Abstract
Nanocrystalline Fe-Nd-B alloys composed of nanoscale magnetically soft and hard phases attracted much attention as a consequence of enhancement in saturation magnetization and remanence ratio, and a theoretical energy product capable of approximately 1 MJ/m3.Amorphous flakes, based on ternary alloys with different ratios of Fe, Nd, and B were prepared by a single-roller melt-spinning technique at the wheel speed of 40 m/s in this study. Subsequent heat treatments were performed both by a rapid thermal annealing (RTA) process at a heating rate of 600℃/min and a conventional rapid annealing (CRA) process at a heating rate of 100℃/min in order to control the grain size. The results showed that a nanocomposite two-phase magnet alloy can be obtained either by RTA process or by CRA Process. Alloys with higher iron content respond much better to the RTA process which results in more reproducible magnetic properties, due to the high heating rate and short soaking time to avoid overgrowth of α-Fe grains.Alloying effect was also made in this study. The addition of extra transition metals, such as Ti, V, Zr, Nb, or/and Cu, to replace iron effectively refines the crystallized grams, and enhances the remanence except Zr addition, thus maximum energy product. For high iHc purpose, Ti or Zr addition is a good choice; for best Br and (BH)max combination, V or Nb addition is. From the mass production point of view, Nb or Cu+Nb addition exhibits similar magnetic properties in a very wide RTA temperature range (550~750℃).The exchange coupling and coercivity mechanism of these alloys were investigated by magnetic viscosity and δM-plots. The results showed that an overgrowth of soft phases results in larger nucleation field than intrinsic coercivity of RTA treated alloys.Ac initial susceptibility studies on toroids made of as-spun amorphous and RTA treated two-phase nanocomposite Fe88Nd6B6 alloy ribbons were carried out at 30-650 ℃. A new method to quantitatively estimate the amount of residual amorphous phase of heated ribbons has been deduced by the χac measurement. Furthermore, the full width at half maximum (FWHM) of χac peak corresponding to the Curie temperature of Nd2Fe14B phase was found to correlate quantitatively with the extent of exchange coupling interaction between the hard and soft nano-phases. The sum of FWHM and remanence ratio is almost a constant, 1.11 to 1.14 for the Fe88Nd6B and 1.02~1.05 for the Nd6Fe86CuNbB6 flake.The spin reorientation transition temperature of the Nd2Fe14B nano-phase in the two-phase nanocomposite Fe88Nd6B6 alloy shifts to 110K much lower than 135K for the Nd2Fe14B single crystal.