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, whi ch is richer in Nd than the stoichiometric Nd2Fe14B composition, has been d etermined for ribbon using convergent-beam electron diffraction, high-resol ution electron microscopy, X-ray diffraction and energy-dispersive X-ray sp ectroscopy. Three crystalline phases have been identified, namely the tetra gonal, ferromagnetically hard Nd2Fe14B phase, the fcc paramagnetic phase Nd 73Fe27, which has been reported previously, and a second paramagnetic Nd-ri ch phase having a tetragonal structure with a Nd : Fe ratio of 13:1 and O i n solution, which has hitherto not been reported. In addition, a small volu me fraction of an amorphous grain-boundary phase was found in the ribbon sp un at a lower roll speed. These minority grain-boundary phases substantiall y increase the intrinsic coercivity of the alloy, compared with the stoichi ometric composition, as a result of the decoupling of the Nd2Fe14B hard mag netic grains and the damping of the nucleation of reverse magnetic domains.