Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet Phase transitions in Fe-27Ga alloys: Guidance to develop functionality I.S. Golovin a , A.M. Balagurov c , A. Emdadi a,b,* , V.V. Palacheva a , I.A. Bobrikov c , V.V. Cheverikin a , E.N. Zanaeva a , D. Mari d a National University of Science and Technology “MISIS”, Leninsky ave. 4, 119049, Moscow, Russia b Chair of Mechanical Design and Manufacturing, Brandenburg University of Technology Cottbus-Senftenberg, Konard-Wachsmann-Allee 17, Cottbus, D-03046, Germany c Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, 141980, Russia d École Polytechnique Fédérale de Lausanne, Institute of Condensed Matter Physics, Lausanne, CH-1015, Switzerland ARTICLE INFO Keywords: Functional alloys (magnetostrictive) Phase transformation Magnetic properties Diffraction (neutron) ABSTRACT Several aspects of the formation of functional Fe-Ga alloys are considered in this paper: (i) the mechanism of phase transition from a metastable ordered bcc-derivative phase to an equilibrium fcc-derivative ordered phase, (ii) the nature of corresponding anelastic effects, (iii) the usage of isothermal annealing for the formation of an intrinsic composite microstructure with a different ratio between the bcc-derivative metastable and fcc-deri- vative equilibrium phases that have different magnetic properties, and (iv) the effect of an additional doping by Tb to stabilize bcc-derivative phases with high positive values of magnetostriction. In situ neutron diffraction, measurements of magnetostriction and magnetization, mechanical spectroscopy were applied to achieve the goals of this study using Fe-27Ga type alloys. 1. Introduction Fe-Ga alloys exhibit unique functional properties, such as magne- tostriction that can be varied from the highest positive values among the iron-based alloys to negative values including zero magnetostriction if proper compositions and heat treatments are chosen [1–3]. This unusual behaviour is related to a complex phase transformation se- quences in the Fe-Ga alloys. In earlier studies, the phase transforma- tions in Fe-Ga alloys were studied by X-ray diffraction [4–6], which provides structural information limited to the near-surface sample area. More recently, we used electron backscatter diffraction and in situ neutron diffraction to characterize the D0 3 and L1 2 phases that origi- nate from the fcc and bcc phases in the Fe-27Ga type bulk alloy [7]. Different ratios between these phases, characterized by magnetostric- tion values of different signs, were achieved at 400 °C using isothermal annealing that produces an intrinsic composite in the alloy. Depending on the relative fraction of the metastable D0 3 and stable L1 2 phases, the magnetostriction values of the alloy, λ S , were varied from positive to negative, including the value of λ S = 0 for the alloy with L1 2 /D0 3 = 2/ 1[2]. The phase transition from metastable D0 3 to stable L1 2 is ac- companied with a well-pronounced transient anelastic effect [8]. Doping Fe-Ga with rare earth elements do not only enhance magne- tostriction in Fe-17Ga alloys [9–11] but also stabilize the metastable bcc-derivative phase in as-cast Fe-27Ga alloys and slows down the ap- pearance of close-packed phases at heating [12,13]. These facts demonstrate a possible way to control the adjustment of magnetos- triction in Fe-Ga alloys. In this paper we studied three interconnected subjects: (i) crystal- lographic details of the phase transition from metastable D0 3 to stable L1 2 phase, as well as the influence of Tb on this transition, (ii) tailoring magnetostriction in intrinsic Fe-27%Ga natural composites, and (iii) anelastic effects accompanying phase transitions (from 400 to 800 °C) in Fe-27Ga. This work presents a way of adjusting the microstructure of the Fe-27Ga alloy so that the magnetostriction of the material can be tuned in a controlled way to meet the demands of a given application. 2. Materials and methods Several Fe-27 at.%Ga and Fe-27.4 at.%Ga-0.5%Tb alloys were pro- duced by rapid solidification in a copper mold using Fe (commercial purity) and Ga (99.99%) by arc melting under the protection of high- purity inert argon gas using an Arc 200 mini vacuum furnace (Arcast Inc, USA). In this paper, all the compositions are in atomic percent. We used energy dispersive spectroscopy to measure the chemical compo- sitions of the cast buttons with ± 0.2% accuracy as 27.4 to 27.8%Ga in binary alloys and Fe-27.4%Ga-0.5%Tb. According to the literature [for example 2, 3] and our results [13], only as-cast samples, i.e. samples in the metastable state, produce high magnetostriction, while heat treatment (homogenizing) leads to a de- crease in the magnetostriction due to the formation of the phase with https://doi.org/10.1016/j.intermet.2018.05.016 Received 6 December 2017; Received in revised form 8 April 2018; Accepted 26 May 2018 * Corresponding author. National University of Science and Technology “MISIS”, Leninsky ave. 4, 119049, Moscow, Russia. E-mail addresses: ali.a.emdadi@gmail.com, emdadi@b-tu.de (A. Emdadi). Intermetallics 100 (2018) 20–26 0966-9795/ © 2018 Elsevier Ltd. All rights reserved. T