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Tensile characteristics and shape memory effect of Ni56Mn21Co4Ga19 high-temperature shape memory alloy
Available online at www.sciencedirect.com
Scripta Materialia 58 (2008) 918–921 www.elsevier.com/locate/scriptamat
Tensile characteristics and shape memory effect of Ni56Mn21Co4Ga19 high-temperature shape memory alloy Yunqing Ma,* Shuiyuan Yang, Cuiping Wang and Xingjun Liu Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China Received 25 December 2007; revised 7 January 2008; accepted 8 January 2008 Available online 16 January 2008
Ni56Mn21Co4Ga19 alloys have been successfully hot-rolled to smooth plates of 0.5 mm thickness, and the mechanical and shape memory characteristics were investigated by tensile tests. The results show that the tensile stress and strain of dual-phases Ni56Mn21Co4Ga19 alloy are 491 MPa and 8.17%, respectively. Its martensitic transformation starting temperature is 412 °C, and the recoverable strain due to shape memory effect is 2.1% under a residual strain of 4.3%. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: NiMnGa; Martensitic phase transformation; Tensile; High-temperature shape memory alloy
During the past few decades, many shape memory alloys (SMAs) have been developed as new functional materials, with commercial applications in pipe couplings, medical implants, various actuators, etc. [1,2]. However, the highest operation temperature of these alloys is limited to about 100 °C due to the low martensitic transformation temperatures or poor thermal stability [1,2]. Several alloy systems have been investigated as high-temperature shape memory alloys (HTSMAs), such as Cu-based alloys, NiAl, TiNi(Hf, Zr) and TiNiPd [3–6], but till now some problems still remain unsolved in these alloys. There is a pressing need to develop HTSMAs. In the early work of Chernenko et al. [7], the high sensitivity of the martensitic transformation temperature of Ni–Mn–Ga alloys to composition was revealed and the existing martensitic transformation temperature as high as 350 °C showed their potentials as HTSMAs. Recently, single crystalline Ni–Mn–Ga HTSMAs were investigated to exhibit excellent shape memory effect (SME) [8], superelasticity [9], as well as high thermal stability [10]. All these have made Ni–Mn–Ga alloys promising HTSMAs. However, the brittleness of polycrystalline Ni–Mn–Ga alloys is a crucial obstacle for their further development. Ishida et al. have reported that introducing a ductile c phase is an effective method to improve the ductility of B2-type intermetallic compounds, as shown in Fe doped Ni–Al SMAs [11] and b + c dual-phase Co–Ni–Al alloys [12]. Accordingly, several papers * Corresponding author. E-mail: [email protected]
revealed that the introduction of a ductile c phase into Ni–Mn–Ga alloy by Fe addition has a positive effect on its ductility [13,14], implying that the ductility of the Ni–Mn–Ga alloys can be improved by adding a fourth element. Therefore, in this paper a dual-phase Ni56Mn21 Co4Ga19 alloy was studied by Co addition. It could be successfully hot-rolled to smooth plates with thickness of 0.5 mm, and the mechanical and shape memory properties were investigated by tensile tests. Polycrystalline buttons of Ni56Mn21Co4Ga19 alloy were arc-melted five times under argon atmosphere to ensure uniformity. The purities of the nickel, manganese, gallium and cobalt are 99.9%, 99.7%, 99.99% and 99.9%, respectively. The metal buttons were sealed into vacuum quartz ampoules and annealed at 900 °C for three days. Then the buttons were heated to 900 °C and conducted to hot-rolling at a reduction of about 0.5 mm per pass. During the hot-rolling, some little cracks were generated. However, hot-rolling could be continued after cleaning these cracks. Finally, the button ingots were successfully hot-rolled to thin plates of 0.5 mm thickness. The obtained plate exhibits smooth surfaces and regular edges, as shown in Figure 1. The phase structure was identified at room temperature by a Panalytical X’pert PRO with Cu Ka radiation. The microstructure was observed by optical microscopy. Samples for optical observation were mechanically polished and etched in a solution of 99 ml methanol + 2 ml nitric acid + 5 g ferric chloride. The martensitic transformation temperatures were determined by differential scanning calorimetry (DSC) (Netzsch STA 404) at
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.01.013
Y. Ma et al. / Scripta Materialia 58 (2008) 918–921
Figure 1. Surface morphologies of button ingot and plate after hotrolling of Ni56Mn21Co4Ga19 alloy.
