L Journal of Alloys and Compounds 315 (2001) 243–250 www.elsevier.com / locate / jallcom NdDyFeBZr high-coercivity powders prepared by intensive milling and the HDDR process a, a a a a b b * ¨ A. Bollero , B. Gebel , O. Gutfleisch , K.-H. Muller , L. Schultz , P.J. McGuiness , G. Drazic , b S. Kobe a ¨ IFW Dresden, Institut f ur Metallische Werkstoffe, Postfach 270016, D-01171 Dresden, Germany b Jozef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia Received 18 August 2000; accepted 19 October 2000 Abstract A comparative study of the processing of highly coercive, isotropic NdFeB-type magnet powders was carried out using two different techniques. Nd Fe B alloys with additions of Dy (1 at.%) and Zr (0.1, 0.3 and 0.7 at.%) were processed by: (a) a combination of 15 77 8 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 ZrB lamellar phase was observed using transmission electron microscopy; this phase appears to have a grain-growth- 2 inhibiting effect during HDDR processing of NdDyFeBZr materials. A maximum coercivity of 1.9 T was obtained for the HDDR- processed material. At recombination temperatures above 8008C explosive grain growth was observed using Kerr microscopy.  2001 Elsevier Science B.V. All rights reserved. Keywords: Rare earth alloys; Transition metal alloys; Permanent magnets; Hydrogen absorbing material; Magnetic measurements; Mechanical alloying 1. Introduction processing NdFeB alloys into highly coercive powders [1]. In this study, intensive milling (where the as-cast alloy is From 1983, when high-performance Nd Fe B-type used as the starting material, unlike mechanical alloying 2 14 magnets were successfully prepared in Japan and the USA, which begins with the constituent elements) and the HDDR the production of rare-earth (RE) magnets, and NdFeB- process have been used. type magnets in particular, has seen a spectacular growth. Intensive milling leads to an amorphous or nanocrystal- The wide range of applications for these magnets, from line alloy powder which must be subsequently annealed to everyday applications like loudspeakers to the high-tech of reform the Nd Fe B hard magnetic phase [2]. The 2 14 the aerospace industry, and new areas of application like resulting magnetically isotropic particles have a very fine magnetic resonance imaging (MRI), have fueled this microstructure comparable to that obtained by melt-spin- increase in production. The search for optimized com- ning. The as-milled powders can also be used as a positions and improved processing techniques, which has precursor for hot pressing and subsequent die upsetting [3]. lead to improved properties and low-cost NdFeB-type The HDDR process applied to alloys consisting mainly magnets, have been the challenges facing researchers over of the Nd Fe B phase begins with the hydrogen decrepi- 2 14 recent years. Melt-spinning, mechanical alloying and the tation of the alloy as a result of the absorption of hydrogen hydrogenation-disproportionation-desorption-recombina- at room temperature [4]. A subsequent heating of the tion (HDDR) process are three different methods of material at 8008C under 1 bar hydrogen pressure leads to the disproportionation of the Nd Fe B phase into a very 2 14 fine mixture of neodymium hydride, iron and ferroboron according to the reaction: *Corresponding author. Tel.: 149-351-4659-669; fax: 149-351-4659- 537. E-mail address: a.bollero@ifw-dresden.de (A. Bollero). Nd Fe B 1 (2 1 d )H ⇔2NdH 1 12Fe 1 Fe B 1DH 2 14 2 21d 2 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388(00)01290-1