Phase formation, crystallization kinetics and properties of (Fe,Co)-based bulk metallic nanocomposites

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The interesting soft magnetic properties of multicomponent Fe- or (Fe,Co)-based bulk metallic glasses (BMGs) have been a great stimulus for research regarding their synthesis and physical properties. Several studies on amorphous ferromagnetic ribbons demonstrated that in some cases the soft magnetic properties can be enhanced by annealing. Despite their unique properties, the amorphous ribbons still have limited application. Much more suitable are bulk soft magnetic glassy alloys because they can be cast directly in the final shape with thicker dimensions than the rapidly quenched ribbons. The good magnetic properties are not the only requirements for applications. A suitable combination between magnetic, mechanical and/or corrosion resistance may make such kind of materials very attractive for applications. The BMGs have a much higher hardness, yield strength, fatigue and wear resistance than the crystalline materials. They lack the deformability, due to the intrinsic brittleness of the glassy structure, but it can be improved by nanocrystallization. Very recent studies show that small additions of elements which promote the nanocrystallization of soft magnetic nanophases may enhance also the ductility (deformability). There the mechanism of magnetic softening is supposed to be similar to what is observed in the case of nanocrystalline Finemet-type ribbons, but it is not yet fully investigated and understood. The Finemet ribbons become brittle upon nanocrystallization, while the BMGs may become deformable. The different mechanical behaviour resides in the different scale of the nanocomposites: the ribbons have a very limited thickness (usually less than 50 µm), while the BMGs are real 3D samples. The metastable nanophases are more likely to appear in metalloid-rich compositions. However, the details of the magnetic softening mechanism(s) and mechanical behaviour are not fully understood so far. Also, magnetic softening by controlled devitrification was never done on a large scale in the case of BMGs. In some cases, the metastable phases developed in BMGs seem to be different from those grown in ribbons and they form only if a particular short-range order exists in the bulk amorphous precursors. The purpose of this project is to find out how these metastable phases can be stabilized and what kind of particular short-range order is necessary to be achieved in order to create them. The main goal is a simultaneous optimization of glass forming ability (GFA) and nanocrystallization for improving the soft magnetic and mechanical behaviour of bulk alloys. In parallel, new bulk soft magnetic nanocomposites can enlarge the application field: they are suitable for use as miniaturized motion sensors, magnetic cores, magnetic valves, magnetic clutches etc