Crystallization kinetics of Ti-doped Nd₂Fe₁₄B/α-Fe nanocomposite permanent magnets

Abstract

Nanocomposite magnet consisting of a fine mixture of magnetically hard and soft phase has received much attention for potential permanent magnet development. One of the important requirements for alloys to exhibit excellent magnetic properties is a nanocrystalline grain size. The soft and hard magnetic phases can simultaneously achieve ideal nanoscale composites. The effect of Ti additions in the amorphous crystallization process of the exchange-coupled nanocomposite Nd₂Fe₁₄B/α-Fe magnet prepared by melt spinning is investigated. The results show that Ti can change the crystallization kinetics of the NdFeB melt-spun ribbons. The Ti can increase the activation energy of α-Fe and contrarily reduce the activation energy of a metastable 1∶7 phase, so the growth speed of α-Fe decreases and the metastable 1∶7 phase can stably precipitate from the amorphous phase. When the annealing temperature increases, a metastable 1∶7 phase is decomposed into the α-Fe phase and the Nd₂Fe₁₄B phase. The microstructure observation shows that the grains of the alloys doped with Ti are fine and uniform, with an average grain size of about 20 nm, and no particularly large α-Fe particles appear. The optimal magnetic property is (<i>BH</i>)_max = 12 MG·Oe (1 G = 10^–4 T, 1 Oe = 79.57795 A/m) when Ti addition is 1.0%.