Sintering of Ferromagnetic Materials at Lower Temperatures in Hydrogen. II. Nd–Fe–B Alloys

Abstract

The use of the hydrogenation and disproportionation (HD) and desorption and recombination (DR) route (HDDR) for sintering Nd₂Fe₁₄B-based ferromagnetic alloys, such as Nd_11.7Fe_81.1Zr_1.2B₆ and Nd₁₆Fe_73.9Zr_2.1B₈, was studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The dependence between the production conditions—grinding of the alloys into powders, compaction pressure of the powders, hydrogen pressure and temperature at the first stage of sintering in hydrogen (HD), and temperature at the second stage of sintering in vacuum (DR)—and the porosity and microstructural particle size of the sintered materials was evaluated. The powders were ground in hydrogen in a planetary-ball mill at 200 rpm for 1 h and compacted at 2, 5, and 6 t/cm². The first sintering stage was carried out at a hydrogen pressure of 0.05 MPa and a temperature of 760°C, and the second stage at 850 and 950°C. The powders were found to sinter at the first stage. The porosity of the sintered materials decreased with increasing compaction pressure. The grain size of the ferromagnetic Nd_11.7Fe_81.1Zr_1.2B₆ phase in the sintered materials ranged from 100 to 300 nm. The physical mechanism behind the reduction in the sintering temperature was attributed to an increase in the diffusion rate of alloy components resulting from hydrogen-induced phase transformations, such as disproportionation and recombination, and to the presence of a hydrogen solid solution at both stages of the process, HD and DR. A very important aspect of this research is that the powders were sintered under low hydrogen pressure required to produce magnetically anisotropic materials. Problematic aspects of the properties shown by the sintered materials, particularly microstructural heterogeneity, were analyzed, and approaches to their solution, through homogenizing the particle size of the powders and optimizing the HDDR parameters (hydrogen pressure, temperature, reaction time), were proposed. The process advantages of the new sintering method compared to similar techniques included the temperature lower by more than 100°C, the potential for producing nanostructured anisotropic materials, and the use of technically simpler and cheaper sintering furnaces.