Double-peak age strengthening of magnetostriction in heterogeneous Fe-Ga alloy

Artificial aging is a powerful approach to strengthen most structural alloys owing to the precipitation of mechanically harder particles, and a large portion of structural alloys exhibit a “harder-to-softer-to-harder” transition when extending the aging time, an effect known as “double-peak age strengthening”. Beyond this, here we report a “double-peak age strengthening” effect of functional properties—magnetostriction (magnetic-field-induced elastic strain), well corresponding to two successive microhardness softening states, as exhibited in a proof-of-principle reference Fe₇₃Ga₂₇ alloy. During the isothermal aging process, the body-centered-cubic (BCC) supersaturated solid solution undergoes concurrent ordering of the matrix and successive precipitations of Ga-depleted tetragonal L6₀ and Ga-enriched ω/M (ω is a hexagonal phase ; M is a monoclinic Fe₁₃Ga₉-type phase) nanoparticles with extending aging time. The first magnetostriction peak arises from the nucleation of coherent L6₀ nanoprecipitates, following which the magnetostriction drops due to the formation of ω nanoprecipitates as well as the phase-transition-induced dislocations. The second peak appears when the coherent M nanoparticles precipitate and the phase-transition-induced dislocations are reduced, prior to the formation of incoherent L6₀/L1₂ phases. Upon reaching these two critical conditions, the saturation magnetostriction is nearly 3 times higher than that of the solution-treated counterpart. Such double-peak age strengthening effect of magnetostriction was observed at a metastable phase-transition state, which may provide more freedom for tuning the functional properties of heterogeneous alloys.