From metastable to stable structure: the way to construct functionality in Fe-27Ga alloy

Highlights: • Interrupted isothermal annealing leads to the formation of an intrinsic composite. • Structure and magnetostriction depend on temperature and time of annealing. • Doping by Tb stabilizes bcc-derivative phases in Fe-27Ga alloy. Isothermal annealing treatment was used to form a natural two-phase ferromagnetic composite structure with a different ratio between the bcc-derivative metastable D0₃ and fcc-derivative equilibrium L1₂ phases. Depending on the ratio of these phases, magnetostriction values of the alloy can be varied considerably between +100 and −50 ppm, while magnetic field dependence of the magnetostriction depends on dispersion of the L1₂ phase. Microalloying of Fe-27Ga type alloy with Tb significantly stabilizes the metastable D0₃ phase and leads to the formation of a new phase enriched of Tb and Ga at the grain boundaries of as cast alloy. A competition between the precipitation of the Tb-enriched phase and the nucleation of the L1₂ phase at the grain boundaries of the D0₃ phase can reasonably explain the decelerating effect of Tb on the kinetics of the phase transitions in Fe-27Ga alloys. The Tb-enriched phase precipitates during casting, and therefore, at subsequent annealing these precipitations prevent the nucleation of the L1₂ phase on the grain boundaries of the as cast (D0₃) structure.