Structural Evolutions of High-performance Fe-based Amorphous And Nanocrystalline Alloys


Student thesis: Doctoral Thesis

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Award date7 Aug 2023


Amorphous alloys have attracted increasing attention due to their excellent physical, chemical, and mechanical properties. Compared to crystalline alloys, amorphous alloys have no long-range translational and rotational symmetry. However, how the unique disordered structures influence their properties remains a long-standing issue. In this thesis, the recently developed high-performance FeSiBPCu amorphous and nanocrystalline alloys, which are promising for next-generation electronic materials because of their remarkable soft magnetic properties and low cost, were chosen as research objects. By combining a series of state-of-the-art techniques, including in-situ high-energy synchrotron X-ray diffraction, small-angle neutron scattering (SANS), atom probe tomography (APT), high-resolution transmission electron microscopy (TEM) and differential scanning calorimetry (DSC), the structural evolutions from the nanoscale to atomic scale were well investigated during heating and annealing processes. The effects of structural evolutions on thermal stability, relaxation behavior, crystallization behavior, and soft magnetic properties were discussed, and the underlying mechanisms were elucidated.

The core-shell nanostructure of α-Fe precipitates was investigated in a recently developed high-performance Fe83.3Si4B8P4Cu0.7 nanocrystalline soft magnetic alloy utilizing SANS and APT. SANS measurements under a saturating magnetic field provided evidence of core-shell structures at the nanoscale. Moreover, the core-shell structure was directly visualized from APT analysis, where P-enriched shells were found around Fe-enriched cores due to the large negative heat of mixing between Fe and P elements and the slow diffusion of P atoms. The core-shell nanostructures induced by nanoscale partitioning are effective in stabilizing the residual amorphous matrix and suppressing crystal growth. Our findings provide insight into the microstructure of Fe-based soft magnetic alloys and have direct implications in alloy design for the improvement of magnetic properties.

In-situ high-energy synchrotron X-ray diffraction experiments were conducted during the heating and isothermal annealing process to explore the atomic structural evolutions in the reciprocal and real space of Fe-based amorphous alloys. Anomalous structural changes occurring at medium-range orders are in accordance with the Curie temperature Tc. The structural rearrangements can be related to the Invar effect. In addition, atomic structural evolutions induced by thermal relaxation and their impacts on soft magnetic properties were investigated. After isothermal annealing, the coercivity Hc decreases from about 14 to 6 A/m, which can be attributed to the cooperative atomic rearrangements in the short-to-medium range orders. These results advance the understanding of the correlation between atomic structural evolutions and soft magnetic properties and thus facilitate the modulation of soft magnetic amorphous alloys.

The synthesis of amorphous and nanocrystalline alloys with outstanding properties greatly benefits from the control of the crystallization process. The kinetics of crystallization in NANOMET alloys was systematically investigated utilizing in-situ high-energy synchrotron diffraction, DSC, high-resolution TEM, and APT. It is revealed that the as-quenched amorphous alloy processes a two-stages crystallization behavior, and the nucleation is easier compared to crystal growth by the Kissinger analysis of calorimetric curves. The crystallization pathway and atomic structural evolution were further characterized by in-situ synchrotron diffraction upon annealing. Kolmogorov-Johnson-Mehl-Avrami (KJMA) analysis of the intensity data demonstrates that the crystallization process is dominated by a three-dimensional diffusion-controlled growth with a decreasing nucleation rate. The size distribution of nanocrystalline precipitates derived from the high-resolution TEM images illustrates an increase in the average grain size with a longer annealing time. Additionally, the crystalline products and their evolution sequence have been identified. Only bcc α-Fe grains are characterized by the Rietveld refinement in the first crystallization stage. At higher annealing temperatures, the residual amorphous matrix transforms to the tetragonal Fe2B and orthorhombic Fe3B in the second crystallization stage, which can be further supported by APT analysis. These experiment results enrich the structural evidence for the fundamental understanding of crystallization behaviors and provide guidance for the development of high-performance Fe-based amorphous and nanocrystalline soft magnetic alloys.