Analytical TEM study of Al-doped, “two-phase” Nd–Fe–B sintered magnets

The microstructure and the coercivity of sintered Nd₂₀Fe_(73.5-x)B_6.5Al_x (x = 0, 2.5) permanent magnets are influenced by the Al concentration. In Al-containing magnets we found a homogeneous distribution of Al in the hard magnetic phase and the occurrence of an intergranular Nd(Fe, Al)₂, phase between the hard magnetic grains. Our analytical TEM study revealed that in “Al₂O₃-doped” magnets the crystal structure of the Nd-rich intergranular phase partly changes from fcc (a = 0.52 nm) to hep (a = 0.39 nm, c = 0.61 nm), whereas the content of iron simultaneously decreases from 5–10 at.% to 1.5–4 at.%. A platelet-shaped phase, embedded in the Nd-rich intergranular phase, was determined as a Nd₅Fe₂ (B, O)_x, phase. The influence of the microstructure on the coercivity in Al-doped magnets is discussed.