 1073 K for 3 h show the parent phase with the L21 structure. However, in addition to the diffraction peaks of parent phase, some extra peaks are observed for the alloy. The XRD pattern of the
Figure 3. Microstructure of the Ni53Mn23.5Ga18.5Ti5 alloy were agedassist at different temperature for 3 h (a) 823 K; (b) 873 K; (c) 973 K; (d) 1073 K; (e) 1173 K.
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t=1173 K 800
ageing-free
A(440)
A(422)
A(400)
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t=973 K
(201)
A(220)
Intensity (cps)
T=1073 K
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NiMnGa 823K-3h 873K-3h 973K-3h 1073K-3h 1173K-3h
200
t=873 K 0
t=823 K 20
1600 40
60
80
1400
100
2theta (deg.)
ageing-assist
Figure 4. XRD patterns of the Ni53Mn23.5Ga18.5Ti5 alloy aged-assist at different temperature for 3 h.
Ti-rich phase is rather similar to that of Ni3Ti reported by our previous study [14]. The lattice parameters of this phase are calculated to be a = b = 0.51199 nm, c = 0.82669 nm, i.e. slightly larger than that of the Ni3Ti phase but very close to that obtained from unaged samples. It is assumed that the presence of Mn atoms in the Ti-rich phase may account for the increase in the lattice parameters. Also, this result has been further confirmed by TEM analysis. Figure 5 shows the TEM image of precipitates and the corresponding selected-area electron diffraction patterns (SAED) in the Ni53Mn23.5Ga18.5Ti5 alloy aged at 973 K for 3 h. It can be seen that the second particles and dislocation line are distributed in the matrix, as shown in Figure 5a. These two SAED patterns, shown in Figures 3c and 5b, can be indexed as a hexagonal structure with a = b = 0.51199 nm, c = 0.82669 nm, space group D63/mm, which agrees well with the XRD results. The SAED of the alloy precipitates displays three extra superlattice spots between the two primary spots, indicating the existence of a four-layered modulated structure. In order to investigate the effect of ageing on the mechanical properties, compression tests were carried out at room temperature. All the samples were compressed to fracture. Figure 6 shows the compressive stress–strain curves of Ni53Mn23.5Ga18.5Ti5 alloy at room temperature. It can be seen that the compressive
Figure 5. (a) Bright field image of the Ti-rich precipitates in 973 K/3 h aged-assist Ni53Mn23.5Ga18.5Ti5 alloy; (b, c) EDPs from precipitates.
Stress (MPa)
1200 1000 800 600
NiMnGa 823K for 3 h 873K for 3 h 973K for 3 h 1073K for 3 h 1173K for 3 h
400 200 0 -200 0
2
4
6
8
10
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Figure 6. The compressive stress–strain curves of Ni53Mn23.5Ga18.5Ti5 alloys at room temperature.
strength of Ni–Mn–Ga alloy is obviously enhanced by ageing-free treatment. With an increase in ageing temperature, the compressive strength increases remarkably. The highest compressive strength of 900 MPa is obtained in alloy aged at 1073 K for 3 h, which is about 600 MPa higher than that of the Ni–Mn–Ga alloy. Furthermore, it is noted that the ageing can improve the compressive ductility. However, the compressive strength and strain of ageing-free alloys at 973 and 1173 K for 3 h decreases slightly. Lower compressive strength and compressive strain are obtained in alloy aged-free at 823 K for 3 h, which is due to the small number of precipitates, and the small size of these precipitates. Therefore, the strengthening effect of aged-free alloy is not obvious. Thus, Ni53Mn23.5Ga18.5Ti5 alloy aged-free at 873 K for 3 h exhibits the best mechanical properties. This clearly demonstrates that a proper amount of Ti addition significantly enhances the compressive strength and improves the ductility of Ni– Mn–Ga alloys. Furthermore, it can be concluded that appropriate ageing treatment can improve the breaking strength of Ni–Mn–Ga alloy, when the compressive strain is kept constant. On the other hand, alloying and a dispersed distribution of the second phase may partly explain why the alloy is strengthened. The existence of the Ni3Ti phase effectively hinders the movements of the dislocation and the propagation of the cracks, which is one of the reasons for the enhancement in the strength. Figure 6 also shows the compressive stress–strain curves of aged Ni53Mn23.5Ga18.5Ti5 alloy at room temperature. It can be seen that compared to aged-free alloy, the aged samples clearly have higher compressive strengths and slightly lower compressive strains. With increasing ageing temperature, the compressive strength increases at first and subsequently decreases. The highest
