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

Abstract

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.