Fabrication and soft magnetic properties of rapidly quenched Co-Fe-B-Si-Nb ultra-thin amorphous ribbons

Ultra-thin soft magnetic amorphous ribbons of Co-Fe-B-Si-Nb alloy were synthesised by a single step rapid-quenching approach to acquire advantage of improved material performance and lower costs over commercial amorphous alloys. The amorphous ribbons of approximately 5.5 µm thicknesses were quenched by a single roller melt spinner in a single-step production process and characterised for their structural and magnetic properties. The disordered atomic structure of amorphous ribbons was confirmed by the X-ray diffraction. A surface morphology study revealed the continuity of ultra-thin ribbons without pores over a large scale. The amorphous alloy showed the ultra-soft magnetic properties in the as-quenched state. The observed thickness dependency of the magnetic properties was attributed to the increased surface roughness and possibly due to a lack of densely packed atomic structure resulting from the extremely high cooling rates experienced by ultra-thin ribbons. We propose that in-situ thinning process of amorphous ribbons significantly reduces the basic material cost and eliminates the need for post-processing steps; hence it provides the opportunity for mass production of high-performance soft magnetic amorphous ribbons at relatively lower costs

Amorphous and nanocrystalline soft magnetic materials: from design to application

Advanced soft magnetic materials are required to match high-power density and switching frequencies made possible by advances in wide band-gap semiconductors. Magnetic materials capable of operating at higher operating frequencies have the potential to reduce the size of power converters significantly. Amorphous magnetic alloys lack long-range atomic order and consequently exhibit high electrical resistivity, no macroscopic magnetocrystalline anisotropy and no microstructural discontinuities, e.g., grain boundaries or precipitates, on which magnetic domain walls can be pinned. Consequently, they show excellent performance in DC and AC magnetic fields due to their low hysteresis and eddy current losses. Much work on this class of materials has been carried out, yet there are some critical issues in design, fabrication and optimisation that need to be addressed. First, one of the main challenges is producing amorphous alloys with a lower content of non-magnetic elements. The alloys comprising more abundant magnetic elements show low amorphisation capability, so non-magnetic metals have been added to these alloys to tackle this issue. However, adding these elements can significantly reduce the saturation magnetisation and increase the cost of alloy. To overcome these hurdles, kinetic, thermodynamic and topological parameters have been exploited to predict and tune the amorphisation capability of Co-Fe-B alloys. Based on this, seven alloys with very attractive magnetic properties have been designed and fabricated by melt spinning. Five out of seven alloys are amorphous, highlighting the efficiency of the design procedure, among which there is an alloy with ultra-low coercivity, 2.9 A/m. This substantiates the efficiency of the amorphous alloy design policy. Additionally, the crystallisation behaviour of alloys correlates significantly with their amorphisation ability. In this regard, the eutectic mode of crystallisation can be a sign of higher amorphisation ability, whereas alloys that crystallise through other mechanisms can offer lower amorphisation ability. Two in-situ crystallised alloys offer low power loss and high saturation magnetisation. The surface crystallised alloy exhibits not only very low coercivity but also surprisingly 20% lower power loss compared to the best amorphous sample. The mechanism behind this observation is investigated in more detail. The surface crystallisation can be used to induce anisotropy and decrease the power loss of melt-spun ribbons, eliminating the need for magnetic annealing. In addition, it has been shown that the amorphisation capability of an alloy can be tuned, based on the as-mentioned parameters in order to produce an in-situ nanocrystallised alloy with remarkable Bs = 1.57 T. The precipitation of the kinetically-favoured metastable Co7Fe3 nanocrystalline phase in the amorphous matrix is responsible for the high Bs of this alloy. The atomic structure of the amorphous matrix is evaluated based on Mössbauer spectroscopy, and the distribution of the hyperfine magnetic field implies that cobalt atoms form clusters and boron atoms undergo only short-range ordering. Therefore, designing alloy compositions using the as-mentioned predictive parameters can give one the opportunity to manipulate the microstructure of alloys to greatly benefit from their optimised properties. To further lower the cost of amorphous alloys, ultra-thin soft magnetic amorphous ribbons are fabricated via a single-step rapid-quenching process. The ribbons, with the thicknesses of 5.5 µm, fabricated by a high speed melt spinner offer ultra-soft magnetic properties. It is shown that in-situ thinning process results in a substantial reduction in the cost of the material. It also eliminates the need for post-processing steps, providing the opportunity for mass production of high-performance soft magnetic amorphous ribbons at a relatively low cost. Despite the low hysteresis and eddy current loss of amorphous metals, the measured losses, especially at high frequencies, are often in excess of these classical loss contributions. To gain a better understanding of this observation, the measured total power loss is resolved into hysteresis, eddy current, and anomalous losses. It is found that the main contributor to the power loss is the anomalous loss that can be reduced substantially through annealing in a transverse magnetic field at temperatures lower than the ribbons crystallisation temperature. It has been shown that transverse magnetic annealing not only alters the mechanism of magnetisation, from domain wall motion to magnetisation rotation, but it results in domain pinning due to the increased number of domains by decreasing their width. The latter promotes the magnetisation rotation mechanism. These effects account for a 75% decline in the total power loss in the melt-spun ribbons, making them desirable candidates for power converters working at mid- and high frequency. In addition, soft magnetic nanocomposites can be produced through annealing the amorphous ribbons, with a specific composition, at temperatures higher than crystallisation temperature. The nanocrystallisation of Co23B6, with network-like FCC structure, in the amorphous matrix, leads to substantial progress in DC and AC magnetic properties of ribbons. These metastable complex phases with 116 atoms per unit cell are not well understood. Therefore, it is essential to investigate the kinetics and thermodynamics of this kind of phase transformation in order to identify the substantial effects of the resulting phase on magnetic properties. This has been done using CALPHAD and nucleation theory. As a result of transverse magnetic annealing leading to the combined effects of nanocrystallisation and coherent magnetisation rotation, the anomalous loss declines by 70% in the ribbons annealed at 525 °C. Therefore, this soft magnetic nanocomposite can be utilised as the core of efficient power convertors

