Maria Daniil

Fe-based nanocrystalline soft magnetic alloys for high-temperature applications

We report on improved high-temperature soft magnetic properties in Fe88−2xCoxNixZr7B4Cu1 nanocrystalline alloys. Substituting 5.5 at. % Co and Ni for Fe enhances the magnetization by 5% at ambient temperature and by 30% at 650 °C. The Curie temperature of the residual amorphous phase is also raised significantly (from 67 °C for x=0 to 298 °C for x=5.5), resulting in low coercivities (<30 A m−1) for Fe77Co5.5Ni5.5Zr7B4Cu1 over the temperature range 50–500 °C. The higher magnetization and Curie temperature as compared with other Fe-based alloys, and smaller Co content as compared with (Fe,Co)-based alloys, make this alloy attractive as an affordable high-temperature soft magnetic material.

Chapter Four - Nanocrystalline Soft Magnetic Alloys Two Decades of Progress

Abstract Since their introduction over two decades ago, nanostructured soft magnetic alloys have demonstrated excellent magnetic softness and improved performance over a wide range of environmental conditions. This chapter gives an updated account of progress in alloy design, synthesis, structure, and performance of this class of alloys. Existing data are reviewed to highlight recent advances and promote new perspectives for future work in this area.

Magnetic properties and thermal stability of (Fe,Co)-Mo-B-P-Si metallic glasses

A series of ferromagnetic metallic glasses with compositions (Fe,Co)78Mo1(B,P,Si)21 are shown to possess good thermal stability and soft magnetic performance. The thermal stability inside the supercooled liquid temperature region was evaluated using Kissinger analysis of primary crystallization, time-temperature-transformation (TTT) diagrams, and the extent of the supercooled liquid region (ΔTx). The phosphorus-free alloy, Fe78Mo1B15Si6, had an activation energy (Ea) of 414 kJ/mol, ΔTx ∼ 50 K, and began devitrifying after about 1 min at 730 K. By way of comparison, the phosphorus-containing alloy, Fe78Mo1B13P6Si2, had an Ea of 440 kJ/mol, ΔTx ∼ 45 K, and began devitrification after 10 min at 730 K. High saturation magnetization (μ0Ms ∼ 1.45-1.55 T) and low coercivity (Hc ∼ 20 A/m) are demonstrated across the composition range. Core loss measurements of toroidal cores are shown to be less than 12 W/cm3 at 1 T, maximum induction amplitude (under both sinusoidal and square waveforms). Trends were established...

Exchange anisotropy in the nanostructured MnAl system

In this letter, we report on the achievement of exchange anisotropy magnitude in a nanostructured Mn55Al45 alloy fabricated by rapid solidification with large exchange bias values (HE ≈ 13 kOe at 10 K) and a blocking temperature of TB ∼ 95 K. Field-cooled magnetization loops show a prominent exchange bias for T < TB signaling the simultaneous presence of antiferromagnetic and ferromagnetic phases at these temperatures. Structural probes confirm a majority presence of the high-temperature metastable hexagonal ɛ-MnAl in the as-solidified state with an intriguing double-Bragg peak structure indicative of phase separation. The observed exchange bias is hypothesized to originate from an intimate mixture of antiferromagnetic and nanoscaled ferromagnetic phases or dual mictomagnetic phases, approximating a cluster glass with well-defined variations in the local Mn concentration of the composition and leading to Mn-rich and Mn-poor regions with antiferromagnetic and ferromagnetic characters, respectively.

Effect of selective Co addition on magnetic properties of Nd2(FeCo)14B/α-Fe nanocomposite magnets

Nd2Fe14B/α-Fe-based hard/soft nanocomposite magnets with Co addition have been prepared by ball-milling and warm compaction. It was found that Co addition into the magnetically hard phase improves magnetic properties significantly, especially the remanence ratio and coercivity. The effect on the magnetic properties of the selective Co addition may be attributed to enhanced interdiffusion across the hard/soft interface that improves the interface conditions for effective interphase exchange coupling. By optimizing the Co content in the Nd15Fe79−xCoxB6 hard phase, an energy product value about 21 MG Oe can be obtained in the isotropic Nd2(FeCo)14B/α-(FeCo) nanocomposite magnets compared with 15 MG Oe of Nd2Fe14B/α-Fe nanocomposite magnets prepared under the same conditions with the same grain size and microstructure, owing to the strengthened intergranular exchange interactions.

