Collective Behavior of Nanograins in Co-Substituted Fe-Based Nanocrystalline Alloys

The need for more efficient energy conversion and/or distribution systems is a challenging and strong demand nowadays. Among other materials, amorphous and/or nanocrystalline soft magnetic alloys are a viable alternative for both storage and transportation of the energy. In the recent years, they turned out to become competitive with silicon electrical steels and various ferrites for niche applications, mainly the ones involving working at high frequencies and temperatures [1]. The literature reports many attempts to improve the soft magnetic properties of Fe-based amorphous and/or nanocrystalline rapidly solidified materials, by modifying either the composition or annealing conditions. For example, it was shown that by replacing 22 at.% of Fe with Co, the Co-substituted FINEMET alloy can be used at 500 °C, the relatively low coercivity being preserved even at such high temperatures [2]. In order to study further these correlations and to understand why an amorphous and/or nanocrystalline material with nearly-zero magnetostriction has a large output response when subjected to a mechanical stress or vibration, in this work we will present comparatively our latest results on the collective behavior of nanograins in Co-substituted FINEMET and VITROPERM 800 rapidly quenched alloys, having nominal compositions (Fe1-x Cox)73.5 Cu1 Nb3 Si13.5 B9 and (Fe1-x Cox)73.5 Cu1 Nb3 Si15.5 B7, respectively (x = 0, 0.25, 0.5, 0.75 and 1), in the as-quenched state and after annealing at temperatures between 500 and 600 °C. Our study mainly focusses on how Co influences the precipitation and anisotropies of the nanograins, as well as the temperature variation of magnetic and magnetoelastic properties of the 2 systems. In addition, we were interested to understand why the small compositional variations of Si and B in FINEMET and VITROPERM 800 alloys are inducing a strongly different magnetoelastic behavior in the as-quenched amorphous samples, with small positive magnetostriction values for FINEMET samples and zero magnetostriction for VITROPERM 800 ones. The as-quenched samples are fully amorphous as one can see from the TEM images shown in Fig. 1, while the ones subjected to annealing are nanocrystalline, with grains of 15–30 nm, randomly dispersed within the amorphous matrix, depending on the annealing temperature and Co content. The optimum magnetic properties are obtained at different annealing temperatures (between 510 and 550 °C), depending on Co content, as shown in Fig. 1; the larger the Co content, the lower is the optimum annealing temperature. The total substitution of Fe with Co is strongly influencing the microstructure and is hardening the material (Fig. 2). The substitution of Fe with Co followed by optimum annealing reduces drastically the saturation magnetostriction due to the more random distribution of internal micro-stresses in Co-substituted samples compared with the ones containing Fe only, but also due to the different orientation of the anisotropies of Fe(Co) grains relative to the matrix. The optimum magnetic properties are obtained for samples with Co contents ranging from 25 to 50 at.%, annealed at temperatures in the range of 530–540° C, when the nanograins reach their optimum sizes (between 15 and 25 nm) and the percolation limit increases to 60–70%. In this case the collective behavior of the nanograins reaches the maximum strength, this being also influenced by the presence of Co in the DO3 nanograins, which slightly shifts the nanograins structure from bcc towards fcc or even hcp. Such a specific behavior is also strongly dependent on the Si to B contents, a larger content of Si in VITROPERM 800 playing a more significant role in the exchange interactions between the grains through the amorphous residual matrix. Financial support from the ITN-FP7 Marie Sklodowska-Curie program “VitriMetTech” N. 607080 and 3MAP NUCLEU Program (2018) is thankfully acknowledged.