Improved magnetic performance of Cobalt-based ribbons by nanocrystallization through magnetic annealing

Phase transformation driven soft magnetic properties have been correlated through different stages of nano-crystallization of Co-based amorphous alloys, via transverse magnetic annealing, by combining structural, magnetothermal, domain imaging, and AC/DC magnetometry techniques. The nano-crystallization starts by nucleation and growth process of soft magnetic meta-stable, thermodynamically favored, Co23B6 phase with less nucleation activation energy compared to other stable phases. In the second crystallisation stage, Co2B and Co3B, as a semi-hard magnetic phase, are identified in the alloys, magnetically annealed at 525 and 550 °C, respectively. Field-induced anisotropy dominates over the residual contributions of magneto-crystalline anisotropy of the phases, precipitated after field annealing at 510, 515, and 525 °C. The anomalous loss is significantly reduced as by annealing in a transverse magnetic field due to the reorientation of the preferred magnetisation axis, and consequently, change in dominant magnetization reversal mechanism from domain wall motion to magnetization rotation. In addition, magnetic annealing causes a measurable decrease in the domain width, which, in turn, promotes pinning and inhibits domain wall motion, thus further favours coherent domain rotation as the primary mechanism of magnetization. The combined mechanism of nanocrystallisation and coherent magnetisation rotation accounts for a 70% decrement in the anomalous loss in the so-processed ribbons at 525 °C, which renders them attractive for applications in mid- and high-frequency power supplies and inverters

Substantial thinning of melt-spun ribbons by an optimised and high-yield ball-milling process

Melt-spun Fe-based ribbons are widely used as the core of transformers and inductors due to their high flux density and low coercivity. However due to their high thickness (∼19 µm) these ribbons are prone to large eddy current losses at MHz frequencies. Despite low yield, ball milling has been widely used to break such ribbons down to thinner flakes to suppress the eddy current losses at high frequency. In this work, we demonstrated an optimized ball milling process with increased yield for flakes in the desired size range (2–4 µm). We have demonstrated that reducing pre milling annealing temperature from 450 to 350 °C increases the yield in desired size range from 2% to 5% and further increasing batch size from 10 to 20 g increases the yield to 21%. The coercivity of the milled flakes increases from 139 to 1352 A/m due to the ball milling process. A post-mill annealing at 350 °C in Ar atmosphere decreases the coercivity to 341 A/m. X-ray diffraction analysis showed no sign of crystallization during ball milling. The result presented here demonstrates an efficient approach to fabricate ultra-thin flakes out of soft magnetic ribbons for high-frequency applications

Novel predictive methodology of amorphisation of gas-atomised Fe-Si-B alloy powders

The present work is focused on developing amorphisation capability criteria to predict regions with high amorphous forming ability (AFA) in the Fe-Si-B phase diagram. First, the AFA of Fe-Si-B alloy powders was evaluated by conventional empirical glass forming parameters, which eventually did not guide to the best AFA alloy. Then, AFA analysis was extended to the ternary phase diagram, calculated using CALPHAD, along with superimposed mathematical model based on topological instability factor (λ), estimated critical cooling rate (RC) and critical particle size (dC), to confine the phase diagram regions with larger AFA. The alloy with the highest AFA shows optimum atomic size mismatch when λ = 0.204. Furthermore, the optimal region in the phase di- agram to design alloys with high AFA is where Fe2B is the first solid phase under equilibrium solidification. Within these two limits, the alloys with lower liquidus temperatures show the highest AFA for the gas-atomised powder

Novel predictive methodology of amorphisation of gas-atomised Fe-Si-B alloy powders

The present work is focused on developing amorphisation capability criteria to predict regions with high amorphous forming ability (AFA) in the Fe-Si-B phase diagram. First, the AFA of Fe-Si-B alloy powders was evaluated by conventional empirical glass forming parameters, which eventually did not guide to the best AFA alloy. Then, AFA analysis was extended to the ternary phase diagram, calculated using CALPHAD, along with superimposed mathematical model based on topological instability factor (λ), estimated critical cooling rate (RC) and critical particle size (dC), to confine the phase diagram regions with larger AFA. The alloy with the highest AFA shows optimum atomic size mismatch when λ = 0.204. Furthermore, the optimal region in the phase di- agram to design alloys with high AFA is where Fe2B is the first solid phase under equilibrium solidification. Within these two limits, the alloys with lower liquidus temperatures show the highest AFA for the gas-atomised powder