Effects of gadolinium and silicon substitution on magnetic properties and microstructure of Nd–Fe–B–Nb bulk nanocomposite magnets

The magnetic properties, phase evolution and microstructure of Fe70−xMxB19Nd7Nb4 (M=Si, Gd, Si+Gd; x=0–2.5 at%) bulk nanocomposite permanent magnets in the form of rods produced by annealing the amorphous precursor have been investigated systematically. Microstructural examination, three-dimensional atom probe microanalysis, δM-plots, X-ray diffraction analysis and magnetometer studies deduced that good magnetic properties in the magnets originate from the homogenous microstructure consisting of exchange coupled, soft magnetic (α-Fe, Fe3B) and hard magnetic (Nd,Gd)2Fe14B nanophases. Optimally annealed Fe70B19Nd7Nb4 rod magnets exhibit magnetic properties of Br=0.61T, iHc=876kA/m and (BH)max=50.2kJ/m3. Gadolinium and silicon addition to quaternary Fe70B19Nd7Nb4 alloy increased the mass fraction of hard magnetic phase, strengthened the exchange coupling interactions and enhanced the magnetic properties. Gadolinium and silicon segregated into hard magnetic phase which led to enhance coercivity up to 1115kA/m. Enhancement in the coercivity is mainly resulted by hard phase increment as well as domain wall pinning, while strengthening of exchange coupling is caused by grain size refinement and increase in Curie temperature of the magnetic phases. The Fe67B19Nd7Gd2Nb4Si1 magnetic rods of 1.2mm in diameter demonstrated the best magnetic properties such as intrinsic coercivity, iHc of 1115kA/m, remanence, Br of 0.57T and maximum energy product, (BH)max of 65.7kJ/m3. •Magnetic and microstructural properties of Fe70−xMxB19Nd7Nb4 (M=Si, Gd, Si+Gd; x=0–2.5 at%) magnets are studied.•Amorphous rods of 1.2mm in diameter are synthesized by injection casting.•Magnetic properties are improved by substitution of Gd and Si.•Magnet exhibits magnetic properties as: jHc of 1115kOe, Br of 0.57T, (BH)max of 65.70kJ/m3.