Anelasticity of phase transitions in Fe-Ga alloys

Reference Speaker Authors
(Institution)
Abstract
404-004
Igor S. Golovin Golovin, I.S.(National University of Science and Technology); Palacheva, V.V.(National University of Science and Technology); Emdadi, A.(National University of Science and Technology); Balagurov, A.M.(Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research); Bobrikov, I.A.(Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research); Heintz, A.(École polytechnique fédérale de Lausanne); Mari, D.(Ecole Polytechnique Fédérale de Lausanne); 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. Phase transitions (A2 ? D03 ? L12 ? D019 ? B2) in the as-cast Fe-27Ga are carefully studied using SEM EBSD, MFM, XRD techniques, in situ neutron diffraction, magnetostriction tests and mechanical spectroscopy in the range from room temperature up to 850°C. Special attention is paid to influence of rare Earth elements (Tb) on the phase transitions. Isothermal annealing of the alloy was applied to obtain different ratios between bcc and fccderivative phases. A very strong anisotropic broadening of diffraction peaks was observed and related to micro-deformations in crystallites. The EBSD analysis shows that residual strains are inhomogeneously distributed within the transformed fcc grains and are concentrated at grain boundaries and triple junctions. Upon heating this first order transition leads to transient IF effects related to D03 ? L12 and D019 ? B2 transitions. In contrast, the L12 ? D019 transition in Fe-27Ga and second order D03 ? B2 transition in Fe-26Al and several Fe-Ga alloys with 13-21Ga are not accompanied with significant micro-deformations in crystallites and these transitions lead to a smaller IF transient effect (?-peak). We also demonstrate that the phase transition from an ordered bcc-derivative D03 phase to an fcc-derivative ordered L12 phase first leads to disordering of the D03 phase to obtain an A2 structure followed by an A2 to ?1 transition with final A1 phase ordering to achieve the L12 structure. Interrupted isothermal annealing leads to the formation of an intrinsic composite microstructure with a given ratio between bcc-derivative and fcc-derivative phases that have different magnetic properties. Depending on the ratio of these phases, magnetostriction values of the alloy are characterized by ?S from +100 to -50 ppm including ?S = 0 at a magnetic field H > 100 kA/m for alloy with L12:D03 = 2:1. Tb significantly slows down formation of closed packed structures and contributes into increase in absolute values of magnetostriction. The results of this work present the details of the phase transformations occurring in the Galfenol and related transient anelastic effects, and they also pave the way for tuning the microstructure of the alloy in such a way that the magnetostriction of the material can be tuned in a controlled way to meet the demands of a given application.
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