Memory of texture during HDDR Processing of NdFeB

The hydrogenation disproportionation desorption recombination (HDDR) process can yield highly anisotropic and 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_balB_6.3 and Nd_12.5Fe_balGa_0.3Nb_0.3B_6.3 alloys have been processed and hydrogen partial pressures and dwell times during disproportionation and recombination have been varied systematically. The effect of these processing parameters on the microstructural and magnetic properties is described. Intermediate processing stages were characterized by Rietveld analysis, field emission gun scanning electron microscopy, transmission electron microscopy using EDX and diffraction modes. Crystallographic relationships between parent, disproportionated and recombined phases are described and it is suggested that the iron boride phase could act as the anisotropy-mediating phase throughout the different stages of the process. The nucleation and growth of the various phases are controlled by the hydrogen partial pressure during the exothermic disproportionation, thereby strongly effecting the final degree of texture. This d-HDDR (dynamic-) process yields a highly stable memory carrier, thus, allowing to maximize texture without Co addition. It is proposed that Nb is useful for the stabilization of the boride phase, whereas Ga is beneficial during initial recombination.