Atomic scale investigation of clustering and nanocrystallization in FeSiNbB(Cu) soft magnetic amorphous alloys

Beschreibung

Fe-based metallic glasses are one of the most successful classes of metallic amorphous and nanocrystalline alloys to have ever been investigated because of their excellent and unique soft magnetic properties. Formation of soft magnetic nanocrystals in FeSiNbBCu-type metallic glass proceeds by heterogeneous nucleation upon annealing above the glass transition temperature. Within the framework of this thesis, the primary crystallization of soft magnetic nanocrystals in an amorphous Fe73.5Si15.5Cu1Nb3B7 alloy upon rapid (4s and 10s) and conventional (5 min and 30 min) annealing was studied by atom probe tomography. Pre-clustering of Cu atoms in the amorphous matrix induces heterogeneous nucleation of Fe3Si nanocrystals. Accordingly, the clustering of Cu atoms starts at least 50 °C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization resulting in a reduction of available nucleation sites during Fe-Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Interestingly, rapid annealing when performed under a small applied stress of ~50 MPa, introduces preferential orientation of magnetic domains without inducing any noticeable change neither to the morphology nor to the structure of Fe-Si nanocrystals and Cu clusters. As a result, a nucleation dominated growth mechanism of nanocrystallization is introduced as an alternative method of producing fine grained advanced soft magnetic materials.

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Pradeep Konda Gokuldoss

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Atom Probe Tomography Magnetic Materials Metallic glass Nanocrystals

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