G. F. Dong et al. / Scripta Materialia 59 (2008) 268–271
compressive strength of 1403 MPa is obtained in alloy aged at 873 K for 3 h; this is about 500 MPa higher than that of the ageing-free alloy. This is the highest compressive strength reported to date in the Ni–Mn–Ga alloy system. The highest compressive strain obtained in alloy aged at 1173 K for 3 h is approximately 12%, which is about 2% higher than that of the aged-free samples and 3.5% higher than that of the alloy aged at 873 K for 3 h. This can be attributed to the increase in crystal defects (vacancies, faults) and to the interface between martensite and austenite induced by stress, which increases the number of nucleation sites for precipitation. Therefore, the average size of the secondary particles decreases and their volume fraction increases gradually. This increases the interphase density and makes plastic deformation difficult, resulting in the enhancement of the strength. In addition, the critical stress DT needed to form a dislocation line through the precipitates can be expressed as [20] DT ¼ af 1=2 r1 ; where f is the volume fraction of second phases, r is the average diameter of secondary particles, and a is a constant. Hence, according to this equation, the critical stress DT needed to form a dislocation line through the precipitates is increased by ageing, which enhances the strength and makes plastic deformation difficult. Moreover, it can be seen that the strengthening effect of the alloy with ageing is stronger than that in unaged alloy. However, the reason for the decrease in the strength and the increase in the strain in the alloy by ageing at 1173 K for 3 h is not clear, and needs further investigation. The effect of ageing on the microstructure and mechanical properties of Ni53Mn23.5Ga18.5Ti5 alloy has been studied. The results show that, compared to ageing-free samples, the ageing increases the amount and decreases the size of the Ni3Ti precipitates. Moreover, the compressive strength of the alloys is obviously enhanced, and the compressive strain decreases slightly during ageing, which can be attributed to the strengthening of the Ni3Ti precipitates and dislocation. In addition, it is found that Ti addition can significantly enhance the compressive strength and improve the ductility of both aged and unaged Ni–Mn–Ga alloy.
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This work is supported by Natural Science Foundation of China (No. 50531020). [1] K. Ullakko, J.K. Huang, C. Kantner, R.C. O’Handley, V.V. Kokorin, Appl. Phys. Lett. 69 (1996) 1966. [2] J. Tellinen, T. Suorsa, A. Jaaskelainen, I. Aalito, K. Ullakko, in: M. Bremen (Ed.), Basic Properties of Magnetic Shape Memory Actuators, Actuator 2002, Bremen, 2002. [3] O. Heczko, A. Sozinov, K. Ullakko, IEEE Trans. Mag. 36 (2000) 3266. [4] A. Sozinov, A.A. Likhachev, N. Lanska, K. Ullakko, Appl. Phys. Lett. 80 (2002) 1746. [5] A. Sozinov, A.A. Likhachev, N. Lanska, K. Ullakko, V.K. Lindroos, J. Phys. IV 112 (2003) 955. [6] S.J. Murray, M.A. Marioni, S.M. Allen, R.C. O’Handley, T.A. Lograsso, Appl. Phys. Lett. 77 (2000) 886. [7] V.A. Chemenko, E. Cesari, V.V. Kokorin, I.N. Vitenko, Scripta Metall. Mater. 33 (1995) 1239. ¨ osa, A. GonzaN ˜ lez-Comas, E. ObradoN ˜ , A. [8] Ll. MaA Planes, V.A. Chernenko, V.V. Kokorin, E. Cesari, Phys. Rev. B 55 (1997) 11068. [9] M. Matsumoto, T. Takagi, J. Tani, T. Kanomata, N. Muramatsu, A.N. Vasil’ev, Mater. Sci. Eng. A 273–275 (1999) 326. [10] R.C. O’Handley, S.J. Murray, M. Marioni, P.G. Tello, S.M. Allen, J. Magn. Magn. Mater. 