Co-Ni-Al and Co-Ni-Al-Fe Ferromagnetic Shape Memory Alloy

Co-Ni-Al and Co-Ni-Al-Fe Ferromagnetic Shape Memory Alloy Microstructures and Magnetic Anisotropy Properties of Co-Ni-Al and Co-Ni-Al-Fe ferromagnetic shape memory alloy Abstract This study investigated the microstructure, magnetic anisotropy and the trend of magnetic field induced strain in Co-Ni-Al and Co-Ni-Al-Fe ferromagnetic shape memory alloys. At room temperature, a trunk-type ÃŽ ³ phase precipitates in the matrix phase and the grain boundaries in each specimen. The parent phase in each specimen is identified as L10-type martensitic phase with a (1-11) twinning plane, which prefer growth in (110) orientation after directional solidification. The magnetic anisotropy constant can evaluate 1.13Ãâ€"106erg ·cm-3 and 1.36Ãâ€"106erg ·cm-3 by Suckmith-Thompson method, respectively. The trend of twin martensitic rearrangement had evaluated by O’handley model and the result was revealed that the magnetic anisotropy energy in specimens was far greater than Zeeman energy difference across the twin boundaries and the twin martensitic can rearrangement to obtain strains in applied magnetic field. Key words: magnetic anisotropy; ferromagnetic shape memory alloys; twin martensitic; Suckmith-Thompson method; strains in applied magnetic field 1 . Introduction Ferromagnetic shape memory alloys (FSMAs) exhibit large magnetic field induced strain (MFIS) and rapid response in the application of an external magnetic field, which was considered as potential candidate materials for magnetic controlled actuators and sensors[1, 2]. Several FSMAs exist including Ni-Mn-Ga[3-8], Co-Ni-Ga[9, 10], Ni-Mn-Al[1], Ni-Fe-Ga[2] and Co-Ni-Al[11-17] etc. Of these alloys, ÃŽ ²-base Co-Ni-Al alloys was drawn much attention because of their better ductility and low cost of constituent elements[18, 19]. In Co-Ni-Al alloys, dual-phase structure arises is of a great advantage for practical applications, due to tailor of mechanical properties of the ÃŽ ² phase and ÃŽ ³ phase. Generally, ÃŽ ² phase (B2, B.C.C.) in polycrystalline material is extremely hard and brittle, but the presence of ÃŽ ³ phase (A1, F.C.C.) can significantly improve the ductility with alloy[20, 21]. On the other hand, B2-type ÃŽ ² phase has transformed to the L10-type thermo-elastic martensite when temperature cooling below the phase transformation temperature and a large MFIS were found in Co-Ni-Al alloy due to the rearrangement of twin martensite variants in external magnetic field[22, 23]. In MFIS process, the magnetic anisotropy energy can lead the variant rearrangement in order that the magnetic easy axis was aligned parallel to the magnetic field direction when the magnetic anisotropy energy was larger than the energy driving variant rearrangement[24]. So, to obtain the magnetic anisotropy and the trend of twin martensite boundary mobility in FMSAs was very important. In this study, the microstructure and magnetic anisotropy in Co-Ni-Al and Co-Ni-Al-Fe were investigated. Furthermore, in order to establish out a useful direction in ferromagnetic shape memory alloy designs, the trend of magnetic field induced strain with ferromagnetic element Fe added in Co-Ni-Al alloy was discussed. 2. Experimental Procedure The samples with the composition Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 (at%) were prepared by arc-melting furnace using purity elements (>99.99%) under pure argon atmosphere. Ingots were melted four times to ensure the homogeneity and then suction cast into rods with a diameter of 3mm and a length of 70mm. The rods were grown used the liquid metal cooling directional solidification method in Al2O3 crucible at pulling rate of 100ÃŽ ¼m/s and temperature gradients of 800à ¢Ã¢â‚¬Å¾Ã†â€™/cm. In order to obtain microstructure of the specimens, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were examined. XRD were examined in the Philips PW170 using CuKÃŽ ± 1 radiation at a scanning angle of 10 °-90 ° and a scanning speed of 3 °/min. TEM was performed on a Philips CM12 and a Tecnai F20 super twin field emission gun TEM equipped with a Gatan imaging filter system. Specimens for TEM analysis were thinned by twin jet electro-poli shing in a solution of 5% perchloric acid and 95% ethanol. The magnetization was examined for selected samples using the Vibrating Sample Magnetometer (Lake Shore 7407) with a maximum magnetic field of 1.5T at room temperature. 