Phase and structural changes of Nd12Fe82B6 alloy during mechanical milling in both an argon and a hydrogen atmosphere

Nd12Fe82B6 alloy powders were mechanically milled for various times up to 20 h in both an argon and a hydrogen atmosphere. The phase and structural changes of the alloy powders during milling were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) analyses. The results show that mechanical milling of the Nd12Fe82B6 alloy in hydrogen leads to disproportionation of the Nd2Fe14B phase in the alloy into H5Nd2, Fe2B, and α-Fe phases. These phases are of nano-structure, with an average crystallite size of below 20 nm. After milling in hydrogen for 20 h, the Nd12Fe82B6 alloy powders are fully decomposed, and the average crystallite size of the resulting phases are decreased to not more than 10 nm. In comparison, no phase transition takes place when milling is performed in argon, but the crystallite size of both the Nd2Fe14B and the α-Fe phase decreases steadily with increasing milling time. After milling in argon for 20 h, the average crystallite size is decreased to about 20 nm. Further, the Nd12Fe82B6 alloy powders milled in hydrogen have a smaller average particle size than those milled in argon, attributed to the hydrogen-induced decrepitation and embrittlement of the Nd12Fe82B6 alloy powders upon hydrogenation during the early stage of milling in hydrogen.