Growth kinetics and TiC precipitation phenomena in Nd–Fe–B–Ti–C melts in dependence on cooling parameters and composition

The formation of dendritic α-Fe and grain growth during heat treatment is a crucial problem in industrial production of high performance Nd–Fe–B magnets. The solidification behaviour and phase selection phenomena of stoichiometric Nd–Fe–B alloys with Ti and C additions (0.5–3 at.%) were investigated using the electromagnetic levitation and melt spinning techniques. It was ascertained from in-situ temperature–time measurements that the growth velocity of the hard magnetic Nd2Fe14B phase is reduced by Ti and C additions. The cooling rate prior to solidification influences drastically the morphology of the TiC precipitates, which affects the nucleation of the properitectic γ-Fe in the undercooled stage. High quenching rates promote the formation of fine TiC precipitates. The solubility of Ti in the hard magnetic Nd2Fe14B1 phase was found to be negligible. TEM investigations of melt spun Nd–Fe–B–Ti–C ribbons showed that nano-size TiC particles are situated in the intra- or inter-granular region. The fine dispersed TiC precipitates act as nucleants for the formation of the hard magnetic Nd2Fe14B grains. The correlation between the increasing nucleation rate and the considerable grain refinement is pronounced. Annealing at elevated temperatures promotes further TiC precipitation at the grains boundaries leading to a stable fine grained microstructure.