Depth-dependent decomposition and property of large magnetostriction Fe-Ga alloys

[Display omitted] •Surface of Fe-Ga bulk has faster decomposition kinetics than interior.•From surface to interior, fraction of equilibrium FCC decreases.•From surface to interior, magnetostriction keeps increasing. Introducing dispersive and coherent nanoprecipitates by early-stage decomposition has been recognized as a promising strategy to achieve super-functional properties, such as the large and sensitive magnetostriction in Fe-Ga alloys. Considering that the diffusion-controlled decomposition is very sensitive to material position, where the atomic diffusion rate can be several orders faster at material surface than that at material interior, it is necessary to investigate how the transformation depth affects the microstructure and the macroscopic property. Here, we investigated the depth-dependent phase transformation and magnetostriction of isothermally-aged Fe75Ga25 alloy, which evolves a diffusion-controlled phase transformation from metastable body-centered-cubic (BCC) to equilibrium face-centered-cubic (FCC) intermediated by face-centered-tetragonal (FCT) phase. X-ray diffraction (XRD) and Transmission electron microscop (TEM) results revealed that from the surface to the interior, both volume fraction and size of the FCC phase gradually decrease, together with the gradual structure transformation from FCC to FCT. When compared with the solution-treated state prior to aging, ordering of the BCC matrix also happens at the sample interior. The gradual changes in precipitates’ structure and morphology result in the gradual magnetostriction enhancement from the material surface to the interior since the FCC phase has negative magnetostriction constant, being opposite to that of the BCC matrix. Our work not only reveals that the over-transformed surface layer of the bulk Fe-Ga material should be removed for fabricating large-magnetostriction materials but also suggests that the depth-dependent microstructure should be carefully controlled when applying the precipitation strategy to develop high-performance materials.