Transmission electron microscope characterization of cast and hot-worked R-Fe-B:Cu(R=Nd,Pr) permanent magnets

The hard magnetic properties of hot-worked R-Fe-B permanent magnets are improved by the addition of a minor amount of Cu (up to 2 at. %). Transmission electron microscope investigations show that the reason for the increase of the coercivity is the fact that Cu is mainly dissolved in the intergranular region separating the hard magnetic 2:14:1 grains. The results obtained are similar in Nd-Fe-B:Cu and Pr-Fe-B:Cu magnets. The intergranular region has been observed to consist of a eutectic consisting of two phases: the Nd-or Pr-rich phase and the orthorhombic NdCu- or PrCu-phase. This corresponds to the eutectic decomposition of the liquid intergranular phase during cooling. It is suggested that the decrease of the melting temperature of the intergranular phase leads to a better wetting behavior and separation of the hard magnetic grains, and therefore to an enhancement of the coercivity.