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Abstract
Silicon steels with Si≈3wt.% and C≤0.04wt.% are widely used in transformers as soft magnetic materials (easy to magnetize and demagnetize). However, they suffer from low permeability and high core loss. Recently, Fe-based amorphous/nanocrystalline alloys have drawn attention for their promising applications as cores of transformers, generator and motors because of their low core loss, low coercivity Hc and high permeability compared to conventional silicon steels. However, the problem of the Fe-based amorphous alloy is its lower saturation magnetization Bs as compared to commercial silicon steels.
In this work, a variety of Fe-based amorphous samples with main composition of Fe-Si-B(Cu) which is based on Finement system were produced using melt-spinning method. The microstructure, thermal behavior, magnetic properties and mechanical behavior of the produced samples were studied by means of high resolution TEM (HRTEM), X-ray diffraction method (XRD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), transmission kikuchi diffraction (TKD), vibrating sample magnetometer (VSM), B-H loop tracer and nanoindentation techniques.
The DSC and DMA results showed that the addition of Cu decreases the glass transition temperature Tg and activation energy of the α-Fe(Si) nanoparticles, indicating the facilitation of the crystallization process and deterioration of the glass forming ability. Anomalous behavior was observed at the final stage of crystallization in Cu-containing sample which can be attributed to the inhomogeneous nature of crystallization. The absence of Cu resulted in a coarse dendritic microstructure after annealing whilst much refiner and equiaxed grains were attained in Cu-containing alloys. The saturation magnetic flux density Bs value of as high as 1.61(T) and coercivity Hc value of as low as 10.6 (A/m) were achieved in the amorphous specimen with the chemical composition of Fe78.6Si1.8B17.75Cu1.85 (at.%). Also, it was shown that annealing at temperature around the glass transition point could effectively decrease the coercivity.
Single-step nanoindentation (loading-unloading) study revealed that annealing process effectively raised the hardness H and reduced elastic modulus (Er) due to the annihilation of free volume (decrease of interatomic spacing) and interaction of shear bands with nanoparticles; however, unnecessary prolonged annealing time and increasing annealing temperature reduced the hardness and reduced modulus considerably owing to the significant grain growth and probably Fe2B precipitation within the matrix.
Generally, amorphous alloys do not show strain hardening; however, multi-step nanoindentation (loading-partial unloading-reloading) exhibited a peculiar strain hardening behavior which is comparable to those crystallized sample even stronger. It is suggested shear bandings during nanoindentation would increase the temperature to a level that β-relaxation (local movement of smaller atoms), could happen. In addition, stress itself can also induce relaxation which is known as mechanically induced relaxation. It appears that cyclic loading of the amorphous sample could have encouraged the atomic-level rearrangements and led to stress relaxation in a very small and confined area. And, vice-versa, cyclic loading resulted in the localized rejuvenation of the stress-relaxed sample.