Magnetic shape-memory alloy

Magnetic shape-memory alloys (MSMAs), or ferromagnetic shape-memory alloys (FSMAs), are ferromagnetic materials which exhibit large strains under the influence of an applied magnetic field due to martensitic phase transformation. Magnetic shape-memory alloys, with near-stoichiometric Ni₂MnGa being the most studied example, differ from other magnetostrictive materials, such as Terfenol-D and Galfenol, as they produce much larger strains by twinning, sometimes as large as 9%, under relatively low bias magnetic fields. The mechanism is based on the magnetic anisotropy of the material.

MSMAs produce a similar phase transformation between martensite 1 and martensite 2 (the two variants), as other shape memory alloys (SMAs) which change phase between austenite and martensite with the application of thermal energy. Few models have been developed which describe the constitutive response of MSMAs. Typically, thermodynamic modeling is used to describe the materials behavior.

When finding strain for MSMA the total strain equals the sum of the parts: ε^total = ε^elastic + ε^{reorientation} where ε^{reorientation} is defined as ε^{reorientation} = ε^r,max*ξ. ξ and ε^r,max are defined as variant two volume fraction and maximum reorientation strain, respectively.

ξ may be found analytically from a driving force function found from the Gibbs free energy using the relations of a polynomial or trigonometric hardening function. Variant 2 volume fraction (the variant which expands the specimen when exposed to magnetic energy) is a function of the magnitude of bias magnetic field, applied stress, heat, magnetic anisotropy energy, and other material properties such as magnetization saturation.

Due to the nature of MSMA, a shift in the direction of magnetization is produced when applying a stress to a fully strained element exposed to a bias field. The magnitude of this shift is dependent on the strength of the applied field and material properties. Using Faraday's law of induction, it is evident that MSMAs may be used for energy harvesting using a pickup coil, or inductor.

^{[1] [2]}

Compared to the typical shape-memory alloy (SMA), MSMA can actuate at higher frequencies (up to 1 kHz). However, MSMAs are stiff, very brittle and only recommended for low temperature and actuation force applications.

References

1. B. Kiefer and D. C. Lagoudas, "Magnetic field-induced martensitic variant reorientation in magnetic shape memory alloys", Philosophical Magazine, vol. 85, nos. 33-35, 21 Nov. -11 Dec. 2005, 4285-4329
2. Karaman et al., "Energy harvesting using martensite variant reorientation mechanism in NiMnGa magnetic shape memory alloy", Applied physics letters, 90, 172505 (2007)