The effect of quench rate on the microstructure and coercivity of some Nd-Fe-B based ribbons

Abstract

The microstructure of some melt spun NdFeB and NdFeBNb ribbons has been examined using transmission electron microscopy. The correlation between microstructure and coercivity has been investigated as a function of quench rate. In general the coercivity of these ribbons increased as the grain size became smaller, reaching an upper limit just before the ribbon became amorphous at the higher quench rates. However, in a narrow range of wheel speeds (≈8–9 m/s using a Cu wheel) we observed a microstructure containing fairly large grains (≈0.5 μm) of the hard magnetic Nd 2Fe 14B phase surrounded by an amorphous matrix. These ribbons had coercivities of less than 16 kA/m and we attribute the fall in coercivity to the existence of a ferromagnetic amorphous grain boundary. In the ribbons containing Nb additions we observed fine coherent precipitates in the hard magnetic phase. Similar precipitates have been observed in sintered NdFeB magnets containing Nb additions. The precipitate size decreased with increasing quench rate and attained dimensions comparable with the domain wall width in the hard phase (≈5 nm) at wheel speeds in the range 5–7 m/s. We attribute the enhanced coercivities of the under-quenched Nb-containing ribbons to the presence of coherent precipitation in the hard magnetic grains. The coercivity of the ternary ribbons fell in this range of wheel speeds.