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 γ 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 1.13×10⁶erg·cm^-3 and 1.36×10⁶erg·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.

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Of these alloys, β-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 β phase and γ phase. Generally, β phase (B2, B.C.C.) in polycrystalline material is extremely hard and brittle, but the presence of γ phase (A1, F.C.C.) can significantly improve the ductility with alloy^{[20, 21]}.

On the other hand, B2-type β 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 Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 (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 100μm/s and temperature gradients of 800℃/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 CuKα 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-polishing 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 γ phase, which precipitates in the matrix phase and the grain boundaries in each specimen. The γ phase grows in Co_1.36Ni_1.21AlFe_0.12 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 (T_Ms) higher than room temperature.

Fig.2 gives the XRD patterns of Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12. The spectrum peaks of the parent phase in each specimen is identified as L1₀ structure (martensite phase) with the small amount of the coexisting γ 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 Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 and the spectrum peak of γ phase appears less orientation when Fe add in Co_1.36Ni_1.21Al alloy.

Fig.3 shows TEM photographs and selected-area diffraction pattern of samples. It can be seen that martensite, whose transformation from β 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 (H_c) and saturation magnetization (M_s) with Co_1.36Ni_1.21Al alloy was about 102Oe and 43.72emu/g, respectively. Compared, the value of M_s was promoted from 43.72emu/g to 57.64emu/g and the H_c decrease from 102Oe to 53Oe in Co_1.36Ni_1.21AlFe_0.12.

The axial magnetic anisotropy constant K_u of the sample was determined by the magnetization curves measured along and perpendicular the axis. The magnetic anisotropy energy E_m was calculated by equation^[25] (1):

E_m≈K₂^’sin²θ+K₄^’sin⁴θ (1)

Where θ 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 K_u is approximately equal to the sum of K₂^’ and K₄^’ as shows in equation (2):

K_u≈K₂^’+ K₄^’ (2)

After correcting the demagnetizing field, the value of magnetic anisotropy constant K₂^’, K₄^’ and K_u can evaluate by the Suckmith-Thompson method ^[24]using the equation (3):

2 K₂^’/M_s²+(4K₄^’/M_s⁴)M²=H_e/M (3)

Where M_s is the saturation intensity; M is the magnetization and H_e is the effective field. From equation (3), the anisotropy constants can obtain from the graph of M² and H_e/M: the slope being is 4 K₄^’/M_s⁴ and the intercept of Y-axis is 2 K₂^’/M_s².

Fig.5 is the graph of M² and H_e/M of specimens Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 and the values of magnetic anisotropy constant K₂^’, K₄^’ and K_u were calculated in Table 1. However, the value of K_u in Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 approach same level compare with tradition FSMAs (NiMnGa^{[26, 27]}, K_u=-2.03×10⁶ erg·cm^-3) and the lager value of K_u 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 (M_sH) 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 h_a to express the relationship between Zeeman energy and magnetic anisotropy energy. The dimensionless field parameter h_a can evaluate by the equation (4):

h_a=M_sH/2K_u (4)

When h_a<1, where magnetic anisotropy energy is stronger than Zeeman energy, the twin boundaries in variants martensitic phase will rearrangement and obtain large strains. If h_a≮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 h_a were calculated and the result list in Table 2. Obviously, the values of h_a in Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 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 h_a as shows in Table 2. It was suggesting that Co_1.36Ni_1.21AlFe_0.12 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 Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 alloys, the microstructure and magnetic anisotropy and the trend of rearrangement twin boundary were investigated.

A trunk-type γ 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 K_u of Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 alloys were evaluated to be 1.13×10⁶erg·cm^-3 and 1.36×10⁶erg·cm^-3, respectively. The trend of twin martensitic rearrangement has evaluated using O’handley model. The result is revealed that the dimensionless field parameter h_a of Co_1.36Ni_1.21Al and Co_1.36Ni_1.21AlFe_0.12 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.

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