Modified HDDR procedures applied to NdFeB alloys

The magnetic properties of Nd_16.2Fe_78.2B_5.6, Nd_12.6Fe _81.4B_6.0 and Nd_12.6Fe_69.4Co _11.0Ga_1.0B_6.0 alloys after the application of modified HDDR (hydrogenation-disproportionation-desorption-recombination) procedures have been investigated and particular emphasis has been given to the development of magnetic anisotropy and high coercivity in these different types of materials. Conventional HDDR processing leads to anisotropic material only in the case of the Nd_12.6Fe_69.4Co_11.0Ga_1.0B _6.0 alloy. Combining solid-disproportionation with a controlled recombination under partial hydrogen pressure at 900°C, a significant degree of anisotropy and a good coercivity were achieved for the two ternary alloys. These findings indicate the possibility of more than one mechanism and the relevance of the rod-like solid-disproportionated structure for the inducement of anisotropy. Another modification of the standard procedure, the reactive milling technique, leads to isotropic Nd_16.2Fe_78.2B_5.6 powder with grain sizes, both in the disproportionated and recombined states, significantly smaller than those of conventional powder and a very high coercivity of μ_0JH_c=1.88T