Crystallographic alignment and magnetic anisotropy inducement from as-deformed disproportionated Nd(Fe,Co)B alloy with different deformation ratios

Abstract

The process of cold deformation and desorption-recombination vacuum annealing was proposed to produce anisotropic nanocrystalline Nd(Fe,Co)B magnet by using as-milled disproportionated nano-structured alloy powders as the precursor material. The effect of deformation ratio on crystal alignment and magnetic anisotropy was experimentally studied using X-ray diffraction and transmission electron microscopy, and the underlying mechanisms were clarified by referring to correlation of critical driving energy with nucleation activation energy for the reaction. Cold deformation plays an essential role in the formation of anisotropic magnet via the induction of desired texture by vacuum annealing treatment, composed of oriented Nd2(Fe,Co)14B nano-crystals with well defined c-axis alignment, maintaining a coherency relationship with as-deformed disproportionated α-(Fe,Co) precursor described by [110]α-(Fe,Co)∥[001] Nd2(Fe,Co)14B. The formation of [00l] Nd2(Fe,Co)14B orientation stems from preferential nucleation and growth of Nd2(Fe,Co)14B crystals induced by (110) texture of α-(Fe,Co) phase during desorption-recombination process. Increasing the deformation ratio results in the improvement of remanence and energy product, but moderate loss in coercivity, and the former reason can be ascribed to stronger crystallographic texture along [00l] direction (c-axis alignment), while the latter is related to the absence of intergranular phase, or stray field and weaker exchange coupling caused by grain coarsening, which was further elaborated by magnetization behavior study. The optimum magnetic properties of (BH)max: 140.91 kJ/m3, Hci: 650 kA/m and Br: 0.94 T were achieved by vacuum annealing as-deformed samples with deformation ratio of 70% at 780 °C for 30 min, due to pronounced anisotropy and a fully recombined microstructure with uniform grains of about 45 nm in average size. The evident anisotropy and enhanced remanence of nano-grained magnet prove advantageous for the fabrication of textured nanocrystalline permanent magnet.