Research Progress on Annealing Embrittlement of Fe-based Nanocrystalline Soft Magnetic Alloys
ZHANG Guozhong,LI Yanhui,WU Licheng,ZHANG Wei
Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province),School of Materials Science and Engineering,Dalian University of Technology,Dalian 116024,China
Abstract: Since Fe-based nanocrystalline soft magnetic alloys were discovered in 1988, they have attracted increasing attention due to the excellent properties including low coercivity, high permeability, low core loss, low magnetostriction coefficient, and especially outstanding soft magnetic properties in high frequency. Among them, Finemet alloys have been widely used as core materials in high-frequency transformer and other devices. In recent years, a series of new nanocrystalline soft magnetic alloys with high saturation magnetic flux density have been developed, which will promote the miniaturization and energy saving of the power and electronic equipment. The Fe-based nanocrystalline soft magnetic alloys are usually fabricated by annealing the amorphous ribbons. During annealing, the ribbons become brittle, which increases the difficulty to the processing and forming of the iron cores, and also causes the devices prone to failure in service, hence restricting their extensive applications in industry. Therefore, plenty of work on the annealing embrittlement of the alloys have been conducted, and a series of progress has been made. The process of the annealing embrittlement of the Fe-based nanocrystalline alloys includes two stages, i.e., the ductile-to-brittle transition due to the structural relaxation of the amorphous precursor annealed below the primary crystallization temperature, and the further increase in the brittleness caused by the precipitation of α-Fe phase after crystallization. The relative strain at fracture (εf) and critical stress intensity factor (KQ) are commonly adopted to evaluate the brittle degree of the alloy ribbons. The εf is mainly employed to study the annealing embrittlement of the ribbons because it can quantitatively characterize the degree of the brittleness, and the test is simple, although the εf values are discrete to a certain degree. The increase in the volume fraction of the α-Fe phase usually aggravates the annealing embrittlement, while refining the α-Fe grain is beneficial to restrain the embrittlement. The hardness of the alloys rises linearly with increasing the volume fraction of the α-Fe phase when the volume fraction is below 70%. In this case, the hardness can also indirectly reflect the degree of the embrittlement. Alloying appropriate element, optimizing annealing process and/or adopting new heat treatment methods can refine the microstructure of nanocrystalline alloys, and to some extent, suppress the tendency of the annealing embrittlement. In this paper, the research progress in the mechanism and evaluation methods for the annealing embrittlement of the Fe-based nanocrystalline soft magnetic alloys, the correlation among the alloy composition, heat treatment process, microstructure and annealing embrittlement, together with the approaches for inhibiting the annealing embrittlement has been reviewed. In addition, the problems and the orientations of the research on the annealing embrittlement have been discussed as well.
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2020, Vol. 34 
