Improvement in coercivity,thermal stability,and corrosion resistance of sintered Nd-Fe-B magnets with Dy₈₀Ga₂₀ intergranular addition

To investigate the coercivity, corrosion resistance, and thermal stability of Nd-Fe-B magnets, their properties were investigated at room and high temperature before and after doping with Dy₈₀Ga₂₀ (at.%) powder. The coercivity of the magnets increased from the undoped value of 12.72 kOe to a doped value of 21.44 kOe. A micro-structural analysis indicates that a well-developed core-shell structure forms in the magnets doped with Dy₈₀Ga₂₀ powder. The improvement in magnetic properties is believed to be related to the refined and uniform matrix grains, continuous grain boundaries, and a hardened (Nd, Dy)₂Fe₁₄B shell surrounding the matrix grains. Additionally, the doped magnets exhibit an obvious improvement in thermal stability. For the magnets with added Dy₈₀Ga₂₀ powder, the temperature coefficients of remanence (α) and coercivity (β) increased to -0.106%℃^-1 and -0.60%℃^-1 over the range 20-100℃, compared to temperature coefficients of -0.117%℃^-1 (α) and -0.74%℃^-1 (β) in the regular magnets without Dy₈₀Ga₂₀ powder. The irreversible loss of magnetic flux (Hirr) was investigated at different temperatures. After being exposed to 150℃ for 2 h, the Hirr of magnets with 4 wt.% Dy₈₀Ga₂₀ decreased by ~95% compared to that of the undoped magnets. The enhanced temperature coefficients and Hirr indicate improved thermal stability in the doped Nd-Fe-B magnets. The intergranular addition of Dy₈₀Ga₂₀ also improved the corrosion resistance of the magnets because of the enhanced intergranular phase. In a corrosive atmosphere for 96 h, the mass loss of the sintered magnets with 4 wt.% Dy₈₀Ga₂₀ was 2.68 mg/cm², less than 10% of that suffered by the undoped magnets (28.1 mg/cm²).