E. Ferrara

Study of the brittle behaviour of annealed Fe-6.5 wt%Si ribbons produced by planar flow casting

Abstract Rapidly solidified Fe-6.5wt%Si ribbons are fully ductile in the as-quenched state. However, the annealing process, required for optimization of the magnetic properties, generally leads to final brittle materials. In this article, a study is made of the embrittlement process, through mechanical tests and structural investigations on ribbons subjected to different cooling rates (1 °C min −1 ⩽T⩽ 1500 °C min −1 ). A ductile-to-brittle transition is observed for T∼ 1000 °C min −1 , where long range B 2 ordering takes place and the dislocation character changes from unitary to superlattice. An increase in the work hardening coefficients is correspondingly found, in association with enhanced hindering of the dislocation motion within the grains. The restricted glide and cross-slip capability of the dissociated superdislocations is consequently identified as the chief mechanism responsible for the buildup of internal stresses and eventual brittle fracture of the material.

Loss measurements on amorphous alloys under sinusoidal and distorted induction waveform using a digital feedback technique

A newly developed digital feedback wattmeter, allowing power loss measurements in soft laminations under generic induction waveform is presented. The system is intrinsically free of auto‐oscillations, typical drawback of analog feedback circuits, and can therefore be operated in a wide frequency range (0.5 Hz–100 kHz). Loss measurements performed by this setup under sinusoidal and distorted induction waveforms on Co based amorphous ribbons are reported. It is shown that the classical approximation to the prediction of power losses under distorted induction largely fails to account for the experimental results. A novel theoretical approach, based on the statistical theory of losses, is discussed and successfully applied to the experiments. In particular, it is shown that knowledge of the loss components under sinusoidal induction at a given magnetizing frequency permits one to make an accurate prediction of the effect of distortion at that frequency as well as other ones. Illustrative applications at 1 kHz...

Magnetic properties of rapidly quenched soft magnetic materials

Abstract The discovery and large scale investigation of rapidly solidified alloys have brought to light, in the last two decades, a new scenario in basic and applied magnetism. With a range of compositions and microstructural states unavailable to conventional materials, the rapidly quenched alloys have in fact demonstrated unprecedented versatile properties, provided unique opportunities in applications and stirred up the domain of physical theories. In this paper we discuss the basic magnetic features of three classes of rapidly quenched soft magnetic materials: amorphous, nanocrystalline, and high Si alloys. Attention will be focussed on the technically relevant properties correlated with the magnetization process—coercivity, losses, permeability—and their relationship with the structural material properties. The fundamental role of local and macroscopic anisotropies in determining the overall magnetic behavior of these alloys will be highlighted and present trends in the predictive approach to the magnetization process and its dependence on the microstructural parameters will be discussed. Both the static and the dynamic magnetic behaviors will be considered and the loss dependence on the magnetizing frequency will be assessed against modelling.

Magnetic and mechanical properties of rapidly solidified FeSi 6.5 wt% alloys and their interpretation

Abstract The structural, mechanical and magnetic properties of FeSi 6.5 wt% rapidly solidified alloys have been investigated following recrystallization annealing and different rates of cooling through the B 2 + DO 3 ordering region (1 ≤ T ≤ 1500° C min −1 ). A transition from ductile to brittle behavior is observed for T −1 , chiefly due to B 2 ordering and the associated formation of superlattice dislocations, having reduced glide and cross-slip capability. The magnetic behavior appears, however, to be weakly dependent on T , with the energy losses minimized for average grain size around 100–150 μm.

Domain Wall Processes, Rotations, and High-Frequency Losses in Thin Laminations

We have investigated and modeled the magnetization process in thin amorphous and nanocrystalline ribbons from DC to 1 GHz. These transverse anisotropy laminations, their thickness ranging between 6 and 20 μm, display excellent broadband magnetic behavior, ensuing from the dominant role of magnetization rotations. Combination of fluxmetric, aftereffect, and high-speed magneto-optical experiments put in evidence that the domain wall processes, the obvious source of losses at low and medium frequencies in spite of negligible contribution to the magnetization reversal, fully damp on attaining the MHz range. Here the energy dissipation chiefly descends from the rotations and conforms to the so-called classical regime. To describe the high-frequency spin dynamics, the coupled Maxwell and Landau-Lifshitz-Gilbert equations are therefore considered. We have worked out a numerical solution of such equations by a finite element approach, based on a very fine time discretization and a computing scheme preserving the magnetization modulus. From the calculation of hysteresis loop and eddy current density at each mesh point, the separate contributions to the rotational losses by the eddy currents and the spin damping mechanism are obtained. The overall energy loss behavior versus frequency is thus eventually predicted in terms of separate contributions by the domain wall processes and the rotations.