226–230 (2001) 945. [11] J. Pons, V.A. Chernenko, R. Santamarta, E. Cesari, Acta Mater. 48 (2000) 3027. [12] S.J. Murray, M. Marioni, P.G. Tello, S.M. Allen, R.C. O’Handley, J. Magn. Magn. Mater. 226–230 (2001) 945. [13] L. Gao, W. Cai, A.L. Liu, L.C. Zhao, J. Alloys Compd. 425 (2006) 314. [14] S.H. Guo, Y.H. Zhang, Z.Q. Zhao, J.L. Li, X.L. Wang, J. Chin. Rare Earth Soc. 21 (2003) 668. [15] Z.Q. Zhao, H.X. Wu, F.S. Wang, O. Wang, L.P. Jiang, X.L. Wang, Rare Met. 23 (2004) 241. [16] K. Tsuchiya, A. Tsutsumi, H. Ohtsuka, M. Umemoto, Mater. Sci. Eng. A 378 (2004) 370. [17] G.F. Dong, W. Cai, Z.Y. Gao, Scripta Mater. (2007), doi:10.1016/j.scriptamat.2007.11.034. [18] G.F. Dong, W. Cai, Z.Y. Gao, J. Alloys Compd., in press (available online 9 November 2007). [19] Y.H. Wen, W. Zhang, N. Li, Acta Metall. Sin. 42 (11) (2006) 1217–1220. [20] J.S. Pan, J.M. Tong, M.B. Tian, Elements of Materials Science, Tsinghua University Press, Beijing, 1988.
Scripta Materialia 59 (2008) 268–271 www.elsevier.com/locate/scriptamat
The effect of ageing on the microstructure and mechanical properties of Ni53Mn23.5Ga18.5Ti5 ferromagnetic shape memory alloy G.F. Dong,a C.L. Tan,b Z.Y. Gao,a Y. Feng,a J.H. Suia and W. Caia,* a
National Key Laboratory Precision Hot Processing of Metals, School of Materials Science and Engineering, P.O. Box 405, Harbin Institute of Technology, Harbin 150001, China b Department of Applied Physics, P.O. Box 125, Harbin University of Science and Technology, Harbin 150080, China Received 16 January 2008; revised 13 February 2008; accepted 14 February 2008 Available online 23 April 2008
The effect of ageing on the microstructure and mechanical properties of five-layered tetragonal Ni53Mn23.5Ga18.5Ti5 alloy has been studied. The results show that ageing increases the amount and decreases the size of Ni3Ti precipitates. In addition, the compressive strength of alloys is clearly enhanced and compressive strain is slightly decreased by ageing, which can be attributed to the strengthening of the Ni3Ti precipitates and dislocation. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ni53Mn23.5Ga18.5Ti5 alloy; Ageing; Dislocation strengthening; Mechanical properties; Ferromagnetic shape memory alloys
Ni–Mn–Ga ferromagnetic shape memory alloy (FSMA) has attracted much attention due to its large magnetic-field-induced strain (MFIS) and its high response frequency [1,2]. The martensitic and magnetic transformations of this alloy system have been extensively studied in recent years [3–10]. The maximum strain of FSMAs is limited by the crystal structure of the material, which is known to be composition dependent [11]. Ni–Mn–Ga compositions exhibiting tetragonal and orthorhombic martensite have shown up to 6% and 10% strain, respectively [3–6,11,12]. Unfortunately, the brittleness, low strength and poor processability of Ni–Mn–Ga alloys greatly limit their application. In order to improve the mechanical properties without sacrificing the magnetic and thermoelastic properties, the modification of Ni–Mn–Ga FSMAs by adding a fourth element has become a new field of research. Several rare earth elements, such as Tb, Sm, Dy and Nd, have been added to ternary Ni–Mn–Ga alloys. Their effects on the phase transformation behavior and magnetic and mechanical properties have been studied [13–16]. Recently, we reported that adding Ti to a polycrystalline Ni53Mn23.5Ga23.5 alloy yielded a sig* Corresponding author. Tel.: +86 451 86418649; fax: +86 451 86415083; e-mail: [email protected]
