Effect of melt-spinning roll speed on the nanostructure and magnetic properties of stoichiometric and near stoichiometric Nd–Fe–B alloy ribbons

Nanostructured NdFeB hard magnetic material exhibiting enhanced magnetic properties can be produced by direct chill or melt spinning followed by recrystallization. The surface condition and nanostructure of stoichiometric, Fe82.3Nd11.8B5.9, and near stoichiometric, Fe83.2Nd10.9B5.9, alloy ribbons and the effect of melt-spinning parameters on the structure are investigated using optical, scanning and transmission electron microscopes. The formation of gas pockets on the roll surface of the ribbons during melt spinning can prevent heat transfer which can result in local coarse grains. A decrease in the ribbon thickness and mean Nd2Fe14B grain size and improvement in magnetic properties, including remanence enhancement, were observed on increasing the roll speed, prior to vitrification and consequent collapse of properties at higher speeds. An orientation relationship was found between the Nd2Fe14B and α-Fe phases for coarse grain samples melt spun at low roll speeds. Dark field TEM also showed that some of the α-Fe precipitates had identical orientations showing epitaxy with the Nd2Fe14B matrix phase. A preferred growth orientation of [0 0 1] was observed for some Nd2Fe14B grains.