REPLACEMENT OF ND BY AN INTERMETALLIC PHASE IN THE INTERGRANULAR REGION OF FE-ND-B SINTERED MAGNETS

The microstructure of Fe-Nd-B based magnets consists of mainly Fe14Nd2B (phi) grains embedded in a eutectic matrix of mainly Nd plus some Fe4B41.1B4 (eta) and Fe-Nd phases stabilized by impurities. The addition of Al or Ga to Fe-Nd-B material leads to the formation of quite different ternary intergranular phases based on Fe, Nd and Al or Ga. The main phase stabilized by these additions is a phase (delta) with the composition Fe70-xNd30Tx (for T = Al: 8 less-than-or-equal-to x less-than-or-equal-to 25; for T = Ga: 3 less-than-or-equal-to x less-than-or-equal-to 8). In standard magnet compositions the pure Nd is still dominant in the intergranular region and causes corrosion problems due to high sensitivity to oxygen and humidity. By choosing an appropriate gross composition for the master alloy based on the knowledge of the phase relations, a new microstructure can be created consisting of phi and an intergranular Fe-Nd (Al or Ga) phase almost without the residual Nd phase. The magnetic parameters of this material are comparable to or even better than those of conventional Fe-Nd-B materials. The delta phase crystallizes peritectically at the surface of phi and leads to a better separation of the grains. The new two-phase material of two intermetallic phases shows better corrosion resistance combined with high coercivities of more than 15 kOe (1200 kA/m) after sintering without annealing, due to the formation mechanism of delta and the avoidance of Nd. Ga additions show better results than Al because of the very limited solubility of Ga in phi.