State Key Laboratory of Metastable Materials Science and Technology, Yanshan University
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DOI:
10.7502/j.issn.1674-3962.2015.11.05
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Abstract:
Nanocomposite exchange coupled magnets are most likely next-generation magnets because of their potentially high energy products. Microstructural control is the key to achieve high energy products. This paper reviews our studies on microstructural control of nanocomposite magnets. The study of the kinetics of phase transition in amorphous Nd-Fe-B demonstrates that the mechanism underlying the coarse grain size (20-100 nm) of α-Fe phase in nanocomposite magnets is a high nucleation activation energy (En) and a low growth activation energy (Eg), that is, the difficult nucleation and easy growth processes of the α-Fe phase, Eg /En < 1. The growth process of soft and hard phases is dependent on atomic diffusion mediated by vacancy-type thermal defects, in which the growth of α-Fe grains in size is dominantly controlled by the diffusion of Fe atoms mediated by thermal vacancies. Room-temperature severe plastic deformation (SPD) affects the grain size (10-20 nm) and volume fraction of soft phase significantly and inhibits the formation of metastable intermediate phases in the alloy. Temperature gradient, high pressure and hot deformation at high stress can induce the easy-axis alignment of Nd2Fe14B hard-phase grains, yielding anisotropic α-Fe/Nd2Fe14B nanocomposite magnets. The exchange-coupling strength between soft and hard phase grains and the coercivity of magnets can be enhanced through the modification of interfacial structure and chemistry. Keywords: Nanocomposite permanent magnets; Nanocrystals; Microstructure; Interface; Crystal orientation; Severe plastic deformation; High pressure.