Structures and magnetic properties of Nd15Fe77B8 alloy produced by twin-roller quenching

The relationship between the magnetic properties and microstructures of flakes of a Nd15Fe77B8 alloy produced by twin-roller quenching was studied. Flakes produced at low to intermediate roller speeds (1.4–8 m/s) showed a dendritic structure of Nd2Fe14B oriented normal to the chilled surface, while flakes produced at higher roller speeds showed fine, equiaxed Nd2Fe14B grains. The flakes with a dendritic structure were found to be magnetically anisotropic and showed a high-energy product (BH)max (15.2 MGOe) in the direction perpendicular to the chilled surface. The flakes that had fine equiaxed grains were magnetically isotropic. The maximum coercivity of the flakes exceeded 20 kOe. The microstructural transition from columnar dendrites to equiaxed grains is explained in terms of morphological changes in the solidification of undercooled melts. Optimization of twin-roller quenching parameters for the production of desirable microstructures in Nd-Fe-B magnets is discussed.