THERMODYNAMIC, KINETIC AND STRUCTURAL MECHANISMS CONTROLLING THE FORMATION OF NANOCRYSTALLINE ND-FE-B MATERIALS

Two off-stoichiometric Nd-Fe-B alloys near the intermetallic Nd2Fe14B have been investigated. Their nominal compositions are Nd13Fe79B8 and Nd16Fe76B8. The decomposition of fully amorphous melt-spun alloys under isothermal and continuous heating has been studied by X-ray diffraction, electron microscopy and differential scanning calorimetry. The main mechanisms that control the microstructure and magnetic hardening in the alloys are quite different. For amorphous Ndl3Fe79B8 nanocrystalline crystallization products with optimized coercivity require heat treatments at high heating rates (200 K min-1 or more) up to temperatures of about 1000 K. For amorphous Nd16Fe76B, full nanocrystalline crystallization products are obtained at about 850 K, but further annealing at temperatures above 875 K leads to Nd2Fe14B grains completely surrounded by an Nd film. The mechanisms that control the final microstructure are explained.