M.F. de Campos

Chemical composition and coercivity of SmCo5 magnets

The effects of alloy composition and heat treatment on the intrinsic coercivity iHc of SmCo5 magnets were studied. Alloys having six chemical compositions near that of stoichiometric SmCo5 were used to produce magnets via the usual powder metallurgy techniques. Magnets were either as sintered (1150 °C) or sintered (1150 °C) and treated at 850 °C. The substantial increase in iHc due to the 850 °C heat treatment occurs reversibly and with a negligible change in lattice parameters, Curie temperature Tc and anisotropy field HA. Quantitative metallography and thermomagnetic measurements showed that the microstructural constituent present in addition to the SmCo5 phase in hyperstoichiometric alloys is composed of both the Sm2Co7 phase and the Sm5Co19 phase. Measurements of magnetization in dc magnetic fields up to 33 T suggest that HA is around 53 T, considerably higher than previously reported values. The coercivity of the magnets is discussed in terms of thermal equilibrium populations of lattice defects.

Effect of Frequency on the Iron Losses of 0.5% and 1.5% Si Nonoriented Electrical Steels

The effect of grain size on iron losses were compared for two electrical steels with 0.5 and 1.5 wt% Si. The results confirm that the optimum grain size for minimizing the energy losses decreases when the electrical resistivity decreases or when the frequency increases. Experimental results are compared to a model which considers the influence of grain size. The recrystallization texture of the alloys varies little with grain size and consists mainly of the fibers {111}languvwrang and lang110rang//RD

Remarks on the Co-rich region of the Co-Sm diagram

The Co-rich region (0 to 30 at.% Sm) of the Co-Sm phase diagram was reevaluated. The homogeneity range of Co5Sm was redetermined with optical metallography, Curie temperature measurements, and lattice parameters data. Some comments on the phases Co7Sm2, Co19Sm5, Co5Sm, and Co17Sm2 are presented. The homogeneity range of other Co5RE and Co17RE2 (RE = rare-earth) compounds is discussed.

Anisotropy of the magnetic losses components in semi-processed electrical steels

Making use of magnetic losses separation techniques, it is shown that most of the anisotropy of the total magnetic losses of semi-processed steels with different grain sizes is concentrated in the high induction region of the quasi-static hysteresis loop.

Effect of grain size, deformation, aging and anisotropy on hysteresis loss of electrical steels

The investigation of the effect of cold deformation, anisotropy, aging and grain size on the shape of the hysteresis curve of non-oriented electrical steels shows that most of the hysteresis energy is dissipated in the high-induction region (above the maximum permeability induction). It indicates that more attention should be given to the energy dissipation mechanisms in that region, such as the domain annihilation and nucleation.

Effect of Plastic Deformation on the Excess Loss of Electrical Steel

The interpretation of the effect of plastic deformation on the calculated excess loss component (anomalous-loss) supports the concept of loss separation. Magnetic losses and Barkhausen noise of nonoriented electrical steel sheets were measured on Epstein strips taken from a single coil of 0.8% Si nonoriented electrical steel. Sheets were extracted in the annealed condition, without any skin pass and with a grain size of 18 μm. This material was cold rolled in order to obtain sets of samples with true strain from 2% up to 29%. X-ray diffraction was used to estimate the dislocation density. The analysis of magnetic properties was performed by Barkhausen noise measurements and also by analyzing the hysteresis loops obtained from Epstein frame measurements for different inductions and different frequencies (including the quasi-static regime for hysteresis loss measurements). These data allowed us to observe that most of the well known total loss increase with plastic deformation is due to an increase in the hysteresis loss component, while excess loss decreases to become negligible. This behavior can be explained if it is assumed that the plastic deformation lead to an increase in the number of domain walls per unit volume, thereby decreasing the excess loss. Barkhausen peak area increases with plastic deformation, reproducing results taken from samples of different silicon content.

Modeling of sharp change in magnetic hysteresis behavior of electrical steel at small plastic deformation

In 2.2% Si electrical steel, the magnetic hysteresis behavior is sharply sheared by a rather small plastic deformation (0.5%). A modification to the Jiles–Atherton hysteresis model makes it possible to model magnetic effects of plastic deformation. In this paper, with this model, it is shown how a narrow hysteresis with an almost steplike hysteresis curve for an undeformed specimen is sharply sheared by plastic deformation. Computed coercivity and hysteresis loss show a sharp step to higher values at small strain due to an n=1∕2 power law dependence on residual strain. The step is seen experimentally.

A model relating remanence and microstructure of SmCo5 magnets

Abstract The question of remanence in SmCo5 sintered magnets is reviewed, giving special attention to the influence of microstructure. In SmCo5 magnets, the microstructural constituents are the matrix phase SmCo5, another ferromagnetic phase (Sm2Co7), oxides (Sm2O3), a recently reported carbide SmCoC2 and pores. A method to estimate alignment via the ratio Jr/Js from remanence measurements and microstructural analysis is proposed. The results agree very well with the degree of alignment calculated from Schulz pole figures. During sintering it was observed that the nucleation of SmCo5 on the Sm2Co7 phase is epitaxial. Hexagonal Sm2Co7 is present in our sintered magnets but rhombohedral Sm2Co7 may also be present. The effect of chemical composition (Sm content and oxygen content) on remanence and on the ratio Jr/Js was evaluated. In the sintering step, the densification occurs more slowly for compositions with higher samarium content (or higher Sm2Co7 volume fraction). A model able to calculate the best chemical composition to maximize coercivity and remanence is presented and discussed.

Modeling of Plastic Deformation Effects in Ferromagnetic Thin Films

To explain the magnetic behavior of plastic deformation of thin magnetic films (Fe and permalloy) on an elastic substrate (nitinol), it is noted that unlike in the bulk, the dislocation density does not increase dramatically because of the dimensional constraint. As a result, the resulting residual stress, even though strain hardening is limited, dominates the observed magnetic behavior. Thus, with the field parallel to the stress axis, the compressive residual stress resulting from plastic deformation causes a decrease in remanence and an increase in coercivity; and with the field perpendicular to the stress axis, the resulting compressive residual stress causes an increase in remanence and a decrease in coercivity. These elements have been inserted into the model previously developed for plastic deformation in the bulk, producing the aforementioned behavior, which has been observed experimentally in the films.

The (SmZr)Co

Detailed microstructural characterization of magnets and homogenized as-cast alloys, which included X-ray diffraction Rietveld analysis, has indicated that the so-called platelet or lamellae phase is (SmZr)₁(CoFeCu)₃ with the PuNi₃ structure and lattice parameters a~0.5 nm and c~2.4 nm. The structural and magnetic properties of the (SmZr)Co₃ phase were investigated. The microstructure shows two phases differing in their Zr/Sm ratio. Magnetization curves for the samples (Sm_0.33Zr _0.67)Co₃, (Sm_0.33Zr_0.67)Co _2.97Fe_0.03, and (Sm_0.67Zr_0.33)Co₃ are consistent with the two-phase microstructure observed. Room temperature coercivity values of these samples are low (ap1 kOe.)