Effect of copper and titanium addition on microstructures and magnetic properties of sintered Nd₂₂Fe₇₁B₇ magnets

Alloying elements Cu and Ti were added to the intergranular regions of sintered NbFeB magnets and their effects on microstructure and magnetic properties were investigated. The results show that small amounts of Cu/Ti additives can enhance the coercivity greatly and have a little effect on the remanence of the magnets, while the Ti content is less than 1.2%. The improvement of the coercivity can be attributed to the segregation of Cu and Ti on the surface of main phase, which inhibits the growth of main phase grains during sintering. At the same time, the segregation of Cu and Ti can also prevent the magnetic coupling of Nd₂Fe₁₄B grains to a certain degree and impede effectively the propagation of reversed domain walls through the magnetic phase grains. As the Ti content increases and exceeds 1.2%, a strip-like Ti-rich phase appears in the intergranular region, resulting in the dramatic reduction of the remanence of NdFeB magnets. Compared with the single alloying method, the combined alloying of intergranular regions is more efficient to modify the properties of NbFeB magnets.