Preparation, microstructure, and magnetic properties of a nanocrystalline Nd12Fe82B6 alloy by HDDR combined with mechanical milling

Nanocrystalline Nd12Fe82B6 (atomic ratio) alloy powders with Nd2Fe14B/α-Fe two-phase structure were prepared by HDDR combined with mechanical milling. The as-cast Nd12Fe82B6 alloy was disproportionated via ball milling in hydrogen, and desorption–recombination was then performed. The phase and structural change due to both the milling in hydrogen and the subsequent desorption–recombination treatment was characterized by X-ray diffraction (XRD). The desorption–recombination behavior of the as-disproportionated alloy was investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The morphology and microstructure of the final alloy powders subject to desorption–recombination treatment were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results showed that, by milling in hydrogen for 20 h, the matrix Nd2Fe14B phase of the alloy was fully disproportionated into a nano-structured mixture of Nd2H5, Fe2B, and α-Fe phases with average size of about 8 nm, and that a subsequent desorption–recombination treatment at 760 °C for 30 min led to the formation of Nd2Fe14B/α-Fe two-phase nanocomposite powders with average crystallite size of 30 nm. The remanence Br, coercivity Hc, and maximum energy product (BH)max of such nanocrystalline Nd12Fe82B6 alloy powders achieved 0.73 T, 610 kA/m, and 110.8 kJ/m3, respectively.