Internal structure evolution of L12 variants in aged Fe-Ga alloys

Recently, ferromagnetic Fe-Ga composites prepared by aging the metastable body-centered-cubic (BCC) precursor within the ordered face-centered-cubic (FCC) L1(2) phase regime have been found to yield novel properties, such as highly thermal-stable magnetization, stress-insensitive magnetic permeability, and sign-changed-magnetostriction. Accurate control of the macroscopic properties requires a thorough understanding of the microstructure evolution during the aging process. Here we performed an aging-time dependent study on a Fe73Ga27 alloy to investigate the transformation process and the associated internal structure evolution. The differential scanning calorimetry (DSC) measurements reveal that the BCC to L1(2) transformation is diffusion-controlled. The detailed transmission electron microscope (TEM) investigations reveal that this transformation has also displacive feature, exhibiting shear-induced twin-related L1(2) variants with multilayer internal structure prior to approaching phase equilibria. The twin boundary and sublayer interface are found to contain {111}-L1(2) stacking faults. Especially, the stacking faults within individual L1(2) variants disappear after long-term aging. These findings suggest that this transformation is not only driven by the large free energy difference between the metastable BCC and the equilibrium L1(2) phases but also driven by reducing the stacking fault energy, which may help to understand its slow transformation kinetics and to discover novel properties through engineering the microstructure of Fe-Ga alloys. (C) 2020 Elsevier B.V. All rights reserved.