nificant improvement in the compressive strength and ductility of the alloy with appropriate ageing treatments [17,18]. On the other hand, a significant improvement in the shape memory effect of Fe13.5Mn4.86Si3.82Ni0.16C alloy was achieved by deformation ageing [19]. Therefore, it is important to further investigate ageing-free and aged Ni–Mn–Ga–Ti alloys. The purpose of our study was thus to investigate for the first time the effect of ageing on the microstructure and mechanical properties of polycrystalline Ni53Mn23.5Ga18.5Ti5 alloy. The results show that the addition of Ti can significantly enhance the compressive strength and improve the ductility of Ni–Mn–Ga alloy by appropriate ageing. The nominal composition of the alloy Ni53Mn23.5Ga18.5Ti5 was prepared with high-purity elements melted four times in an arc-melting furnace under an argon atmosphere. The master rod was sealed in a quartz tube under a vacuum, then annealed at 1273 K for 5 h and quenched in iced water for homogeneity. Subsequently, aged-free samples were heated at 573, 673, 773, 873, 973, 1073 for 3 h at each temperature, and quenched into water to introduce second phases. Samples were aged at the same temperatures. X-ray diffraction (XRD) measurements were performed to determine the crystal structure using a Rigaku D/max-rB with Cu Ka radiation. Microstructures of the alloys were studied by a MX2600FE scanning electron microscope
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.02.058
Figure 1. Microstructure of the Ni53Mn23.5Ga18.5Ti5 alloy were agedfree at different temperatures for 3 h (a) 823 K; (b) 873 K; (c) 973 K; (d) 1073 K; (e) 1173 K.
20
40
M(440) M(440) M(440) M(404) M(440)
80
M(404)
M(422) 60
M(224)
A(422)
M(242) M(224)
M(224)
M(242)
M(404)
M(040) M(404)
M(400)
M(222) A(400)
M(404)
T=823K
M(222)
T=873K
(201) M(220) M(202)
T=973K
M(022)
T=1073K
M(220)
T=1173K
Intensity (a.u.)
equipped with an X-ray energy dispersive spectroscopy (EDS) analysis system. Samples for transmission electron microscopy (TEM) were electrochemically polished in a solution of 10% perchloric acid and 90% ethanol at 253 K and examined in a Philips CM-12 electron microscope operated at 120 kV. Compression testing was conducted at ambient temperature on an Instron-1186 Model machine at a strain rate of 0.05 mm s1. Figure 1 illustrates the secondary electron image of ageing-free Ni53Mn23.5Ga18.5Ti5 alloys at 823, 873, 973, 1073 and 1173 K for 3 h at each temperature. Typical unitary martensitic morphology is observed at room temperature in the Ni53Mn23.5Ga18.5Ti5 specimen aged at 823 K, as shown in Figure 1a. Straight plate twinned martensite variants are clearly observed at room temperature, whereas all the aged-free Ni53Mn23.5Ga18.5Ti5 specimens contain Ti-rich precipitates, as shown in Figure 1b–e, respectively. Moreover, it is found that both the amount of the Ti-rich precipitates and the size of the Ti-rich precipitates increases with increasing ageing temperature in the aged-free Ni53Mn23.5Ga18.5Ti5 alloy. Figure 2 illustrates the X-ray diffraction patterns of Ni53Mn23.5Ga18.5Ti5 alloy aged-free at various temperatures for 3 h at room temperature. The typical martensite peaks can be detected within the Ni53Mn23.5Ga18.5Ti5 alloys are aged at 823, 873, 1073 and 1173 K for 3 h each, respectively. In addition, the austenite peaks appear in the Ni53Mn23.5Ga18.5Ti5 alloys aged at 973 K for 3 h. According to the XRD results, the crystal structures can be indexed as five-layer tetragonal martensite with lattice parameters a = b = 0.5958 nm and c = 0.5521 nm aged at 823 K, a = b = 0.5950 nm and c = 0.5674 nm aged at 873 K, a = b = 0.5872 nm and c = 0.5638 nm aged at 1073 K, a = b = 0.5955 nm and c = 0.5480 nm aged at 1173 K. However, typical austenite peaks can clearly be seen in samples aged at 973 K with cubic lattice parameters a = b = c = 0.5844 nm. In both the martensitic and austenitic phases, some minor peaks can be detected with increasing ageing temperature, indicating that a new type of second phase precipitates in the aged-free samples. Moreover, our previous results showed that adding 5 at.% Ti to Ni53Mn23.5Ga23.5 alloy resulted in Ni3Ti phase precipitating in the aged-free samples. Therefore, the precipitates are identified as Ni3Ti type according to our previous results [17].
