Nanoprecipitation induced giant magnetostriction: A time-resolved small-angle neutron scattering study of the vacancy-assisted kinetics

Abstract

Solid-state precipitation is an effective strategy for tuning the mechanical and functional properties of advanced alloys. Structure design and modification necessitate good knowledge of the kinetic evolution of precipitates during fabrication, which is strongly correlated with defect concentration. For Fe-Ga alloys, giant magnetostriction can be induced by the precipitation of the nanoscale tetragonal L60 phase. By introducing quenched-in vacancies, we significantly enhance the magnetostriction of the aged Fe81Ga19 polycrystalline alloys to ∼305 ppm, which is close to the level of single crystals. Although vacancies were found to facilitate the generation of the L60 phase, their impact on the precipitation mechanism and kinetics has yet to be revealed. This study combined transmission electron microscopy (TEM) and time-resolved small-angle neutron scattering (SANS) to investigate the precipitation of the L60 phase during the isothermal aging at 350 and 400 °C, respectively. The evolution of L60 nanophase in morphology and number density in as-cast (AC) and liquid nitrogen quenched (LN) Fe81Ga19 alloys with aging time were quantitatively compared. Interestingly, the nucleation of the L60 phase proceeds progressively in AC while suddenly in LN specimens, indicating the homogenous to heterogeneous mechanism switching induced by concentrated vacancies. Moreover, excess vacancies can change the shape of nanoprecipitates and significantly accelerate the growth and coarsening kinetics. The magnetostrictive coefficient is optimized when the size (long-axis) of L60 precipitates lies between 100 and 110 Å with a number density between 3.2–4.3 × 10–7 Å–3. Insight from this study validates the feasibility of achieving high magnetoelastic properties through precise manipulation of the nanostructure.