Effect of temperature on particle shape, size, and polycrystallinity of Nd-Fe-B powders obtained by hydrogen decrepitation

This work presents a detailed study of hydrogen decrepitation (HD) to obtain monocrystalline Nd-Fe-B powder. The effect of decrepitation temperature has been investigated to optimize both particle size and shape. Differential scanning calorimetry was applied to analyze the hydrogenation kinetics of Nd₂Fe₁₄B and Nd-rich phases in the range of 25 to 300 °C. Thermogravimetry and X-ray diffraction allowed determining the hydrogen absorption of the whole alloy and the matrix phase, respectively. While scanning electron microscopy (SEM) was used to visualize particle shape and size, dynamic image analysis was applied to evaluate quantitatively these properties. The high monocrystallinity of the powder was confirmed by electron backscattering diffraction. The partial pressure of hydrogen required to initiate the hydrogenation reactions decreases when the temperature increases. The hydrogen absorbed by the whole alloy and, in particular, by the Nd₂Fe₁₄B phase decreases with temperature. Below 150 °C, the hydrogen absorbed by the Nd₂Fe₁₄B phase produces a significant transgranular cracking that is undesirable for particle shape. At 300 °C, the fast and limited absorption of hydrogen by the Nd-rich phase causes insufficient intergranular fracture and, hence, polycrystallinity. Between 150 and 300 °C, the controlled fragmentation resulted in monocrystalline particles with a more equiaxial shape, which is a suitable precursor to develop anisotropic ultrafine powders by the hydrogenation, disproportionation, desorption, recombination (HDDR) process.