HDDR behaviour of Nb-containing NdFeB near-stoichiometric alloys

Abstract

The hydrogenation, disproportionation, desorption and recombination (HDDR) behaviours of Nd 11.8Fe 82.3− x Nb x B 5.9 ( x = 0, 1, 2, 3, 5 at.%) alloys were studied by differential thermal analysis. Microstructural and hydrogen absorption/desorption studies were conducted prior to, during and after the HDDR process in order to establish the relationships between the initial microstructure and the changes in the HDDR behaviour and to compare with NdFeB-type alloys without additions. Niobium additions resulted in the formation of Nb 26Fe 32B 42 and Nd 2Fe 17. The area of the disproportionation peak was found to diminish with increasing Nb content in the alloy; this was ascribed to the increasing amounts of Nd-rich material and Nb 26Fe 32B 42 phase which did not disproportionate. The presence of Nd-rich material was indicated by the hydrogen desorption behaviour of the disproportionated alloys. The HDDR behaviour of Nd 11.8Fe 82.3− x Nb x B 5.9 ( x = 2, 5) alloys was investigated using a high resolution scanning electron microscope. The disproportionation of the Nd 2Fe 14B (Φ) matrix phase starts at the grain boundary and at the interface of the Nb 26Fe 32B 42-Φ phases, resulting in the formation of a submicrometre structure consisting of Fe, Fe 2B and Nd-hydride. The HDDR process proved to be a very rapid and efficient method of homogenising the as-cast alloys.