Phase transitions as a tool for tailoring magnetostriction in intrinsic Fe-Ga composites

Abstract

Fe-Ga alloys exhibit unique functional properties such as magnetostriction that can be varied from the highest positive values among iron-based alloys to negative values including zero magnetostriction, if proper compositions and heat treatments are chosen. This remarkable behavior is related to rather complex phase transformation sequences in this alloy family that are still unresolved. In earlier studies, the phase transformations in Fe-Ga alloys were studied by X-ray diffraction, which provides structural information limited to the near-surface sample area. In this paper, we use electron back-scatter diffraction and in situ neutron diffraction to characterize the D03 and L12 phases that originate from the fcc and bcc phases in the Fe-27Ga type bulk alloy, respectively. Different ratios between these phases, characterized by magnetostriction values of different signs, were achieved using an isothermal annealing treatment that produces an intrinsic composite in the alloy. Depending on the relative fraction of the D03 and L12 phases, the magnetostriction values of the alloy, λS, vary from +100 to −50 ppm, including the value of λS = 0 for the alloy with L12:D03 = 2:1 achieved after 600 min annealing at 400 °C, thus demonstrating the controlled adjustment of magnetostriction in these advanced alloys.