Effect of Milling Conditions on the Microstructure and Interphase Exchange Coupling of Nd2Fe14B/α-Fe Nanocomposites

The effects of milling conditions on the microstructure and interphase exchange coupling of Nd2Fe14B+10 wt% α-Fe nanocomposites were investigated. The α-Fe crystallite size is critical for obtaining an efficient interphase exchange coupling. The problem of damaging the Nd2Fe14B crystal structure during milling was addressed by using different milling conditions. Nd2Fe14B+ 10 wt% α-Fe powder samples were prepared through mechanical milling for 6h with Ø = 10mm and Ø = 15mm balls respectively. The restoration of the Nd2Fe14B crystal structure after annealing was confirmed by XRD, with a limited growth of α-Fe crystallites. The magnetic behavior was investigated from hysteresis curves and dM/dH vs. H plots. The samples milled with small balls show a good interphase exchange coupling, however, milling with larger diameter balls determined higher coercivities due to the reduced damaging of the Nd2Fe14B crystal structure during milling. The best exchange coupling was obtained for the samples annealed at 750°C for 1.5minutes, with a maximum coercive field of about 0.65 T. The Nd2Fe14B /α-Fe exchange coupling was analyzed as a function of milling and annealing conditions.