heating and cooling rates of 10 °C/min. The mechanical properties and SME were measured by tensile tests at ambient temperature using a Galdabini Sun 2500 machine at a crosshead speed of 0.2 mm/min. The tensile direction was parallel to the rolling direction. The gauge size of the tensile specimen was 3 mm wide, 0.5 mm thick and 7 mm long according to the relationship of L0 = 5.65 A1/2 [15], where A is the cross-sectional area and L0 is the length of the gauge part. The lengths of the specimens were measured before loading (l0), after unloading (l1) and after heating to 600 °C for 5 min (l2) by a micrometer with an accuracy of 0.01 mm. The prestrain during tensile was defined as epre = Dl/l0, where Dl is the displacement of the crosshead. Residual strain after unloading (er), permanent strain (ep) and strain which was recovered due to the SME (eSME) were obtained as: (l1 l0)/l0 100%, (l2 l0)/l0 100%, and (l1 l2)/l0 100%, respectively. Figure 2 shows the optical micrograph (a) and X-ray diffraction pattern (b) of Ni56Mn21Co4Ga19 plate at room temperature. It can be seen from Figure 2a that the polycrystalline Ni56Mn21Co4Ga19 is composed of dual phases, a gray matrix and a white second phase. The matrix is a martensite phase characterized by typical lamellar twin substructures, identical to those in the single phase Ni54Mn25Ga21 alloy [16]. The white second phase separately distributes in the lamellar martensite phase and on the grain boundaries, and seems elongated along the rolling direction. The volume ratio of the second phase is measured to be about 18%. This dual-phase structure can also be confirmed by XRD measurement as shown in Figure 2b. The reflection pattern can be indexed by two phases: one is tetragonal non-modulated martensite and another is face-centered
919
cubic (fcc) c phase, similar to the c phase in Ni–Fe–Ga alloys [17,18]. The crystal lattice parameters of nonmodulated martensite are calculated to be a = b = 0.7605 nm, and c = 0.6659 nm, and the lattice parameters of the c phase are a = b =c = 0.3608 nm. Since the c phase does not participate in the reversible martensitic transformation, its size, shape, volume and distribution may have important effects on the martensitic transformation and SME of Ni56Mn21Co4Ga19 alloy. Figure 3 shows the DSC curves of Ni56Mn21Co4Ga19 alloy during the cooling and heating. The clear endothermic peak appears on the heating DSC curve is associated with the reverse martensitic transformation from martensite to cubic austenite, where the austenite transformation starting temperature As, peaking temperature Ap and finishing temperature Af are 455 °C, 468 °C and 482 °C, respectively. The exothermic peak indicating the forward martensitic transformation from austenite to martensite occurs on the cooling DSC curve, and the martensite starting temperature Ms, peaking temperature Mp and finishing temperature Mf are 412 °C, 404 °C and 388 °C, respectively. The only one endothermic or exothermic peak appears in the heating and cooling DSC curves, respectively, indicating that one reversible phase transformation appears in heating and cooling. In contrast with that of Ni54Mn25Ga21 alloy [8], Ni56Mn21Co4Ga19 alloy has much higher transformation temperatures and larger hysteresis. The tensile stress–strain curve of Ni56Mn21Co4Ga19 alloy at room temperature was shown in Figure 4. The symbol () represents the fracture point, and the tensile stress and strain are measured to be 491 MPa and 8.17%, respectively. As far as we know, all the mechanical properties concerned with Ni–Mn–Ga alloys before were obtained by compress tests. The addition of Co brings a dramatic improvement of Ni–Mn–Ga alloys in hot-workability and room-temperature ductility, probably due to the formation of the c phase. It is known that Ni–Mn–Ga single crystal has good ductility, and the brittleness of polycrystalline Ni–Mn–Ga is related to the grain boundaries. The c phase particles disperse uniformly among the grain boundaries and martensite variants in dual-phase Ni56Mn21Co4Ga19 alloy, which are helpful to strengthen the grain boundaries and improve the plasticity. Further investigations and
-0.2
Ni56Mn21Co4Ga19
a
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Figure 2. Optical micrograph (a) and X-ray diffraction pattern (b) of Ni56Mn21Co4Ga19 plate.
