Compressive pre-stress effects on magnetostrictive behaviors of highly textured Galfenol and Alfenol thin sheets

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 (H_s) 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 H_s with pre-stress, and present preliminary results from tests conducted to determine the potential role of shape anisotropy in this observation.