The Formation Mechanism and Soft Magnetic Properties of High Bs Fe-based Nanocrystalline Alloys

高飽和磁感應強度Fe基納米晶軟磁合金的形成機制及磁性能研究

Student thesis: Doctoral Thesis

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Award date2 Jul 2019

Abstract

High Bs nanocrystalline alloys (HBNAs) are structurally characterized by nano-sized α-Fe grains in amorphous matrix and functionally characterized by high Bs above 1.80 T with unparalleled magnetic softness. These alloys characterized by high Fe content (above 80 at.%) without large atoms exhibit attractive magnetic properties and low materials cost, which are ideal candidates for next-generation electronic materials or being used as core materials in transformers, motors, sensors and electric vehicles. However, HBNAs always suffer from the harsh requirement of ribbon production and annealing processes due to the low amorphous forming ability (AFA) and poor thermal stability. To obtain high performance HBNAs is of great significance to the wide applications of this kind of soft magnetic materials. Since the magnetic properties are closely related to the nanostructure of HBNAs, it is essential to investigate the formation mechanism of nanocrystals in the high content Fe amorphous alloys during the annealing process. In addition, the effects of compositions and ribbon preparation process on the formation of nanostructure and soft magnetic properties also need to be studied.

From the perspective of atomic radius mismatch, mixing entropy and binary phase diagrams, the effects of constituent elements on AFA and nanostructure refinement were analyzed systematically. The guiding principles for the compositional design of HBNAs were then proposed and the alloys so obtained in this thesis were investigated systematically. The magnetic properties of the designed HBNAs after annealing under different conditions were characterized and the precipitating phases during the annealing process were identified. With the increasing annealing temperature, the coercivity (Hc) first decreases to a minimum due to stress relief and then increases due to the formation of coarse α-Fe grains in the early nanocrystallization process. During the optimal nanocrystallization process, Hc reaches a minimum again due to the formation of uniform nanostructure with fine α-Fe grains. The further increase of the annealing temperature subsequently leads to the formation of intermetallic phases, which greatly deteriorate the magnetic softness. Consequently, the changes in Hc as a function of annealing temperature exhibits as a “w” shape. By comparison, the saturation magnetization (Bs) initially increases with the annealing temperature and then levels off to a plateau value. Therefore, both high Bs and low Hc could be obtained should the nanocrystallization process be optimized.

The nanostructure and elemental distribution show that the nanostructured alloys exhibit a core-shell like structure. The Fe-rich α-Fe(Si) grains are surrounded by the amorphous interphase, which is rich in metalloid elements. Based on the thermal analyses, a “dual phase co-growth” model was developed to understand the kinetics of nano-grain growth, which agrees quite well with the experimental results. Accordingly, the crystallization process of HBNAs can be described as that α-Fe(Si) grains firstly precipitate in the amorphous matrix and rapidly grow to a stable grain size of ~20 nm for a prolonged annealing time due to the soft-impingement and competitive effects. The further increase of annealing temperature/time will lead to the precipitation of intermetallic phases in the intergranular amorphous matrix, resulting in the destruction of the shielding layer and formation of non-uniform nanostructure with coarse α-Fe(Si) grains by grain coalescence. The nanostructure stability of HBNAs is ultimately governed by the thermal stability of the intergranular amorphous matrix. It was found that the increase of compositional complexity by introducing different kinds of metalloid elements is effective in enhancing the thermal stability of HBNAs. Moreover, the increase of Fe/Cu content can significantly increase the number density of α-Fe grains, resulting in an enhanced soft-impingement and competitive effects, which can also improve the thermal stability of HBNAs. The key to obtain fine nanostructure and excellent magnetic softness in HBNAs is to obtain as-quenched precursors with high number density α-Fe nuclei to ensure a synchronous and competitive grain growth during the subsequent nanocrystallization process.

For the HBNAs with low AFA, the formation of primary crystals is noticeable in the as-quenched ribbons. By adjusting the wheel speed during the ribbon production process, fully amorphous as-quenched ribbons and the ribbons with primary crystals can be obtained. Since the cooling rate along the thickness direction changes progressively in the single roller melt-spinning method, the number density and size of the primary α-Fe grains decrease from the free to wheel side of the ribbons. As a result, the wheel side usually exhibits a fully amorphous structure. Our thermal analyses showed that the growth of the primary crystals can readily take place, resulting in a non-uniform crystallization process as large primary crystals grow faster than small ones. To investigate the effects of the primary crystals on the nanocrystallization process, the nanostructure of the free side and wheel side of the ribbons after annealing were further characterized in detail. Interestingly, after annealing, the free side with a high number density and small sized primary crystals exhibit a more uniform nanostructure with fine α-Fe grains than the fully amorphous wheel side, which can be attributed to the synchronous and competitive growth of the primary crystals. Thus, the presence of small primary crystals with a high number density is conducive to obtain fine nanostructure, which in turn enhances the soft magnetic properties and improve thermal stability of HBNAs.