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G. F. Dong et al. / Scripta Materialia 59 (2008) 268–271
100
2θ (deg.)
Figure 2. XRD patterns of the Ni53Mn23.5Ga18.5Ti5 alloy aged-free at different temperature for 3 h.
Figure 3 illustrates the secondary electron image of Ni53Mn23.5Ga18.5Ti5 alloy aged at 823, 873, 973, 1073 and 1173 K for 3 h at each temperature. Typical unitary martensitic phase morphology is observed at room temperature in the Ni53Mn23.5Ga18.5Ti5 specimen aged at 823 and 1173 K. Some water-vapor-like bubbles can also be observed, which is due to the annealing or mechanical polishing process. It was found that the surface morphology of the alloy changed remarkably with increasing ageing temperature. The size of the Ti-rich precipitates clearly increases with ageing temperature. The size of aged Ni3Ti precipitates is significantly less than that of ageing-free samples, and they precipitate homogeneously in the matrix. On the other hand, ageing significantly inhibits growth and promotes uniform precipitation of Ti-rich precipitates. Accordingly, ageing increases the amount of the Ti-rich precipitates compared to that of ageing-free samples under the same conditions. However, Ti-rich precipitates can be returned to solution by ageing at 1173 K for 3 h. Figure 4 illustrates the room temperature XRD patterns of Ni53Mn23.5Ga18.5Ti5 alloys aged at various temperatures for 3 h. The typical martensite peaks can be seen clearly in the Ni53Mn23.5Ga18.5Ti5 alloy is aged at 823 and 1173 K for 3 h. The Ni53Mn23.5Ga18.5Ti5 alloys aged at 873, 973 and 1073 K for 3 h show the parent phase with the L21 structure. However, in addition to the diffraction peaks of parent phase, some extra peaks are observed for the alloy. The XRD pattern of the
Figure 3. Microstructure of the Ni53Mn23.5Ga18.5Ti5 alloy were agedassist at different temperature for 3 h (a) 823 K; (b) 873 K; (c) 973 K; (d) 1073 K; (e) 1173 K.
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G. F. Dong et al. / Scripta Materialia 59 (2008) 268–271 1000
t=1173 K 800
ageing-free
A(440)
A(422)
A(400)
Stress (MPa)
(311) (202)
t=973 K
(201)
A(220)
Intensity (cps)
T=1073 K
600
400
NiMnGa 823K-3h 873K-3h 973K-3h 1073K-3h 1173K-3h
200
t=873 K 0
t=823 K 20
1600 40
60
80
1400
100
2theta (deg.)
ageing-assist
Figure 4. XRD patterns of the Ni53Mn23.5Ga18.5Ti5 alloy aged-assist at different temperature for 3 h.
Ti-rich phase is rather similar to that of Ni3Ti reported by our previous study [14]. The lattice parameters of this phase are calculated to be a = b = 0.51199 nm, c = 0.82669 nm, i.e. slightly larger than that of the Ni3Ti phase but very close to that obtained from unaged samples. It is assumed that the presence of Mn atoms in the Ti-rich phase may account for the increase in the lattice parameters. Also, this result has been further confirmed by TEM analysis. Figure 5 shows the TEM image of precipitates and the corresponding selected-area electron diffraction patterns (SAED) in the Ni53Mn23.5Ga18.5Ti5 alloy aged at 973 K for 3 h. It can be seen that the second particles and dislocation line are distributed in the matrix, as shown in Figure 5a. These two SAED patterns, shown in Figures 3c and 5b, can be indexed as a hexagonal structure with a = b = 0.51199 nm, c = 0.82669 nm, space group D63/mm, which agrees well with the XRD results. The SAED of the alloy precipitates displays three extra superlattice spots between the two primary spots, indicating the existence of a four-layered modulated structure. In order to investigate the effect of ageing on the mechanical properties, compression tests were carried out at room temperature. All the samples were compressed to fracture. Figure 6 shows the compressive stress–strain curves of Ni53Mn23.5Ga18.5Ti5 alloy at room temperature. It can be seen that the compressive
Figure 5. (a) Bright field image of the Ti-rich precipitates in 973 K/3 h aged-assist Ni53Mn23.5Ga18.5Ti5 alloy; (b, c) EDPs from precipitates.
