The influence of free iron on the hydrogen decrepitation capability of some Nd(Pr)–Fe–B alloys

Abstract

Hydrogen decrepitation (HD) followed by jet-milling is now well established as an economic and convenient powder preparation route for the production of sintered Nd–Fe–B based magnets. However, there is often residue in the H-decrepitated and milled material due to the presence of α-iron in the microstructure of the as-cast alloys. The residue is separated from the rest of the material by sieving the powder in order to attain a powder with a suitable average particle size. Thus, the extent of this residue has an overall impact on the total cost of a sintered magnet and it could also affect the final composition of the sintered magnets. In this paper, the influence of α-iron on the amount of the residue in the H-decrepitated and roller milled powder in some Nd(Pr)–Fe–B based alloys has been investigated. The ductile free iron, particularly in the form of large dendrites, was found to act as an obstacle to the propagation of micro-cracks generated by the HD process and was therefore the main cause of the residue. Investigation of a number of as-cast Nd–Fe–B alloys revealed that, with increasing Nd content in these alloys, the size and amount of the α-iron in the as-cast state was reduced. This resulted in a pronounced reduction in the amount of the residue after a standard HD and milling process. In a Nd 13Fe 80.5B 6.5 or a Pr 16.9Fe 79.1B 4-type alloy, the amount of residue was found to be considerable after the standard HD and milling treatment. Homogenising the alloys at 1050°C for different times resulted in the eventual disappearance of the residue after the standard HD and milling treatment. All the residues were found to be very stable in air at room temperature, but producing a clean polished surface in any of these particles, even under dry conditions, resulted in much increased reactivity. Possible means of avoiding α-Fe in the initial alloys have also been explored.