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Figure 3. DSC curves of Ni56Mn21Co4Ga19 alloy showing the forward (on cooling) and reverse (on heating) martensitic transformations.
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Y. Ma et al. / Scripta Materialia 58 (2008) 918–921 2.2
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Figure 4. Tensile stress–strain curve of Ni56Mn21Co4Ga19 alloy at room temperature. The inset shows the SME when prestrained to 5.3%. The arrow represents the strain recovery upon heating to 600 °C.
theoretical analysis on the improvement of ductility of polycrystalline Ni–Mn–Ga are expected. Comparing with the typical tensile stress–strain curves of TiNi SMAs [19], which have three obvious stages that are associated with the elastic deformation of the multivariants, reorientation of martensitic variants and/or detwinning, the elastic and plastic deformation of reoriented martensites, the stress plateau corresponding to the reorientation of martensitic variants and/or detwinning disappears completely for Ni56Mn21Co4Ga19 alloy. This situation is similar to the cases of other HTSMAs, such as NiTiPd [20] and NiTiHf [21], in which the stress plateau completely disappears and high work-hardening is constantly observed, due to the dislocation slip during the reorientation of martensite variants. It is well known that the strain could be recovered due to SME occurrences only through the reorientation of martensitic variants and, in general, a large magnitude of stress plateau means a large shape memory recovery. Accordingly, it can be guessed that Ni56Mn21Co4Ga19 alloy will not exhibit good SME. In order to investigate SME, Ni56Mn21Co4Ga19 plate samples were tensed to different prestrains. The insert of Figure 4 shows the stress–strain curve at prestrain of 5.3%. The residual strain after unloading is 4.3%. The arrowed line in the inset represents the recovery of the strain after heating to 600 °C. The recoverable strain is 2.1% due to the reverse martensitic transformation, and the recoverable rate is 48.8%. Figure 5 shows the recovery strains at different residual strains. It can be seen that the recovery strain increases remarkably with increasing residual strains. Ni56Mn21Co4Ga19 alloy exhibits poor SME as compared with single crystalline Ni54Mn25Ga21 alloy [8]. This can be understood by considering the structural reasons. One important item is the distribution of the c phase in Ni56Mn21Co4Ga19 alloy. It has been demonstrated that in dual-phase Ni–Al–Fe alloys that the pseudoelastic strain recovery and shape memory recovery strain degraded with increasing c phase fraction. Similarly, it is believed that the reorientation of martensitic variants and/or detwinning is hampered by c phase particles dispersed in martensitic matrix, and c phase particles are also barriers to the shape recovery to the parent phase of the Ni56Mn21Co4Ga19 alloy from
Shape Memory Strains (%)
400 Tensile Stress(MPa)
Tensile Stress (MPa)
500
2.0 1.8 1.6 1.4 1.2 1.0 0.8 1.5
2.0
2.5
3.0
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Figure 5. Shape memory strains of Ni56Mn21Co4Ga19 alloy at different residual strains.