Stress (MPa)
1200 1000 800 600
NiMnGa 823K for 3 h 873K for 3 h 973K for 3 h 1073K for 3 h 1173K for 3 h
400 200 0 -200 0
2
4
6
8
10
12
Strain (%)
Figure 6. The compressive stress–strain curves of Ni53Mn23.5Ga18.5Ti5 alloys at room temperature.
strength of Ni–Mn–Ga alloy is obviously enhanced by ageing-free treatment. With an increase in ageing temperature, the compressive strength increases remarkably. The highest compressive strength of 900 MPa is obtained in alloy aged at 1073 K for 3 h, which is about 600 MPa higher than that of the Ni–Mn–Ga alloy. Furthermore, it is noted that the ageing can improve the compressive ductility. However, the compressive strength and strain of ageing-free alloys at 973 and 1173 K for 3 h decreases slightly. Lower compressive strength and compressive strain are obtained in alloy aged-free at 823 K for 3 h, which is due to the small number of precipitates, and the small size of these precipitates. Therefore, the strengthening effect of aged-free alloy is not obvious. Thus, Ni53Mn23.5Ga18.5Ti5 alloy aged-free at 873 K for 3 h exhibits the best mechanical properties. This clearly demonstrates that a proper amount of Ti addition significantly enhances the compressive strength and improves the ductility of Ni– Mn–Ga alloys. Furthermore, it can be concluded that appropriate ageing treatment can improve the breaking strength of Ni–Mn–Ga alloy, when the compressive strain is kept constant. On the other hand, alloying and a dispersed distribution of the second phase may partly explain why the alloy is strengthened. The existence of the Ni3Ti phase effectively hinders the movements of the dislocation and the propagation of the cracks, which is one of the reasons for the enhancement in the strength. Figure 6 also shows the compressive stress–strain curves of aged Ni53Mn23.5Ga18.5Ti5 alloy at room temperature. It can be seen that compared to aged-free alloy, the aged samples clearly have higher compressive strengths and slightly lower compressive strains. With increasing ageing temperature, the compressive strength increases at first and subsequently decreases. The highest
G. F. Dong et al. / Scripta Materialia 59 (2008) 268–271
compressive strength of 1403 MPa is obtained in alloy aged at 873 K for 3 h; this is about 500 MPa higher than that of the ageing-free alloy. This is the highest compressive strength reported to date in the Ni–Mn–Ga alloy system. The highest compressive strain obtained in alloy aged at 1173 K for 3 h is approximately 12%, which is about 2% higher than that of the aged-free samples and 3.5% higher than that of the alloy aged at 873 K for 3 h. This can be attributed to the increase in crystal defects (vacancies, faults) and to the interface between martensite and austenite induced by stress, which increases the number of nucleation sites for precipitation. Therefore, the average size of the secondary particles decreases and their volume fraction increases gradually. This increases the interphase density and makes plastic deformation difficult, resulting in the enhancement of the strength. In addition, the critical stress DT needed to form a dislocation line through the precipitates can be expressed as [20] DT ¼ af 1=2 r1 ; where f is the volume fraction of second phases, r is the average diameter of secondary particles, and a is a constant. Hence, according to this equation, the critical stress DT needed to form a dislocation line through the precipitates is increased by ageing, which enhances the strength and makes plastic deformation difficult. Moreover, it can be seen that the strengthening effect of the alloy with ageing is stronger than that in unaged alloy. However, the reason for the decrease in the strength and the increase in the strain in the alloy by ageing at 1173 K for 3 h is not clear, and needs further investigation. The effect of ageing on the microstructure and mechanical properties of Ni53Mn23.5Ga18.5Ti5 alloy has been studied. The results show that, compared to ageing-free samples, the ageing increases the amount and decreases the size of the Ni3Ti precipitates. Moreover, the compressive strength of the alloys is obviously enhanced, and the compressive strain decreases slightly during ageing, which can be attributed to the strengthening of the Ni3Ti precipitates and dislocation. In addition, it is found that Ti addition can significantly enhance the compressive strength and improve the ductility of both aged and unaged Ni–Mn–Ga alloy.
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