Grain Boundaries In Nd-Fe-B-Based Alloys

In this review we consider the microstructure of grain boundaries (GBs) in the Nd-Fe-B-based alloys for permanent magnets. It is generally accepted that the unique hard magnetic properties of such alloys are controlled by the thin layers of a Nd-rich phase in Nd2Fe14B/Nd2Fe14B GBs. These GB layers ensure the magnetic isolation of Nd2Fe14B grains from each other. It is generally supposed that such GB layers contain metallic Nd or Nd-rich intermetallic compounds. However, the commercial Nd-Fe-B-based permanent magnets frequently contain the tangible amount of neodymium oxide Nd2O3 in the triple junctions between Nd2Fe14B grains. In this review we show that oxide layers are also present in the GBs in Nd-Fe-B-based permanent magnets. The Nd-based phase in GBs can form continous layers between Nd2Fe14B grains (this case is called complete wetting). It corresponds to the zero contact angle between GB and Nd-rich phase. If the contact angle is non-zero, the Nd-based phase forms the chain of particles in the GB. In turn, if the GB between these particles is "dry", this case is called the incomplete wetting. If the GB between the Nd-rich particles contains the thin layer of a Nd-rich phase (either metallic or oxide), this case is called the pseudo-incomplete (or pseudo-partial) GB wetting.