Burgos, N. (Nerea)

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    Effect of temperature on particle shape, size, and polycrystallinity of Nd-Fe-B powders obtained by hydrogen decrepitation
    (2023) Sarriegui-Estupiñan, G.C. (Gabriela Carolina); Checa-Fernández, B.L. (Blanca Luna); Burgos, N. (Nerea); Martín, J.M. (José Manuel)
    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.
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    Effect of neodymium content and niobium addition on grain growth of Nd-Fe-B powders produced by gas atomization
    (2021) Sarriegui-Estupiñan, G.C. (Gabriela Carolina); Martín, J.M. (José Manuel); Burgos, N. (Nerea); Ipatov, M.S. (Mihail S.); Zhukov, A. (Arcady); Gonzalez, J. (Julián)
    Nd-Fe-B powders of different compositions were produced by gas atomization. These powders were annealed between 1000 and 1150 °C for several times to study the microstructural evolution. Differential scanning calorimetry was used to determine the thermal transitions on the as-atomized powders and after slow solidification. The microstructure was studied by high resolution scanning electron microscopy at each stage. Electron back scattering diffraction was used to measure grain size and confirm that gas atomized powders are isotropic. It was observed the formation of necks between the particles, densification, and grain growth due to liquid phase sintering. Grain growth and densification occur in parallel by a dissolution-reprecipitation mechanism. The effect of Nd content and Nb addition on the microstructural changes was analyzed in detail, particularly on grain growth. The degree of sintering increases with Nd content, as this element enhances the formation of the liquid phase. Nb addition leads to the formation of precipitates that delay densification and grain growth at 1000 °C, but promote abnormal grain growth at 1100 °C.