The influence of voxel size on atom probe tomography data

A methodology for determining the optimal voxel size for phase thresholding in nanostructured materials was developed using an atom simulator and a model system of a fixed two-phase composition and volume fraction. The voxel size range was banded by the atom count within each voxel. Some voxel edge lengths were found to be too large, resulting in an averaging of compositional fluctuations; others were too small with concomitant decreases in the signal-to-noise ratio for phase identification. The simulated methodology was then applied to the more complex experimentally determined data set collected from a (Co(0.95)Fe(0.05))(88)Zr(6)Hf(1)B(4)Cu(1) two-phase nanocomposite alloy to validate the approach. In this alloy, Zr and Hf segregated to an intergranular amorphous phase while Fe preferentially segregated to a crystalline phase during the isothermal annealing step that promoted primary crystallization. The atom probe data analysis of the volume fraction was compared to transmission electron microscopy (TEM) dark-field imaging analysis and a lever rule analysis of the volume fraction within the amorphous and crystalline phases of the ribbon.

Shear band formation and fracture behavior of nanocrystalline (Co,Fe)-based alloys

The crystal structure, crystallization, fracture behavior and mechanical properties of (Co1 − x Fe x )89Zr7B4 (x = 0–0.7) nanocrystalline ribbons were investigated. The crystallization peaks of the amorphous ribbons tend to shift to higher temperatures with increasing Fe content. After annealing at 475°C for 3600 s, the main crystallization product is hcp-(Co,Fe) for the Co-rich composition (x = 0), bcc-(Co,Fe) for high Fe contents (x ≥ 0.3) and a mix of bcc, and fcc for intermediate compositions (0.025 ≤ x ≤ 0.15). The relative strain at fracture decreases dramatically (εf < 0.01) for x ≥ 0.15, whereas for lower Fe content it has a maximum (εf > 0.037) at x = 0.025 and 0.050 resulting in excellent resistance against fracture. The brittle ribbons (x ≥ 0.15) showed smooth fracture surface with dimples less than 230 nm in diameter, small localized or absent shear bands and large Vickers hardness (>1200 kg mm−2). On the other hand, the Co-rich ribbons with greater ductility (x = 0.025, 0.05) exhibit a vein pattern filled with voids (features ∼2–11 µm), extensive shear banding and lower Vickers hardness (<1050 kg mm−2).

(Fe,Si,Al)-based nanocrystalline soft magnetic alloys for cryogenic applications

In this work Al and Si are substituted for Fe in a (Fe,Si,Al)–Nb–B–Cu alloy with the goal of improving its magnetic properties at 77 K. The x-ray diffraction patterns for a series of five alloys annealed at 823 K shows a Fe3(Si,Al) ordered phase with some residual amorphous phase. The lowest coercivity at room temperature was observed for the alloy with composition Fe68Si15.5Al3.5Nb3B9Cu1. At cryogenic temperatures, the saturation magnetization of 99.3 A m2/kg, coercivity of 0.45 A/m, and resistivity of 122 μΩ cm for the Fe63Si17.5Al6Nb3B9Cu1 alloy, compare favorably to commercial alloys at 77 K.

Structure and magnetic properties of CoFeZrMBCu soft nanocrystalline alloys

In this article, we report on the effect of substituting Nb, Hf, and Ta for Zr on the crystallization behavior, crystal structure, and magnetic properties of (Co0.95Fe0.05)88Zr7−xMxB4Cu1 (x=0, 1, and 3.5). Samples annealed at temperatures up to 550°C lead to the partial crystallization of very fine bcc-(Co,Fe) and fcc-(Co,Fe) grains, while annealing at 750°C lead to coarser fcc-(Co,Fe) grains and Co–M intermetallic phases. All of the compositions except those with 3.5at.% Nb or Ta show similar low coercive behavior in the as-spun and annealed states. The coercivity of annealed ribbons below 550°C is varied from 0.15to0.46Oe and the magnetization from 128to133emu∕g. High temperature hysteresis loop measurements revealed a nearly constant coercivity for up to 400°C and 20% reduction in magnetic induction, exceeding the performance of Finemet alloys at this temperature.

Nanostructured Soft Magnetic Materials

Reduction of the grain size to less than 20 nm has provided major advances in soft magnetic materials performance, including reduced core losses and coercivities. These promising results have stimulated research efforts, worldwide, in the areas of nanocrystalline alloy design, alloy processing, materials performance evaluation, and transition to various applications. This chapter presents recent advances in nanocrystalline soft magnetic alloy processing methods, phase transformations, microstructure evaluation, magnetic property measurement and analysis, and applications.