Development of a Control Techique for Microstructures in Nd-Fe-B Magnetic Materials produced by HDDR Process

本文用X射线衍射和透射电镜研究了Nd-Fe-B和Nd-Fe-Co-B-Zr合金粉末在氢化-分解-脱附-复合（HDDR）过程中微观结构的变化，以阐明HDDR粉末中Nd_2Fe_ B亚微米晶粒的形成<14>和磁各向异性的出现。结果表明，HDDR过程中NdH_2的微观结构变化始于胞状和球状NdH_2嵌入α-Fe基体中的形成。Nd-Fe-Co-B-Zr合金中的氢分解反应比Nd-Fe-B三元合金中的氢分解反应慢。原始Nd_2Fe_<14>B与分解相NdH_2和α-Fe之间没有特定的晶体学取向关系。在HD过程的早期阶段，NdH_2颗粒中含有大量的硼。随着HD过程的进行，NdH_2颗粒喷出硼，Fe_2B颗粒出现在相对较晚的阶段。随着分解织构中Fe_2B晶粒的形成，磁各向异性降低。在脱附-复合（DR）阶段，<14>NdH_2颗粒与α-Fe基体的界面发生复合，形成边缘状Nd_2Fe_ B相。边缘相是由一些细小的Nd_2Fe_<14>B晶粒组成，这些晶粒的晶体取向相互之间有很强的相关性。进一步放氢后，边缘相长大，形成<14>具有强相关c轴的Nd_2Fe_ B细晶。复合Nd_2Fe_ B晶体的晶粒尺寸<14>与NdH_2颗粒的尺寸和数密度有关。因此，<14>如果能控制HD阶段前的NdH_2颗粒尺寸更小，在α-Fe基体中的分散密度更高，则有望获得超细Nd_2Fe_ B晶体。

The changes in microstructure of Nd-Fe-B and Nd-Fe-Co-B-Zr alloy powders during the hydrogenation-decomposition-desorption-recombination (HDDR) process have been investigated by X-ray diffraction measurements and transmission electron microscope (TEM) observations, in order to clarify the formation of submicron grains of Nd_2Fe_<14>B and the appearance of the magnetic anisotropy in the HDDR powder. It was found that the microstructural changes in the HDDR process start with the formation of cellular and spherical NdH_2 embedded in alpha-Fe matrices. The hydrogenation-decomposition (HD) reaction in the Nd-Fe-Co-B-Zr alloy progresses slower than the one in Nd-Fe-B ternary system. There is no specific crystallographic orientation relationship between the original Nd_2Fe_<14>B and the decomposed phases ; NdH_2 and alpha-Fe. A large amount of boron is contained in the NdH_2 grain in the early stage of the HD process. As the HD process advances, the NdH_2 grain spews out the boron and consequently Fe_2B grains appear at relatively latter stage. The magnetic anisotropy decreases with the formation of the Fe_2B grains in the decomposed texture. In the desorption-recombination (DR) stage, the rim-like Nd_2Fe_<14>B phase is formed by the recombination at the boundaries between the NdH_2 particles and the alpha-Fe matrix. The rim phase is composed of some fine Nd_2Fe_<14>B grains whose crystallographic orientations are strongly correlated with one another. After further hydrogen desorption, the rim phase grows up to form the recombined Nd_2Fe_<14>B fine grains with a strongly correlated c-aixs. The grain size of the recombined Nd_2Fe_<14>B crystal is related to the size and the number density of the NdH_2 particles. It is thus expected to obtain superfine Nd_2Fe_<14>B crystals if the NdH_2 particle before the HD stage can be controlled smaller in size and dispersed more density in the alpha-Fe inatrix.