The effects of Nb addition on magnetic property of Nd10.23Pr2.56Fe68.32Co13.08B5.81 ribbons through modification of the intergranular phase and grain refinement

The (Nd10.23Pr2.56Fe68.32Co13.08B5.81)100-xNbx (x = 0, 0.5, 1, 2, 3) ribbons were prepared by melt-spinning at a wheel speed of 15 m/s. The effects of Nb addition on the microstructure and the magnetic properties of the ribbons were investigated. The X-ray diffraction (XRD) patterns of the ribbons show that the Nd2(Fe, Co)14B (2:14:1) phase is observed in all samples indicating that Nb addition does not change the phase constitution. The transmission electron microscopy (TEM) images show that Nb addition can dramatically reduce the average grain size from 52 ± 5 nm of the Nb-free alloy to 12 ± 2 nm of the Nb-containing x = 3 alloy. Based on the atom probe tomography (APT) result, we identify different mechanisms for grain refinement at different Nb content. When x = 0.5, the distribution of Nb is uniform at the intergranular phase and Nb addition promotes the segregation of rare-earth Nd and Pr at the thick ferromagnetic intergranular phase (≈ 10 nm), which leads to decreasing the average grain size to 38 nm. With further increasing Nb addition to x = 2, the existence of Nb-rich precipitations (2–4 nm in width) at the grain boundaries effectively reduces the grain size to 16 nm. The remanence (Br) and the maximum energy product ((BH)max) are obviously increased by Nb addition and the maximum of the Br and the (BH)max is obtained in the (Nd10.23Pr2.56Fe68.32Co13.08B5.81)98Nb2 (x = 2) alloy. The values of the Br and the (BH)max are 0.90 T and 138 kJ/m3, 8.4% and 22.1% higher than that of the Nb-free alloy. It ascribes to the grain refinement, the changes of the distribution and the composition of the ferromagnetic intergranular phase by Nb addition. Our findings provide a new method of designing prospective permanent alloys with increased magnetic properties by reducing the grain size and tuning the distribution and chemical composition of the intergranular phase.