Effect of boron content on the microstructure and magnetic properties of Ga-doped sintered Nd–Fe–B magnets

High Ga-doped heavy rare earth free sintered Nd‒Fe‒B magnets with the different boron contents (mass fraction) were prepared by powder metallurgy method in this work. The effect of boron content on phase formation, microstructure, and magnetic properties of the magnets was investigated. Results show that the coercivity of the Ga-doped sintered Nd‒Fe‒B magnets decreases as the boron content increases from 0.90% to 1.00%. The magnetic properties deteriorate drastically when the boron content exceeds 0.94%. Microstructure studies show that the significant amount of Nd₆Fe₁₃Ga phases are present and the grain boundary broadens when the boron content is low. The increase of boron content inhibits the formation of Nd₆Fe₁₃Ga phase, which leads to the transformation of grain boundary from non-ferromagnetic to ferromagnetic and the volume fraction decrease of the grain boundary phase. Further micromagnetics simulation confirms that the construction of the non-ferromagnetic grain boundary phase and the wider grain boundary phase can reduce the exchange-coupling interaction between the Nd₂Fe₁₄B main phase grains, which is critical to ensure the high coercivity of the magnets.