deformed martensite [14]. Of course, in the polycrystalline Ni56Mn21Co4Ga19 alloy the existence of the grain boundaries may also result in decrease of the SME, as compared with single crystalline Ni54Mn25Ga21 alloy. In conclusion, Ni56Mn21Co4Ga19 alloy can be successfully hot-rolled to thin plates of 0.5 mm thickness, and the mechanical properties and SME are measured by tensile tests. Ni56Mn21Co4Ga19 alloy exhibits greatly improved hot workability and ductility, high martensitic transformation temperature (Ms = 412 °C), as well as recoverable strain of 2.1% under a residual strain of 4.3%. This work was supported by the Natural Science Foundation of China (No. 50771086), the Program for New Century Excellent Talents in Fujian Province University (NCETFJ), and the Youth Science Foundation of Fujian Province of China (No. 2006F3119). [1] K. Otsuka, X.B. Ren, Intermetallics 7 (1999) 511. [2] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, Cambridge, 1998. [3] H.B. Xu, Mater. Sci. Forum. 394–395 (2002) 375. [4] J.H. Yang, C.M. Wayman, Intermetallics 2 (1994) 111. [5] J.H. Mulder, J.H. Maas, J. Beyer, in: Proceedings of the ICOMAT-92 Monterey, 1992, p. 869. [6] D. Golberg, Y. Xu, Y. Murakami, Scripta Mater. 30 (1994) 1349. [7] V.A. Chernenko, E. Cesari, V.V. Kokorin, I.N. Vitenko, Scripta Metall. Mater. 33 (1995) 1239. [8] H.B. Xu, Y.Q. Ma, C.B. Jiang, Appl. Phys. Lett. 82 (2003) 3206. [9] V.A. Chernenko, V. L’vov, J. Pons, E. Cesari, J. Appl. Phys. 93 (2003) 2394. [10] Y.Q. Ma, C.B. Jiang, G. Feng, H.B. Xu, Scripta Mater. 48 (2003) 365. [11] K. Ishida, R. Kainuma, N. Ueno, T. Nishizawa, Metall. Trans. A 22 (1991) 441. [12] Y. Tanaka, K. Oikawa, Y. Sutou, T. Omori, R. Kainuma, K. Ishida, Mater. Sci. Eng. A 438–440 (2006) 1054. [13] Y.Q. Ma, L.H. Xu, Y. Li, C.B. Jiang, H.B. Xu, Y.K. Lee, Z. Metallkd. 96 (2005) 843. [14] Y. Xin, Y. Li, L. Chai, H.B. Xu, Scripta Mater. 57 (2007) 599. [15] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill, New York, 1986.
Y. Ma et al. / Scripta Materialia 58 (2008) 918–921
[16] Y.Q. Ma, C.B. Jiang, Y. Li, H.B. Xu, C.P. Wang, X.J. Liu, Acta Mater. 55 (2007) 1533. [17] T. Omori, N. Kamiya, Y. Sutou, K. Oikawa, R. Kainuma, K. Ishida, Mater. Sci. Eng. A 378 (2004) 403. [18] Y. Li, C.B. Jiang, T. Liang, Y.Q. Ma, H.B. Xu, Scripta Mater. 48 (2003) 1255.
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[19] Y. Liu, Y.L. Li, Z.L. Xie, K.T. Ramesh, Philos. Mag. Lett. 82 (2002) 511. [20] D. Golberg, Y. Xu, Y. Murakami, K. Otsuka, T. Uekj, H. Horikawa, Mater. Lett. 22 (1995) 241. [21] Y.Q. Wang, Y.F. Zheng, W. Cai, L.C. Zhao, Scripta Mater. 40 (1999) 1327.
Scripta Materialia 58 (2008) 918–921 www.elsevier.com/locate/scriptamat
Tensile characteristics and shape memory effect of Ni56Mn21Co4Ga19 high-temperature shape memory alloy Yunqing Ma,* Shuiyuan Yang, Cuiping Wang and Xingjun Liu Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China Received 25 December 2007; revised 7 January 2008; accepted 8 January 2008 Available online 16 January 2008
Ni56Mn21Co4Ga19 alloys have been successfully hot-rolled to smooth plates of 0.5 mm thickness, and the mechanical and shape memory characteristics were investigated by tensile tests. The results show that the tensile stress and strain of dual-phases Ni56Mn21Co4Ga19 alloy are 491 MPa and 8.17%, respectively. Its martensitic transformation starting temperature is 412 °C, and the recoverable strain due to shape memory effect is 2.1% under a residual strain of 4.3%. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: NiMnGa; Martensitic phase transformation; Tensile; High-temperature shape memory alloy
