Effect of hydrogen and magnetic field on the mechanical behavior of magnetostrictive iron-gallium alloys

Magnetostrictive Fe-Ga and other iron-based alloys are candidates for use in sensing, actuation and large-scale energy harvesting applications. Exposure to aqueous electrochemical environments is anticipated in some of these applications which could potentially introduce hydrogen into the alloy and cause severe ductility reduction due to hydrogen embrittlement. These alloys may also be simultaneously exposed to magnetic field. This study therefore examines the effect of hydrogen and magnetic field on the mechanical behavior of these alloys. This study could also provide an understanding of the relationship between hydrogen embrittlement and magnetoelastic behavior in these alloys. In this work, the effect of hydrogen and magnetic field on the fracture behavior of [100]-oriented Fe-17.5 at.% Ga alloy single crystals and polycrystalline Fe-15 at.% Ga alloy were examined. Three-point bend tests and tensile tests were used to study the fracture behavior. Tests were done in different conditions to understand the effect of hydrogen and magnetic field on the fracture behavior of these materials. Hydrogen loading was done by in-situ electrochemical charging and magnetic field was applied to the samples either by using Nd2Fe14B permanent magnets or by using solenoid coils. Before doing the three-point bend test on the Fe-Ga single crystal samples, tests were done using high-strength AISI 4340 steel to optimize the testing procedures and parameters. In all cases, the samples tested with hydrogen charging show a drastic reduction in ductility and fracture stress values. In the case of [100]-oriented Fe-17.5 at.% Ga alloy single crystal samples tested with hydrogen charging, the presence of applied magnetic field increased the stress required for fracture and a corresponding increase in bending strain values. This is attributed to a decrease of the elastic modulus values on the application of magnetic field in this magnetostrictive alloy. The hydrogen embrittlement was characterized by a change in fracture surface from a ductile type fracture to a brittle cleavage type fracture. Acoustic emission signals collected during the test correspond to the fracture behavior.