3. Results and Discussion 3.1 Microstructures The microstructure images of specimens are shown in Fig.1. It can be seen that a typical dendritic morphology in the specimens and the trunk phase are the Co-rich ÃŽ ³ phase, which precipitates in the matrix phase and the grain boundaries in each specimen. The ÃŽ ³ phase grows in Co1.36Ni1.21AlFe0.12 alloy is smaller indicating that Fe add in Co-Ni-Al alloy has a trend to formationtion more matrix phase. The matrix phase undergoes the martensitic transformation suggesting that the martensitic transformation start temperature (TMs) higher than room temperature. Fig.2 gives the XRD patterns of Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12. The spectrum peaks of the parent phase in each specimen is identified as L10 structure (martensite phase) with the small amount of the coexisting ÃŽ ³ phase (A1 structure), which is in good agreement with the observation of the micrographs. After directional solidification, the martensitic implies preferred (110) orientation in alloy Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 and the spectrum peak of ÃŽ ³ phase appears less orientation when Fe add in Co1.36Ni1.21Al alloy. Fig.3 shows TEM photographs and selected-area diffraction pattern of samples. It can be seen that martensite, whose transformation from ÃŽ ² phase, is tetragonal L10 structure. The twin martensite is spearhead-shaped, which is the presence of many black and white pinstripes regularly piled up. Fig.3b and 3d shows the electron diffraction patterns exhibiting the structural feature of the specimens. The patterns were taken with an incident electron beam parallel to the [011] zone axis and the primary diffraction spots are indexed for the L10 structure twin martensite with a (1-11) twinning plane. 3.2 Magnetic anisotropy The magnetizations of specimens as a function of applied magnetic field at room temperature are shown in Fig.4. The measured M-H curves for the a-plane direction can be saturated easily, while the magnetization for the c-axis is hardly saturated. Obviously, a-plane is the easy direction to magnetic, but c-axis is the hard direction. The value of coercivity (Hc) and saturation magnetization (Ms) with Co1.36Ni1.21Al alloy was about 102Oe and 43.72emu/g, respectively. Compared, the value of Ms was promoted from 43.72emu/g to 57.64emu/g and the Hc decrease from 102Oe to 53Oe in Co1.36Ni1.21AlFe0.12. The axial magnetic anisotropy constant Ku of the sample was determined by the magnetization curves measured along and perpendicular the axis. The magnetic anisotropy energy Em was calculated by equation[25] (1): Em≈K2’sin2ÃŽ ¸+K4’sin4ÃŽ ¸ (1) Where ÃŽ ¸ is the angle between the magnetization and the c-axis; K2’ is the second-order magnetic anisotropy constant and K4’ is the fourth-order magnetic anisotropy constant. The value of magnetic anisotropy constant Ku is approximately equal to the sum of K2’ and K4’ as shows in equation (2): Ku≈K2’+ K4’ (2) After correcting the demagnetizing field, the value of magnetic anisotropy constant K2’, K4’ and Ku can evaluate by the Suckmith-Thompson method [24]using the equation (3): 2 K2’/Ms2+(4K4’/Ms4)M2=He/M (3) Where Ms is the saturation intensity; M is the magnetization and He is the effective field. From equation (3), the anisotropy constants can obtain from the graph of M2 and He/M: the slope being is 4 K4’/Ms4 and the intercept of Y-axis is 2 K2’/Ms2. Fig.5 is the graph of M2 and He/M of specimens Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 and the values of magnetic anisotropy constant K2’, K4’ and Ku were calculated in Table 1. However, the value of Ku in Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 approach same level compare with tradition FSMAs (NiMnGa[26, 27], Ku=-2.03Ãâ€"106 erg ·cm-3) and the lager value of Ku can provide greater magnetic anisotropy energy in applied magnetic field. 