Magnetic heat-treatment induced uniaxial magnetic anisotropy and dynamic precipitation in Fe–Ga magnetostrictive alloys

Introducing coherent tetragonal nanoprecipitates into a cubic matrix has recently been found to significantly increase the magnetostriction of Fe-based alloys. Combining this novel strategy with conventional approaches, such as the introduction of uniaxial magnetic anisotropy (uniaxial alignment of magnetic domains), may improve the magnetostriction performance. On the basis of this consideration, a large-magnetostriction alloy, Fe₇₃Ga₂₇ (at%), was subjected to magnetic heat treatment (MHT) to introduce uniaxial anisotropy and nanoprecipitates simultaneously. The results showed that strong uniaxial magnetic anisotropy (well-aligned magnetic domains) is induced together with the formation of dense nanoprecipitates, hence improving the magnetostriction from 65 ppm for the solution-treated (ST) counterpart to 140 ppm by 115%. However, such magnetostriction enhancement effect is weaker than that observed in the sample subjected to heat treatment for the same time at the same temperature in the absence of a magnetic field. Further microstructural investigations revealed that compared with isothermal annealing, MHT led to dynamic precipitation, the formation of coarser and incoherent tetragonal nanoprecipitates and even a face-centered cubic (FCC) equilibrium phase with intrinsic negative magnetostriction. These results suggest that the magnetic field provides an extra driving force to accelerate precipitation kinetics and that the magnetic annealing conditions should be carefully controlled to prevent the overgrowth of tetragonal nanoprecipitates to enhance the magnetostriction performance.