The magnetic properties of FeSi 6.5 wt% alloys obtained by a SiCl4-based CVD process

Copyright (c) 1996 Elsevier Science B.V. All rights reserved. Grain-oriented Fe−Si alloys have been Si enriched up to 6.5 wt by a chemical vapour deposition method using SiCl 4 . Both longitudinally and transversally cut laminations exhibit a decrease of the energy losses, measured from dc to 400 Hz, upon Si enrichment. The strong decrease of magnetostriction and the increase of electrical resistivity caused by the addition of Si chiefly affect the hysteresis and the classical loss components, respectively, and combine to provide a decrease of the excess losses.

Grain growth and texture in rapidly solidified Fe(Si) 6.5 wt.% ribbons

Abstract The dependence of structural properties of Fe(Si) 6.5 wt.% ribbons on annealing treatments has been investigated in the temperature range 700–1300 °C. Two different annealing methods, respectively characterized by slow (conventional) and fast (infrared) heating rate, have been considered. It is found that abnormal grain growth takes place above a threshold temperature, which depends on the type of annealing, with the average grain size attaining a maximum value around 200 μm. Such a growth is conducive to a cube-on-face (100) crystallographic texture after conventional annealing, whereas after the infrared treatment a cube-on-edge (110) texture is observed. The texture development is here discussed in terms of competition between grain boundary and surface energy minimization, with attention devoted to the role of the presence of oxygen on the surface energy of (100) and (110) crystallographic planes.

Magnetic properties of Ga substituted NdFeB composite

We have studied the effects of Ga substitution for Fe in the crystallization and on the magnetic properties of Nd 4 Fe 78 -x Ga x B 18 (x = 0.5, 1.0, 1.5 or 2.0) composites. The remanence and coercivity were improved with the addition of 0.5% atom-Ga. Field annealing was found to enhance slightly the remanence.

Magnetization Process in Thin Laminations up to 1 GHz

Amorphous and nanocrystalline ribbons with transverse anisotropy have been characterized by magneto-optical, fluxmetric, and transmission line techniques from DC to 1 GHz. The contributions of domain wall displacements and magnetization rotations to the magnetization process, singled out by direct domain observations, consistently fit with the observed dependence of complex permeability and energy losses on frequency. With the domain wall motion suffering progressive hindering with increasing frequency and falling into full relaxation on reaching the MHz range, the magnetization process becomes amenable to a classical description, the concept of loss decomposition being secured. For this description, the conventional rate-independent constitutive relation for the magnetic material proves, however, inadequate. The diffusion equation for the electromagnetic field is therefore coupled with the Landau-Lifshitz-Gilbert equation, taken as a dynamic constitutive magnetic equation, and a solution is worked out by a numerical procedure. Magnetization and eddy-current field are thus obtained versus time at any point of the lamination, account being taken of the exchange field and its restraining action on the skin effect. The so calculated magnetic loss behavior turns out to correctly describe the high-frequency results, while coalescing with the classical eddy-current loss prediction at low frequencies.

Losses in transverse field annealed amorphous ribbons

Abstract Dc and high frequency energy losses have been determined in near-zero-magnetostrictive amorphous ribbons as a function of field-induced longitudinal and transverse magnetic anisotropy (300 J/m 3 ≥ K u ≥ − 150 J/m 3 ). The hysteresis and the total losses at 10 kHz basically follow opposite trends with K u , the first tending to increase and the second to decrease on passing from positive to negative K u values. For high transverse anisotropy, only coherent rotations contribute to the magnetization reversal, but losses do not vanish, because rotations induce domain wall motion through the corresponding evolution of the magnetostatic free charges at the ribbon edges.