NdDyFeBZr - processing and properties

Cast alloys with a range of compositions based on Nd15Fe77B8 with additions of Dy and Zr were produced by a conventional casting method. Zr was added prior to casting in the form of either zirconium or zirconia. The materials were processed using both the hydrogen decrepitation, disproportionation, recombination (HDDR) technique and by mechanical milling in order to develop the fine microstructures necessary for high coercivities. Magnetic measurements on bonded samples produced from the HDDR powders indicated that additions of Zr tended not to affect the recombination temperatures at which the material began to develop high coercivity but was clearly observed to extend the temperature range, by up to 40 C, over which high coercivity material could be produced. Microstructural analysis has revealed that the Zr doped samples were less prone to the explosive grain growth exhibited by the Zr free samples at these temperatures resulting in a much enhanced processing window for these materials. Prehomogenisation of the cast samples prior to HDDR resulted in lower coercivities being obtained for the Zr free materials, where as for the Zr containing alloys broadly similar results to those obtained for cast alloys were obtained, particularly at higher processing temperatures. The mechanical milling experiments indicated that additions of Zr can lead to substantially enhanced coercivities, in comparison with Zr free materials, with careful compositional control. The optimum coercivity was obtained with Zr additions of 0.5% Long term corrosion studies using high humidity conditions carried out on fully dense hot pressed materials using HDDR powders have revealed that small additions of Zr to the melt are able to substantially increase the resistance to corrosion of the resulting magnets. (orig.)