NdDyFeBZr high-coercivity powders prepared by intensive milling and the HDDR process

A comparative study of tilt: processing of highly coercive, isotropic NdFeB-type magnet powders was carried out using two different techniques, Nd15Fe77B8 alloys with additions of Dy (1 at.%) and Zr (0.1, 0.3 and 0.7 at.%) were processed by: (a) a combination of intensive milling-and-annealing and (b) the hydrogenation-disproportination-desorption-recombination (HDDR) process. A marked retarding effect on grain growth due to the Zr addition was observed. intensively milled and annealed samples containing both Zr and Dy additions showed a large improvement in coercivity in comparison with the additive-free alloys, reaching an optimum value of 2.5 T for 0.1 at.% Zr and 1.0 at.% Dy. The improved microstructure resulted in a wide processing window in which high-coercivity material could be produced. A ZrB2, lamellar phase was observed using transmission electron microscopy; this phase appears to have a grain-growth-inhibiting effect during HDDR processing of NdDyFeBZr materials. A maximum coercivity of 1.9 T was obtained fur the HDDR-processed material. At recombination temperatures above 800 degreesC explosive grain growth was observed using Kerr microscopy. (C) 2001 Elsevier Science B.V, All rights reserved.