Extension of the primary solidification region of Nd2Fe14B by levitation of undercooled melts

Undercooled Nd-Fe-B melts with compositions near the Nd2Fe14B phase (Phi phase) were studied by the electromagnetic levitation technique. In situ measurements of the solidification kinetics were accomplished and melt drops with controlled undercooling were quenched onto chill substrates in order to reveal the solidification sequence and microstructure formation processes. Crystal growth velocities of the Phi phase of the order of 1 mm s(-1) were estimated from in situ measurements. Consistent with the nucleation theory, which predicts primary gamma-Fe phase crystallization up to 650 K undercooling for the stoichiometric alloy, gamma-Fe nucleation could not be avoided during undercooling but the quenching of the melt drops led to dissolution of the gamma-Fe nuclei during Phi-phase formation. The gamma-Fe phase growth was suppressed in a seam adjacent to the chill surface, whereas Fe dendrites occurred in the remaining parts of the quenched samples. Reduced solute rejection on growth is believed to be responsible for the preferred metastable Phi-phase formation from the undercooled melt on chill substrates. The investigation gives some clues to understand different microstructures and magnetic properties of powder precursors for permanent magnets obtained from different rapid solidification routes. (C) 1998 American Institute of Physics. [S0021-8979(98)15111-3].