Effect of the disproportionation and recombination stages of the HDDR process on the inducement of anisotropy in Nd–Fe–B magnets

Abstract

Abstract The magnetic properties of HDDR (hydrogenation disproportionation desorption recombination) treated Nd 12.2 Fe 81.8 B 6.0 alloys were investigated. Two different patterns were used for the disproportionation stage: (i) the alloys were heated to a certain processing temperature between 850–950°C in 0.1 MPa of hydrogen (conventional hydrogen treatment: c - HD treatment), and (ii) the alloys were heated under vacuum and hydrogen was only admitted when the processing temperature had been reached ( ν - HD treatment). The alloys were then held at the processing temperature for 1 or 2 h under hydrogen in order to cause complete disproportionation. Either an argon heat treatment, or a hydrogen heat treatment at various constant pressures below 0.1 MPa, was then used to control the hydrogen pressure during the recombination stage ( s - DR treatment), followed by the usual heat treatment in vacuum (conventional heat treatment in vacuum: c - DR treatment) to cause complete recombination. It was found that the magnetic properties of the ν -HD powders were more sensitive to the s -DR treatment time than those of the c -HD powders, which is thought to be related to differences in microstructure observed in the disproportionated state. The best magnetic properties were obtained for a ν -HD powder s -DR treated at 950°C for 20 min: B r =1.4 T, j H c =385 kAm −1 , B r / J s =0.92. It can be said that the inducement of anisotropy is influenced by the hydrogenation and desorption stages of the HDDR process, and that the combination of ν -HD and s -DR treatment can be an effective method of inducing anisotropy in Nd–Fe–B powders.