Effect of post-sinter annealing on structure and coercivity of (Zr, Ti)-doped Nd-Ce-Fe-B magnets

Abstract

Post-sinter annealing effects on the structure and coercivity have been investigated for the (Zr, Ti)-doped Nd-Ce-Fe-B sintered magnets. In them, an in-situ formed Zr-rich Zr-Ti phase with the strip shape is found to be beneficial for the coercivity Hcj, because its distribution along grain boundaries (GBs) after annealing at 700℃ can block the magnetic domain within matrix phases passing through the adjacent RE2Fe14B grains. It, coupled with the optimized distribution of RE-rich phase, accounts for the 21.1 % increase of Hcj before and after annealing at 700 ℃. This temperature much higher than the melting-point (465 ℃) of RE-rich phase is the optimum annealing temperature, at which the RE-rich liquid phase is good for the distribution of Zr-Ti phase along GBs rather than at triple junction regions, optimizing GB microstructure. Such new findings deepen the understanding on microstructure evolutions of the GB modified Nd-Fe-B magnet during annealing and further provide a theoretical basis for the Hcj improvement.