Changes in microstructure of Nd-Fe-Co-B-Zr alloy powder during the hydrogenation-decomposition-desorption-recombination (HDDR) process have been investigated in detail by transmission electron microscopy (TEM). It was found that the microstructural changes in the HDDR process start with the formation of cellular NdH2 and α-(Fe, Co) at a temperature between 933 and 973 K. The starting temperature is about 50 K higher than that of the Nd-Fe-B ternary system. By further annealing at 973 K, colonies of rod-shaped NdH2 and spherical grains of NdH2 grow, embedded in the α-(Fe, Co) matrix. It was confirmed by the selected area electron diffraction (SAED) analysis that there is no specific crystallographic orientation relationship between the original Nd2(Fe, Co)14B and the decomposed phases; NdH2 and α-(Fe, Co). (Fe, Co)2B grains appear above 973 K, but the number density of the grains is very small. These features suggest that none of the NdH2, the α-(Fe, Co) and the (Fe, Co)2B transfers the c-axis of the original Nd2(Fe, Co)14B to the recombined one. The microstructure formed at 1073-1103 K consists mainly of fine spherical NdH2 particles and α-(Fe, Co) matrix. Most of the NdH2 particles are covered with rims. The SAED and EDX analyses confirmed that the rim regions are made of Nd2(Fe, Co)14B crystalline grains which are strongly correlated with one another in the crystal orientation. After a short treatment of hydrogen desorption, the rim-like phases grow up to form a microcrystalline structure of Nd2(Fe, Co)14B.
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