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

The magnetic properties of HDDR (hydrogenation disproportionation desorption recombination) treated Nd_12.2Fe_81.8B_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, _jH_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.