Investigation of the phase constitution and structure of rapidly solidified hard magnetic Nd18Fe76B6 ribbons by transmission electron microscopy

The phase constitution of a high-coercivity melt-spun Nd18Fe76B6 alloy, which is richer in Nd than the stoichiometric Nd2Fe14B composition, has been determined for ribbon using convergent-beam electron diffraction, high-resolution electron microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy. Three crystalline phases have been identified, namely the tetragonal, ferromagnetically hard Nd2Fe14B phase, the fcc paramagnetic phase Nd73Fe27, which has been reported previously, and a second paramagnetic Nd-rich phase having a tetragonal structure with a Nd: Fe ratio of 13 : 1 and O in solution, which has hitherto not been reported. In addition, a small volume fraction of an amorphous grain-boundary phase was found in the ribbon spun at a lower roll speed. These minority grain-boundary phases substantially increase the intrinsic coercivity of the alloy, compared with the stoichiometric composition, as a result of the decoupling of the Nd2Fe14B hard magnetic grains and the damping of the nucleation of reverse magnetic domains.