During the past few decades, many shape memory alloys (SMAs) have been developed as new functional materials, with commercial applications in pipe couplings, medical implants, various actuators, etc. [1,2]. However, the highest operation temperature of these alloys is limited to about 100 °C due to the low martensitic transformation temperatures or poor thermal stability [1,2]. Several alloy systems have been investigated as high-temperature shape memory alloys (HTSMAs), such as Cu-based alloys, NiAl, TiNi(Hf, Zr) and TiNiPd [3–6], but till now some problems still remain unsolved in these alloys. There is a pressing need to develop HTSMAs. In the early work of Chernenko et al. [7], the high sensitivity of the martensitic transformation temperature of Ni–Mn–Ga alloys to composition was revealed and the existing martensitic transformation temperature as high as 350 °C showed their potentials as HTSMAs. Recently, single crystalline Ni–Mn–Ga HTSMAs were investigated to exhibit excellent shape memory effect (SME) [8], superelasticity [9], as well as high thermal stability [10]. All these have made Ni–Mn–Ga alloys promising HTSMAs. However, the brittleness of polycrystalline Ni–Mn–Ga alloys is a crucial obstacle for their further development. Ishida et al. have reported that introducing a ductile c phase is an effective method to improve the ductility of B2-type intermetallic compounds, as shown in Fe doped Ni–Al SMAs [11] and b + c dual-phase Co–Ni–Al alloys [12]. Accordingly, several papers * Corresponding author. E-mail: [email protected]
revealed that the introduction of a ductile c phase into Ni–Mn–Ga alloy by Fe addition has a positive effect on its ductility [13,14], implying that the ductility of the Ni–Mn–Ga alloys can be improved by adding a fourth element. Therefore, in this paper a dual-phase Ni56Mn21 Co4Ga19 alloy was studied by Co addition. It could be successfully hot-rolled to smooth plates with thickness of 0.5 mm, and the mechanical and shape memory properties were investigated by tensile tests. Polycrystalline buttons of Ni56Mn21Co4Ga19 alloy were arc-melted five times under argon atmosphere to ensure uniformity. The purities of the nickel, manganese, gallium and cobalt are 99.9%, 99.7%, 99.99% and 99.9%, respectively. The metal buttons were sealed into vacuum quartz ampoules and annealed at 900 °C for three days. Then the buttons were heated to 900 °C and conducted to hot-rolling at a reduction of about 0.5 mm per pass. During the hot-rolling, some little cracks were generated. However, hot-rolling could be continued after cleaning these cracks. Finally, the button ingots were successfully hot-rolled to thin plates of 0.5 mm thickness. The obtained plate exhibits smooth surfaces and regular edges, as shown in Figure 1. The phase structure was identified at room temperature by a Panalytical X’pert PRO with Cu Ka radiation. The microstructure was observed by optical microscopy. Samples for optical observation were mechanically polished and etched in a solution of 99 ml methanol + 2 ml nitric acid + 5 g ferric chloride. The martensitic transformation temperatures were determined by differential scanning calorimetry (DSC) (Netzsch STA 404) at
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.01.013
Y. Ma et al. / Scripta Materialia 58 (2008) 918–921
Figure 1. Surface morphologies of button ingot and plate after hotrolling of Ni56Mn21Co4Ga19 alloy.
heating and cooling rates of 10 °C/min. The mechanical properties and SME were measured by tensile tests at ambient temperature using a Galdabini Sun 2500 machine at a crosshead speed of 0.2 mm/min. The tensile direction was parallel to the rolling direction. The gauge size of the tensile specimen was 3 mm wide, 0.5 mm thick and 7 mm long according to the relationship of L0 = 5.65 A1/2 [15], where A is the cross-sectional area and L0 is the length of the gauge part. The lengths of the specimens were measured before loading (l0), after unloading (l1) and after heating to 600 °C for 5 min (l2) by a micrometer with an accuracy of 0.01 mm. The prestrain during tensile was defined as epre = Dl/l0, where Dl is the displacement of the crosshead. Residual strain after unloading (er), permanent strain (ep) and strain which was recovered due to the SME (eSME) were obtained as: (l1 l0)/l0 100%, (l2 l0)/l0 100%, and (l1 l2)/l0 