Magnetostrictive behaviour of severe plastically deformed, nanocrystalline materials
Magnetostrictive constants are describing the length changes of materials in magnetic fields. For applications, the goal is to have either low (e.g. in AC fields, transformers) or high (actuator or sensor applications) magnetostrictive response. While pure ferromagnetic elements show magnetostrictive constants of medium magnitude, there are other materials available showing almost no effect (e.g. Permalloy) or a very high one (e.g. Terfenol, Galfenol - FeGa). This study focusses on the influence of the chemical composition on the magnetostrictive behaviour, with an emphasis on nanocrystalline FeCr, FeCu and CoCu alloys processed by severe plastic deformation (SPD).
Using SPD by high pressure torsion (HPT) makes it possible to produce and process almost any desired combination of materials. This can be done by mixing the desired powders, compact and deform them using an HPT-device: In the as-deformed state, the bulk samples show a nanocrystalline microstructure consisting of one (supersaturated) or several phases. In addition to the as-deformed state, some of the samples were stress relieved by a short low temperature annealing, for probing the influence of eigenstresses of severely deformed materials on magnetostrictive results.
To demonstrate the viability of the newly built experimental setup for measuring magnetostrictive properties, pure ferromagnetic metals were measured in different microstructural states and the results show good agreement with literature. The magnetostrictive behaviour of all samples was measured at room temperature in magnetic fields up to 2T using strain gauges. First, the transversal and longitudinal magnetostriction as a function of applied magnetic field and second, the magnetostriction as a function of sample rotation within a constant magnetic field are measured. Exemplarily picking out one result, the addition of small amounts of Cu increases the magnetostriction of FeCu in comparison to literature values of pure Fe.
In addition to their magnetostrictive behaviour, samples are microstructurally investigated with scanning electron microscopy as well as X-ray diffraction techniques and the influence of chemical composition as well as the microstructural state on the results will be thoroughly discussed in this contribution.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant No. 757333).