A metallurgical approach toward alloying in rare-earth permanent magnet systems

摘要

A metallurgical approach was developed toward alloying in rare earth permanent magnet systems to allow for microstructural enhancement and control during solidification and subsequent processing. Compound additions of Group IVA, VA, or VIA transition metals (TM) along with carbon were added to the Nd[subscript]2Fe[subscript]14B system (2-14-1). Transition metal carbides will form in the quintary Nd-Fe-B-TM-C system if the phase stability of the precipitates in the specific multicomponent system is higher than the phase stability of all other phases involving the additive elements and the constituent elements. Transition metal carbide formation was found in the Group IVA (TiC, ZrC, and HfC) and Group VA systems (VC, NbC and TaC). Transition metal carbide precipitates can form at high temperatures in the liquid, during cooling after solidification, or during an appropriate heat treatment. Carbide formation did not occur in the Group VIA system;The alloying ability of each transition metal carbide system was graded using criteria which dealt with phase stability, liquid and equilibrium solid solubility, and high temperature carbide stability. Titanium with carbon additions satisfied all of the proposed alloying criteria and were chosen as the best system for further study. Titanium and carbon have a significant liquid solubility and an equilibrium solid solubility which was extremely low and below detectable limits. No equilibrium solid solubility means that the titanium and carbon additions will ultimately form TiC after an appropriate heat treatment which allows the development of a composite microstructure consisting of the 2-14-1 phase and TiC. Thus, the excellent intrinsic magnetic properties of the 2-14-1 phase remain unaltered and the extrinsic properties relating to the microstructure are enhanced due to the TiC stabilized microstructure which was found to be much more resistant to grain growth;When titanium with carbon are dissolved in the liquid melt or solid phases, such as the glass or the 2-14-1 phase, the intrinsic properties of the phases are changed. Favorable intrinsic changes include; large increases in glass forming ability, a significant reduction in the optimum cooling rate, an increase in the optimum energy product, and an enhancement in the nucleation kinetics of crystallization.