3.3 Dimensionless field normalized by anisotropy The magnetic field induced strains in FSMAs are explained by the rearrangement of twin boundaries in variants martensitic phase under the driving force of the Zeeman energy (MsH) difference across the twin boundaries. Twin boundaries with the large magnetic anisotropy can obtain great magnetic anisotropy energy in applied magnetic field. When the magnetic anisotropy energy is bigger than the energy driving variant rearrangement, the magnetic anisotropy energy can lead the variant rearrangement in order that the magnetic easy axis is aligned parallel to the magnetic field direction. The mechanism for twin-boundary motion shows in Fig.6. O’handley[28] was used dimensionless field parameter ha to express the relationship between Zeeman energy and magnetic anisotropy energy. The dimensionless field parameter ha can evaluate by the equation (4): ha=MsH/2Ku (4) When haaà ¢Ã¢â‚¬ °Ã‚ ®1, the magnetic anisotropy energy is not sufficient to overcome Zeeman energy and the material can’t obtain strain in applied magnetic field. In order to make sure trend of magnetic field induced strain of specimens, the values of ha were calculated and the result list in Table 2. Obviously, the values of ha in Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 alloys was smaller than 1. The magnetic anisotropy energy of specimens is far greater than Zeeman energy difference across the twin boundaries and the twin martensitic can rearrangement to obtain large strains in applied magnetic field. Furthermore, Fe added in Co-Ni-Al alloy can enhance the magnetic anisotropy and reduce the dimensionless field parameter ha as shows in Table 2. It was suggesting that Co1.36Ni1.21AlFe0.12 has lager trend of twin boundary rearrangement and it is a meaningful direction for material design of FSMAs. 4. Conclusion In order to obtain large magnetic field induced strain of MFIS at room temperature in Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 alloys, the microstructure and magnetic anisotropy and the trend of rearrangement twin boundary were investigated. A trunk-type ÃŽ ³ phase precipitates in the matrix phase and the grain boundaries in each specimen. The parent phase in each specimen is identified as L10-type martensitic phase with a (1-11) twinning plane, which prefer growth in (110) orientation after directional solidification. The magnetic anisotropy constant Ku of Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 alloys were evaluated to be 1.13Ãâ€"106erg ·cm-3 and 1.36Ãâ€"106erg ·cm-3, respectively. The trend of twin martensitic rearrangement has evaluated using O’handley model. The result is revealed that the dimensionless field parameter ha of Co1.36Ni1.21Al and Co1.36Ni1.21AlFe0.12 was smaller than 1 and the magnetic anisotropy energy in specimens was far greater than Zeeman energy difference across the twin boundaries. In this condition, twin martensitic can rearrangement and obtains large strains in applied magnetic field. Refernces [1] Fujita A, Gejima F, Ishida K. Magnetic properties and large magnetic-field-induced strains in off-stoichiometric Ni-Mn-Al Heusler alloys[J]. Applied Physics Letters. 2000, 77 (19 ): 3054-3056. [2] Morito H, Fujita A, Ota T, et al. Magnetocrystalline Anisotropy in a single crystal Fe-Ni-Ga ferromagnetic shape memory alloy[J]. MATERIALS TRANSACTIONS . 2003, 44 (4 ): 661-664. [3] Kimura A, Ye M, Taniguchi M, et al. Lattice instability of Ni-Mn-Ga ferromagnetic shape memory alloys probed by hard X-ray photoelectron spectroscopy[J]. Applied Physics Letters. 2013, 103 . [4] Pagounis E, Chulist R, Lippmann T, et al. Structural modification and twinning stress reduction in a high-temperature Ni-Mn-Ga magnetic shape memory alloy[J]. Applied Physics Letters. 2013, 103 . [5] Seiner H, Bodnarova L, Kopecky V, et al. The effect of antiphase boundaries on the elastic properties of Ni-Mn-Ga austenite and premartensite.[J]. Journal of physics. Condensed matter : an Institute of Physics journal. 2013, 25 (42 ): 425402. [6] Pushpanathan K, Santhi R, Chokkalingam R, et al. Martensitic Transformation and Microstructure of Ni-Mn-Ga Magnetic Shape Memory Alloy[J]. Materials and Manufacturing Processes. 