100%, respectively. Figure 2 shows the optical micrograph (a) and X-ray diffraction pattern (b) of Ni56Mn21Co4Ga19 plate at room temperature. It can be seen from Figure 2a that the polycrystalline Ni56Mn21Co4Ga19 is composed of dual phases, a gray matrix and a white second phase. The matrix is a martensite phase characterized by typical lamellar twin substructures, identical to those in the single phase Ni54Mn25Ga21 alloy [16]. The white second phase separately distributes in the lamellar martensite phase and on the grain boundaries, and seems elongated along the rolling direction. The volume ratio of the second phase is measured to be about 18%. This dual-phase structure can also be confirmed by XRD measurement as shown in Figure 2b. The reflection pattern can be indexed by two phases: one is tetragonal non-modulated martensite and another is face-centered
919
cubic (fcc) c phase, similar to the c phase in Ni–Fe–Ga alloys [17,18]. The crystal lattice parameters of nonmodulated martensite are calculated to be a = b = 0.7605 nm, and c = 0.6659 nm, and the lattice parameters of the c phase are a = b =c = 0.3608 nm. Since the c phase does not participate in the reversible martensitic transformation, its size, shape, volume and distribution may have important effects on the martensitic transformation and SME of Ni56Mn21Co4Ga19 alloy. Figure 3 shows the DSC curves of Ni56Mn21Co4Ga19 alloy during the cooling and heating. The clear endothermic peak appears on the heating DSC curve is associated with the reverse martensitic transformation from martensite to cubic austenite, where the austenite transformation starting temperature As, peaking temperature Ap and finishing temperature Af are 455 °C, 468 °C and 482 °C, respectively. The exothermic peak indicating the forward martensitic transformation from austenite to martensite occurs on the cooling DSC curve, and the martensite starting temperature Ms, peaking temperature Mp and finishing temperature Mf are 412 °C, 404 °C and 388 °C, respectively. The only one endothermic or exothermic peak appears in the heating and cooling DSC curves, respectively, indicating that one reversible phase transformation appears in heating and cooling. In contrast with that of Ni54Mn25Ga21 alloy [8], Ni56Mn21Co4Ga19 alloy has much higher transformation temperatures and larger hysteresis. The tensile stress–strain curve of Ni56Mn21Co4Ga19 alloy at room temperature was shown in Figure 4. The symbol () represents the fracture point, and the tensile stress and strain are measured to be 491 MPa and 8.17%, respectively. As far as we know, all the mechanical properties concerned with Ni–Mn–Ga alloys before were obtained by compress tests. The addition of Co brings a dramatic improvement of Ni–Mn–Ga alloys in hot-workability and room-temperature ductility, probably due to the formation of the c phase. It is known that Ni–Mn–Ga single crystal has good ductility, and the brittleness of polycrystalline Ni–Mn–Ga is related to the grain boundaries. The c phase particles disperse uniformly among the grain boundaries and martensite variants in dual-phase Ni56Mn21Co4Ga19 alloy, which are helpful to strengthen the grain boundaries and improve the plasticity. Further investigations and
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a
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M622
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Figure 2. Optical micrograph (a) and X-ray diffraction pattern (b) of Ni56Mn21Co4Ga19 plate.
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Figure 3. DSC curves of Ni56Mn21Co4Ga19 alloy showing the forward (on cooling) and reverse (on heating) martensitic transformations.
920
Y. Ma et al. / Scripta Materialia 58 (2008) 918–921 2.2
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Figure 4. Tensile stress–strain curve of Ni56Mn21Co4Ga19 alloy at room temperature. The inset shows the SME when prestrained to 5.3%. The arrow represents the strain recovery upon heating to 600 °C.
