Intergranular phase transformation in post-sinter annealed Nd–Dy–Fe–Cu–Ga–B magnet: From Ia3¯-cubic to I4/mcm-tetragonal structure

Abstract

High-coercivity Nd–Fe–B permanent magnets crucially depend on the deliberate modulation of intergranular phases, notably exemplified by the I4/mcm-tetragonal Nd₆Fe₁₃Ga intergranular phase in the Nd–Fe–Ga–B magnet. Particularly, for the Nd–Dy–Fe–Cu–Ga–B magnet containing multiple rare earths (RE) and alloying metals (M), understanding the evolution of RE₆(Fe,M)₁₄ intergranular phase becomes more critical in the quest for higher coercivity. Here we design the (Nd,Pr)_29.0Dy_3.0Fe_balCu_0.5Ga_0.5B_0.9N_1.15 (NCo, Al, Zr, wt.%) as-sintered magnets, where the major RE/Cu/Ga-rich Ia$\overline{3}$-cubic intergranular phase is agglomerated in triple junctions. These pristine as-sintered magnets are subjected to annealing over a wide temperature range (390∼900 °C for 3 h) and quenching over a wide time range (0.5∼12 h at 460 °C). Through systematic microstructural characterization and first-principle calculation, the intergranular phase transformation from RE/Cu/Ga-rich Ia$\overline{3}$-cubic to Fe/Ga-rich I4/mcm-tetragonal structure, and accompanying elemental migration is unveiled. During post-sinter annealing, metastable state I firstly occurs, consisting of nanostructured RE/Cu-rich Ia$\overline{3}$-cubic and I4/mcm-tetragonal lamellas, with the emergence of multi-twins and coherent interface. Then it evolves into metastable state II, consisting of lath-shaped Fe/Cu/Ga-rich I4/mcm-tetragonal structure with fluctuating Fe/Cu concentrations. Simultaneously, metastable state III occurs, exhibiting P6-hexagonal platelets with lower Cu content and reduced crystallographic symmetry. Finally, heightened Fe/Ga diffusion into the lattice of tetragonal phase with synchronous Cu discharge generates the thermodynamically more stable Fe/Ga-rich RE₆Fe₁₃Ga phase. The implication of phase transformation pathways on the coercivity is discussed, offering valuable insights into the optimization of RE₆(Fe,M)₁₄ intergranular phase and providing further opportunities for enhanced coercivity.