Anisotropy Mechanism in HDDR Processed NdFeB

Abstract

The Hydrogenation Disproportionation Desorption Recombination (HDDR) process can yield anisotropic and highly coercive NdFeB-type powders with energy densities in excess of (BH)_max=340 kJ/m³. The elucidation of the very unusual phenomenon of texture inducement is of great scientific and technological interest. Here, Nd_12.5Fe_ba1B_6.3 and Nd_12.5Fe_ba1Ga_0.3Nb_0.3B_6.3 alloys have been HDDR processed and hydrogen partial pressures and dwell times during disproportionation and recombination have been varied. The effect of these processing parameters on the microstructural and magnetic properties is detailed. Intermediate processing stages were characterized by XRD (Rietveld analysis), field emission gun scanning electron microscope (FEGSEM), transmission electron microscopy (TEM) using EDX, selected area diffraction and convergent beam electron diffraction modes. The degree of texture in aligned recombined powders has been characterized by magnetic measurements using a VSM and qualitatively by Kerr microscopy. The paper describes the mechanism of texture generation in the recombined powder. Crystallographic relationships between parent, disproportionated and recombined phases are described. It is suggested that it is the ironboride phase which acts as the anisotropy-mediating phase throughout the different stages of the process. The nucleation and growth of this phase is controlled by the hydrogen partial pressure during exothermic disproportionation, strongly effecting the final degree of texture. This d-HDDR (dynamic-) process allows to maximize texture without the Co addition. It is proposed that Nb is useful for the stabilization of the boride phase whereas Ga is beneficial during initial recombination.