Temperature-dependent evolution of REFe2 phase and correlated coercivity responses in post-sinter annealed Nd–Ce–Fe–B magnets

Abstract

Control of the REFe₂ (RE = rare earth) intergranular phase is of vital importance for developing Ce-rich Nd–Ce–Fe–B magnets that can rival the performance of conventional Nd–Fe–B magnets. Here we systematically investigate the evolution of the REFe₂ phase in a multi-main-phase Ce_12.8(Pr, Nd)_19.2Fe_65.33M_balB_0.9 magnet (Ce/total RE = 40 wt. %) triggered by wide-range post-sinter annealing (300–950 °C). Following a scan of the temperature dependence of coercivity, two annealing temperatures of 420 °C and 650 °C yield higher coercivity, but via different mechanisms. 420 °C annealing facilitates the formation of the REFe₂ phase, and partially thickened grain boundary (GB). 650 °C annealing results in the decomposition of the REFe₂ phase, and its re-formation during furnace cooling. In fact, comparing cooling from 650 °C via water-quenching, air-cooling, and furnace-cooling demonstrates that the latter (slowest) cooling rate yields the maximum REFe₂ phase fraction of 5.4 wt.%. Significantly, the formation of this REFe₂ phase is accompanied by the expulsion of Ce from RE₂Fe₁₄B matrix shell, and the concomitant infiltration of Nd into the RE₂Fe₁₄B matrix shell. As a result, a maximum compositional difference between Nd and Ce in the RE₂Fe₁₄B matrix shell is recorded in the 650 °C furnace-cooled sample ([Nd]_shell – [Ce]_shell = 7.1 wt.%), yielding a maximum coercivity of 12.6 kOe. This work unveils the temperature-dependent evolution of the REFe₂ phase in Nd–Ce–Fe–B sintered magnets, and the nature and kinetics of the Ce and Nd redistribution.