Properties of Permanent Magnets of the Nd–Fe–B System Alloyed with Cu, Ti, C, Which Are Obtained by a Powder Method or Sintering of Films in High-Pressure Conditions

Abstract

The review presents a series of experimental investigations studying the influence of alloying Cu, C, Ti elements on structure, phase composition and magnetic properties of permanent Nd–Fe–B magnets, which are obtained by the powder method as well as using a compaction of the products quenched from a liquid state. The composition of alloy is Nd16Fe76−хB8Cuх (х=1–4 at.%). The coercive force Hci and flux linkage W of magnets increase to 1260 kA/m and 58.9 mWb, respectively, as copper content increases up to 2 at.%. Within the same concentration range, from 1 to 4 at.% of cooper, Br gradually decreases from 1.25 T to 1.15 T. Then, at the stage of orientation and compaction, carbon, titanium, and copper powders are added into the powders of original Fe76Nd16B8 alloy, namely, there are 0.1–0.2 at.% of C, 1.3 at.% of Ti, and 0.13 at.% of Cu. Due to mechanical mixing, compacting and sintering at 1373 K with the subsequent annealing at 923 K, the anisotropic magnets with Hci from 1260 kA/m to 1465 kA/m are obtained. In another method, the same alloys with addition of the C, Cu and Ti alloying elements, Nd15,2Fe75,4−хCхB6,7Cu1,3Ті1,3 (х=0,1–1,0 at.%), are subjected to alloying after the quenching from the liquid state. The quenching products are used for the fabrication of bonded magnets. It turned out that bonded magnets possess significant coercivity up to 1400 kA/m at low Вr=0.5 T. Then, to establish the influence of external pressure on structure and properties of sintered compacts, the films (flakes) are placed in a mould and compressed under the pressure of 0.5–12 MPa before sintering. The mould is fixed with bolts. In a compressed state, it is placed in a vacuum oven for sintering. Due to the difference of the coefficients of thermal expansion of bolts and the mould under the heating, the additional pressure on the sample is created and reaches 0.9 GPa, according to theoretical calculations. Decrease of the sintering temperature from 1323 K to 1013 K reduces the size of the NdCu2-type phase. The nanoscale (< 0.05 microns) particles of the NdCu2 phase are obtained. They prevent movement of the domain boundaries, which in turn increases the coercive force of sintered compacts from 200 to 1350 kA/m.

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