Temperature-Induced Phase Transitions in Fe-Ga Alloys for Sensors

Abstract

Over the last few years, Fe-Ga magnetostrictive alloys (also referred to as Galfenol), which are among the most important functional magnetic materials, have found their way into sonar systems and sensors such as displacement and force sensors,  acoustic , tactile and torque sensors [1]. It has already been found that the addition of Ga increases the magnetostriction of bcc Fe by a significant amount. At room temperature, the magnetostriction of Fe–Ga alloys can be ten times greater than that of bcc Fe. Additional advantages of these alloys are their low associated cost, high ductility, high mechanical strength, and ability to withstand high imposed stress levels have drawn considerable attention [2]. Golovin et al. [3] demonstrated that after casting, alloys with  Ga 27 at. percent possess the D0₃ structure and that the first-order L1₂ to D0₁₉ transition occurs at 893 K. Additionally, for the first time, it was observed that the phase transition from one ordered phase to another ordered phase involves disordered states. When temperature transformation processes, most notably cooling from A2 to high temperature phase occur, the phase diagrams of Fe-Ga alloys undergo ordering and disordering structure changes. A faster cooling rate indicates that the alloys prepared at room temperature are a mixture of A2, L1₂, and D0₃ phases [4]. The magnetostrictive, structural, thermal, and magnetic properties of galfenol alloys with various compositions were investigated after they were arc melted and heat treated followed by furnace cooling. The X-Ray Diffraction analysis confirms the body-centered cubic structure of the alloys and the presence of the A2 and D0₃ phases. Additionally, optical microscopy confirmed the presence of equiaxed A2 phase grains which are responsible for the larger grain boundaries [5]. Crystallite size and grain size increment also indicates that phase transition happens in the alloys.  The differential scanning calorimetry confirms the presence of the A2, B2, D0₃, and L1₂ phases. Fig. 1 shows the crystal structures of phases present in the Fe-Ga alloys and sequence of phase transition. The process of transition from one ordered phase to another is characterised by disordering processes. Recorded phase transition sequence is   The decrement in magnetization in particular temperature are due to change in magnetic properties in D0₃, that does not corresponds to phase transitions [6]. A vibrating sample magnetometer was used to determine the saturation and remanent magnetization. When it comes to saturation magnetization, grain size is inversely proportional to it. Particularly, the magnetic properties of Fe-Ga alloys are significantly influenced by the structural ordering and microstructure of the alloys. This demonstrates that our Fe-Ga alloy has been optimised for use as a sensor and actuator applications.