Bulk-nano spark plasma sintered Fe-Si-B-Cu-Nb based magnetic alloys

Highlights

• Iron-based soft magnetic alloys processed by mechanical alloying followed by SPS.

• Microhardness of alloys increases with increasing milling time.

• Crystallite size of iron-based alloys decreases with increasing milling time.

• All alloys exhibited good saturation magnetization and lower coercivity.

Abstract

Iron-based soft magnetic alloys (FeSiB, FeSiBNb, FeSiBCu, and FeSiBNbCu (Finemet)) have been fabricated via mechanical alloying followed by spark plasma sintering (SPS) process. FeSiB alloy powder was obtained by high energy ball milling of an elemental blend Fe, Si, and B powders. The effect of milling time on crystallite size and phase transformation was studied. Additionally, FeSiBCu, FeSiBNb, and FeSiBCuNb alloy powders were milled to study the effect of Cu and Nb on phase transformation, mechanical, and magnetic behavior. The mechanically alloyed powders were sintered via SPS process to achieve full densification. The microhardness and magnetic permeability of sintered FeSiB alloys were found to be increased monotonically with milling time primarily due to the smaller crystallite size and more uniform microstructure. Interestingly, the alloying of Cu or (and) Nb to FeSiB resulted in higher saturation magnetization and lower coercivity mainly due to large volume fraction of α-Fe₃Si nanocrystals. Overall, these alloys exhibit reasonably good soft magnetic behavior along with excellent microhardness. Mechanical alloying followed by spark plasma sintering opens up a new avenue of processing amorphous-nanocrystalline alloys into bulk shape with good mechanical and magnetic properties.

Introduction

Amorphous iron-based soft magnetic materials used in transformers and inductive devices have helped in energy saving, addressing the global warming crisis by minimizing core loss in these devices [1]. What makes iron-based amorphous alloys even more interesting is their applications in every day products such as mobile phones, computer hard disks, sensing devices, etc. [[2], [3], [4]]. Considering the recent problems of increased air pollution from vehicles and demands for energy saving, it is necessary to miniaturize electrical/electronic equipment with improved efficiency. Therefore, there is an increasing critical requirement to develop excellent soft magnetic materials for high frequency applications such as high frequency transformers for fuel cells, gapless transformers, wind power generators, and boosting/down converting inductors in hybrid electrical vehicles.

Fe-Si-B alloys are a group of iron-based soft magnetic materials that possess excellent magnetic properties such as high permeability and saturation magnetization along with low coercivity [5]. These excellent soft magnetic properties are primarily attributed to the formation of nanocrystalline α-Fe(Si) grains dispersed in an amorphous iron matrix [6]. Addition of elements such as niobium and copper further improve the mechanical and magnetic behavior of these alloys, where copper acts as a nucleation site for α-Fe(Si) nanocrystals while niobium reduces growth of nanocrystals. Main et al. [7] stated that Cu-clusters act as nucleation sites for α-Fe(Si) nano crystals since copper is insoluble in iron matrix, at the early stages of crystallization this inhomogeneity of Cu results in creation of more nucleation sites [8] Also, addition of Nb increases the crystallization temperature and helps in stabilizing the glassy phase that delays crystallization.

Amorphous metallic alloys have been of interest for fundamental studies and for practical applications [9]. Amorphous structures exhibit high yield strength, excellent corrosion and wear resistance, and low elastic modulus compared to their crystalline counterparts [[10], [11], [12], [13], [14]]. The main reason that amorphous materials are desirable is their lack of crystal defects such as grain boundaries and dislocations. Alben et al. [15] stated that the low coercivity and high permeability values in amorphous ferromagnetic materials is because the magnetization vectors are parallel to each other in such materials. Among these systems, iron-based amorphous alloys have attracted much attention because of the low cost of iron, their relatively high strength, hardness, and excellent magnetic properties that make them excellent precursors to produce nanocrystalline soft magnetic materials [11,16,17]. FeSiB-based amorphous alloys exhibit superior soft magnetic properties in their partially de-vitrified condition since it has been well established that nanocrystals embedded in an amorphous matrix lead to optimum properties [[18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]. These alloys have attracted huge research and industrial interest [5,[28], [29], [30]]. They exhibit high permeability, while maintaining high saturation magnetization. The microstructure of this alloy mainly consists of a large density of nanometer-sized soft magnetic precipitates, e.g., α-Fe(Si), uniformly distributed within an amorphous matrix [[31], [32], [33], [34], [35], [36], [37]]. Typically, these alloys are prepared via the melt spinning technique and exhibit an amorphous structure due to the very high cooling rates (10⁶–10⁸ °C/s) experienced during such processing [28,38,39]. However, it should be noted that melt spinning fabricates only ribbons and are not suitable for applications where a large volume of soft magnetic materials with complex shapes is required, also nanocrystalline ribbons are quite brittle and cannot be utilized for fabrication of some magnetic compounds [40]. In contrast, mechanical alloying can produce amorphous or nanocrystalline alloy powders from elemental blend suitable for compaction and consolidation via spark plasma sintering (SPS) process.

Considering the recent problems of increased air pollution from vehicles and demand for energy saving, it is necessary to miniaturize electrical and electronic equipment and improve their efficiency. Also, for the permeability engineering applications where both magnetic and mechanical properties are important, stacking of tens of thousands of melt-spun ribbons to form bulk component is impractical. Mechanical alloying (MA) has gained special attention as a powerful non-equilibrium process for fabricating amorphous and nanocrystalline materials, whereas spark plasma sintering (SPS) is a unique technique for processing dense and near net shape bulk amorphous-nanocrystalline alloys with homogenous microstructures.

Mechanical alloying is a process in which powder particles get fractured and fused together repeatedly in a high energy ball mill [41]. This process has a major advantage as compared to conventional alloying processes, it permits different metals exhibiting varying melting temperatures, to form alloys. After the milling process the SPS process can be used for sintering those alloys in to bulk shapes. SPS is a process that uses a pulsed DC current and an uni-axial pressure to densify the powders. The SPS process can be used for processing a wide variety of materials including ceramics, composites or metal powders [42]. The SPS process has an advantage over other conventional sintering techniques such as hot pressing, the high sintering speed prevents grain growth and retains the nanocrystalline structure.

In the present study we investigated the effect of mechanical alloying on the microstructure, phase transformation, mechanical, and magnetic behavior of Fe-Si-B based alloys processed via spark plasma sintering process. We studied the role of Cu and Nb individually as well as combined on the phase transformation and magnetic behavior by processing FeSiBCu, FeSiBNb, and FeSiBCuNb alloys via mechanical alloying followed by SPS. This investigation will open up new avenues for processing and development of bulk amorphous and nanocrystalline alloys with excellent magnetic and mechanical properties.