In situ precipitation of nanoscale α-Fe enables high-performance Nd2Fe14B/α-Fe nanocomposite ribbons

Nanocomposite magnets combining hard Nd2Fe14B and soft α-Fe phases are promising for surpassing the energy product limit of single-phase permanent magnets, but conventional routes using external Fe powders suffer from poor interfacial compatibility and uncontrolled soft-phase coarsening. Here, an in situ precipitation strategy coupled with an ultrahigh melt-spinning speed of 50 m/s is employed to fabricate (Nd2Fe14B)1−xFex (x = 0.1–0.9) ribbons. Increasing Fe content raises both the α-Fe fraction and the amorphous content, with amorphization occurring mainly in the Nd2Fe14B matrix. Strikingly, at an optimal composition of x = 0.5, ultrafine α-Fe grains (12–17 nm) are homogeneously embedded within the Nd2Fe14B nanograins (∼20 nm), yielding a coercivity of 8.6 kOe and enhanced remanence even with high amorphous and α-Fe content. In contrast, excessive Fe (x = 0.7) triggers abnormal growth of some α-Fe particles (120–200 nm), deteriorating the coercivity despite increased maximum magnetization. Micromagnetic simulations corroborate that the exchange coupling is maximized when the soft grain size is confined to ∼10 nm, and the hard grains are appropriately refined. This work demonstrates that synergistic control of composition and rapid solidification kinetics enables the realization of ideal nanostructures, providing a robust pathway for designing high-performance rare-earth permanent magnets.