Synthesis of Nanocrystalline Nd16Fe76B8 by Mechanical Alloying and Heat Treatment

Abstract

The synthesis route for Nd 2 Fe 14 B by mechanical alloying (MA) and subsequent heat treatments was investigated. Elemental Nd, Fe and B powders were mechanically alloyed under Ar and two-stage H2/Ar atmospheres, using a high-energy magneto-mill equipped with rare-earth magnets for the controlled movement of 1″ϕ steel balls. Hydrogen was found to solve the sticking problems of alloying elements via the formation and fracturing of brittle Nd-hydride in the process. Neodymium surrounded by H2 rapidly forms Nd 2 H 5 with excess of H content in the beginning of MA, and the product dehydrogenates slightly in the successive milling up to 200 h in Ar. In Ar-atmospheric MA, on the contrary, amorphous state first results, which changes to a nanocrystalline Nd-rich phase. In both cases, part of α-Fe remains unreacted even after prolonged milling. The lowering and broadening of x-ray peaks reveal that nano-crystallization and/or amorphization proceeded in MA of the elemental powders. In subsequent annealing of as-milled powders, Nd 2 Fe 14 B phase starts to form at 500°C and at 700°C in Ar- and H2/Ar-milling, respectively. The Nd-hydride is thought to form passive laminates among the reactants and inhibit Fe-Nd counter-diffusion.