2012, 28 (1 ): 72-78. [7] Pons J, Santamarta R. Crystal structure of martensitic phases in Ni-Mn-Ga shape memory alloys[J]. Acta Materialia. 2000, 48 (12 ): 3027-3038. [8] Likhachev A A. Magnetic-field-controlled twin boundaries motion and giant magneto-mechanical effects in Ni-Mn-Ga shape memory alloy[J]. Physics Letters A. 2000, 275 (1-2 ): 142-151. [9] Liu J, Zheng H X. High undercooling effect on magnetic shape memory Co-Ni-Ga alloys[J]. Materials Letters. 2006, 60 (13-14 ): 1693-1696. [10] Wuttig M, Craciunescu C. A new ferromagnetic shape memory alloy system[J]. Scripta Materialia. 2001, 44 (10 ): 2393-2397. [11] Scheerbaum N, Kraus R, Liu J, et al. Reproducibility of martensitic transformation and phase constitution in Ni–Co–Al[J]. Intermetallics. 2012, 20(1): 55-62. [12] Liu J, Li J G. Microstructure, shape memory effect and mechanical properties of rapidly solidified Co–Ni–Al magnetic shape memory alloys[J]. Materials Science and Engineering: A. 2007, 454–455: 423-432. [13] Bartova B, Schryvers D, Yang Z, et al. Microstructure and precipitates in as-cast Co38Ni33Al29 shape memory alloy[J]. Scripta Materialia. 2007, 57(1): 37-40. [14] Liu J, Zheng H X, Li J G. Effect of solidification rate on microstructure and crystal orientation of ferromagnetic shape memory alloys Co–Ni–Al[J]. Materials Science and Engineering: A. 2006, 438–440(04): 1061-1064. [15] Liu J, Huang Y L, Li J G. Microstructure and magnetic field induced strain of directionally solidified ferromagnetic shape memory CoNiAl alloys[J]. Scripta Materialia. 2005, 53 (1 ): 29-33. [16] Wang H Y, Liu Z H, Wang Y G, et al. Microstructure of martensitic phase in the Co39Ni33Al28 shape memory alloys revealed by transmission electron microscopy[J]. Journal of Alloys and Compounds. 2005, 400(1–2): 145-149. [17] Fujita A, Kudo T, Kainuma R, et al. Magnetocrystalline anisotropy in a single-variant Co-Ni-Al ferromagnetic shape memory alloy[J]. Materials Transactions. 2003, 44 (10 ): 2180-2183. [18] Khandelwal A, Sharma V K, Chandra L, et al. The magnetic properties across the martensitic transition in the Co38Ni34Al28 alloy[J]. Journal of Magnetism and Magnetic Materials. 2012, 324 (5 ): 729-734. [19] Chatterjee S, Thakur M, Giri S, et al. Transport, magnetic and structural investigations of Co–Ni–Al shape memory alloy[J]. Journal of Alloys and Compounds. 2008, 456(1–2): 96-100. [20] Seiner H, Kopecek J, Sedlak P, et al. Microstructure, martensitic transformation and anomalies in c -softening in Co-Ni-Al ferromagnetic shape memory alloys[J]. Acta Materialia. 2013, 61 (15 ): 5869-5876. [21] Tanaka Y, Oikawa K, Sutou Y, et al. Martensitic transition and superelasticity of Co–Ni–Al ferromagnetic shape memory alloys with ÃŽ ²+ÃŽ ³ two-phase structure[J]. Materials Science and Engineering: A. 2006, 438–440: 1054-1060. [22] Maziarz W. Structure changes of Co–Ni–Al ferromagnetic shape memory alloys after vacuum annealing and hot rolling[J]. Journal of Alloys and Compounds. 2008, 448(1–2): 223-226. [23] Maziarz W, Dutkiewicz J, Santamarta R, et al. Microstructure changes in two phase ÃŽ ²+ ÃŽ ³ Co-Ni-Al ferromagnetic shape memory alloys in relation to Al/Co ratio[J]. The European Physical Journal Special Topics. 2008, 158(1): 137-142. [24] Morito H, Oikawa K, Fujita A, et al. Large magnetic-field-induced strain in Co–Ni–Al single-variant ferromagnetic shape memory alloy[J]. Scripta Materialia. 2010, 63(4): 379-382. [25] Sucksmith W T J E. The Magnetic anisotropy of cobalt [J]. Proceedings of the Royal Society of London Series A-Mathematical and Physical Sciences. 1954, 225(1162): 362-375. [26] Sozinov A L A A U. Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase[J]. Applied Physics Letters. 2002, 80(10): 1746-1748. [27] Sozinov A L A A. Crystal structures and magnetic anisotropy properties of Ni-Mn-Ga martensitic phases with giant magnetic-field-induced strain[J]. IEEE Trancations on Magnetics. 2002, 38(5): 2814-2816. [28] OHandley R C. Model for strain and magnetization in magnetic shape-memory alloys[J]. Journal of Applied Physics. 1998, 83 (6 ): 3263-3270.