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

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Abstract
The microstructure and the coercivity of sintered Nd20Fe(73.5-x)B6.5Alx (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.