theoretical analysis on the improvement of ductility of polycrystalline Ni–Mn–Ga are expected. Comparing with the typical tensile stress–strain curves of TiNi SMAs [19], which have three obvious stages that are associated with the elastic deformation of the multivariants, reorientation of martensitic variants and/or detwinning, the elastic and plastic deformation of reoriented martensites, the stress plateau corresponding to the reorientation of martensitic variants and/or detwinning disappears completely for Ni56Mn21Co4Ga19 alloy. This situation is similar to the cases of other HTSMAs, such as NiTiPd [20] and NiTiHf [21], in which the stress plateau completely disappears and high work-hardening is constantly observed, due to the dislocation slip during the reorientation of martensite variants. It is well known that the strain could be recovered due to SME occurrences only through the reorientation of martensitic variants and, in general, a large magnitude of stress plateau means a large shape memory recovery. Accordingly, it can be guessed that Ni56Mn21Co4Ga19 alloy will not exhibit good SME. In order to investigate SME, Ni56Mn21Co4Ga19 plate samples were tensed to different prestrains. The insert of Figure 4 shows the stress–strain curve at prestrain of 5.3%. The residual strain after unloading is 4.3%. The arrowed line in the inset represents the recovery of the strain after heating to 600 °C. The recoverable strain is 2.1% due to the reverse martensitic transformation, and the recoverable rate is 48.8%. Figure 5 shows the recovery strains at different residual strains. It can be seen that the recovery strain increases remarkably with increasing residual strains. Ni56Mn21Co4Ga19 alloy exhibits poor SME as compared with single crystalline Ni54Mn25Ga21 alloy [8]. This can be understood by considering the structural reasons. One important item is the distribution of the c phase in Ni56Mn21Co4Ga19 alloy. It has been demonstrated that in dual-phase Ni–Al–Fe alloys that the pseudoelastic strain recovery and shape memory recovery strain degraded with increasing c phase fraction. Similarly, it is believed that the reorientation of martensitic variants and/or detwinning is hampered by c phase particles dispersed in martensitic matrix, and c phase particles are also barriers to the shape recovery to the parent phase of the Ni56Mn21Co4Ga19 alloy from
Shape Memory Strains (%)
400 Tensile Stress(MPa)
Tensile Stress (MPa)
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2.0 1.8 1.6 1.4 1.2 1.0 0.8 1.5
2.0
2.5
3.0
3.5
4.0
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Figure 5. Shape memory strains of Ni56Mn21Co4Ga19 alloy at different residual strains.
deformed martensite [14]. Of course, in the polycrystalline Ni56Mn21Co4Ga19 alloy the existence of the grain boundaries may also result in decrease of the SME, as compared with single crystalline Ni54Mn25Ga21 alloy. In conclusion, Ni56Mn21Co4Ga19 alloy can be successfully hot-rolled to thin plates of 0.5 mm thickness, and the mechanical properties and SME are measured by tensile tests. Ni56Mn21Co4Ga19 alloy exhibits greatly improved hot workability and ductility, high martensitic transformation temperature (Ms = 412 °C), as well as recoverable strain of 2.1% under a residual strain of 4.3%. This work was supported by the Natural Science Foundation of China (No. 50771086), the Program for New Century Excellent Talents in Fujian Province University (NCETFJ), and the Youth Science Foundation of Fujian Province of China (No. 2006F3119). [1] K. Otsuka, X.B. Ren, Intermetallics 7 (1999) 511. [2] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, Cambridge, 1998. [3] H.B. Xu, Mater. Sci. Forum. 394–395 (2002) 375. [4] J.H. Yang, C.M. Wayman, Intermetallics 2 (1994) 111. [5] J.H. Mulder, J.H. Maas, J. Beyer, in: Proceedings of the ICOMAT-92 Monterey, 1992, p. 869. [6] D. Golberg, Y. Xu, Y. Murakami, Scripta Mater. 30 (1994) 1349. [7] V.A. Chernenko, E. Cesari, V.V. Kokorin, I.N. Vitenko, Scripta Metall. Mater. 33 (1995) 1239. [8] H.B. Xu, Y.Q. Ma, C.B. Jiang, Appl. Phys. Lett. 82 (2003) 3206. [9] V.A. Chernenko, V. L’vov, J. Pons, E. Cesari, J. Appl. Phys. 93 (2003) 2394. [10] Y.Q. Ma, C.B. Jiang, G. Feng, H.B. Xu, Scripta Mater. 48 (2003) 365. [11] K. Ishida, R. Kainuma, N. Ueno, T. Nishizawa, Metall. Trans. A 22 (1991) 441. [12] Y. Tanaka, K. Oikawa, Y. Sutou, T. Omori, R. Kainuma, K. Ishida, Mater. Sci. Eng. A 438–440 (2006) 1054. [13] Y.Q. Ma, L.H. Xu, Y. Li, C.B. Jiang, H.B. Xu, Y.K. Lee, Z. Metallkd. 96 (2005) 843. [14] Y. Xin, Y. Li, L. Chai, H.B. Xu, Scripta Mater. 57 (2007) 599. [15] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill, New York, 1986.
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