Abstract
Fe-Ga (Galfenol) and Fe-Al (Alfenol) are rare-earth-free magnetostrictive alloys with mechanical robustness and strong magnetoelastic coupling. Since highly textured Galfenol and Alfenol thin sheets along <100> orientations have been developed with magnetostrictive performances of ∼270 ppm and ∼160 ppm, respectively, they have been of great interest in sensor and energy harvesting applications. In this work, we investigate stress-dependent magnetostrictive behaviors in highly textured rolled sheets of NbC-added Fe80Al20 and Fe81Ga19 alloys with a single (011) grain coverage of ∼90%. A compact fixture was designed and used to introduce a uniform compressive pre-stress to those thin sheet samples along a [100] direction. As compressive pre-stress was increased to above 100 MPa, the maximum observed magnetostriction increased 42% in parallel magnetostriction along the stress direction, λ//, in highly textured (011) Fe81Ga19 thin sheets for a compressive pre-stress of 60 MPa. The same phenomena were observed for (011) Fe80Al20 (maximum increase of 88% with a 49 MPa compressive stress). This trend is shown to be consistent with published results on the effect of pre-stress on magnetostriction in rods of single crystal and textured polycrystalline Fe-Ga alloy of similar compositions, and single crystal data gathered using our experimental set up. Interestingly, the saturating field (Hs) does not vary with pre-stresses, while the saturating field in rod-shaped samples of Fe-Ga increases with an increase of pre-stress. This suggests that for a range of compressive pre-stresses, thin sheet samples have larger values of d33 transduction coefficients and susceptibility than rod-shaped samples of similar alloy compositions, and hence they should provide performance benefits when used in sensor and actuator device applications. Thus, we discuss potential reasons for the unexpected trends in Hs with pre-stress, and present preliminary results from tests conducted to determine the potential role of shape anisotropy in this observation.
Highlights
Iron-gallium alloys (Fe100-xGax) and iron-aluminum alloys (Fe100-xAlx), commonly known as Galfenol and Alfenol where 5≤x≤35, are rare-earth-free magnetostrictive alloys
Measurements of magnetostriction with field lines traveling along the [100] direction (λ//) and perpendicular to it (λ⊥) were used to obtain a peak-to-peak magnetostriction number λ// −λ⊥ for each specimen
Data are included for a compressive stress of 60 MPa to illustrate that increases in compressive stress above 49 MPa did not produce a further increase in magnetostriction
Summary
Iron-gallium alloys (Fe100-xGax) and iron-aluminum alloys (Fe100-xAlx), commonly known as Galfenol and Alfenol where 5≤x≤35, are rare-earth-free magnetostrictive alloys. Evidence of shape anisotropy effects contributing to experimental trends is discussed for single crystal and highly textured Fe-Ga specimens of varying aspect ratios These results may offer a foundation for an expansion of Armstrong’s energy model of the magnetic free energy to include a shape anisotropy term.[8] This 3D model is incredibly versatile; the fact that it does not take shape parameters into account limits its applicability to describe magnetostrictive behaviors of thin sheet specimens, the effect of stress on the d33 value (dλ/dH), saturation field Hs and susceptibility of these samples. These authors are the first we are aware of to have observed that in thin sheets, there is no significant increase in saturation field, Hs, with the application of stress, and they recognize the need for further study towards the discrepancy with bulk, single crystal Fe-Ga rods
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