TEM STUDY OF MICROSTRUCTURAL CHANGES IN AN ANISOTROPIC ND-FE-CO-B-ZR MAGNET ALLOY DURING HDDR PROCESS

Changes in microstructure of Nd-Fe-Co-B-Zr alloy powder during the dro genation-decomposition-desorption-recombination (HDDR) process have be en investigated in detail by transmission electron microscopy (TEM). I t was found that the microstructural changes in the HDDR process start with the formation of cellular NdH2 and alpha-(Fe, Co) at a temperatu re between 933 and 973 K. The starting temperature is about 50 K highe r 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, emb edded in the alpha-(Fe, Co) matrix. It was confirmed by the selected a rea electron diffraction (SAED) analysis that there is no specific cry stallographic orientation relationship between the original Nd-2(Fe, C o)(14)B and the decomposed phases; NdH2 and alpha-(Fe, Co). (Fe, Co)(2 )B grains appear above 973 K, but the number density of the grains is very small. These features suggest that none of the NdH2, the alpha-(F e, Co) and the (Fe, Co)(2)B transfers the c-axis of the original Nd-2( Fe, Co)(14)B to the recombined one. The microstructure formed at 1073- 1103 K consists mainly of fine spherical NdH2 particles and alpha-(Fe, Co) matrix. Most of the NdH2 particles are covered with rims. The SAE D and EDX analyses confirmed that the rim regions are made of Nd-2(Fe, Co)(14)B crystalline grains which are strongly correlated with one an other in the crystal orientation. After a short treatment of hydrogen desorption, the rim-like phases grow up to form a microcrystalline str ucture of Nd-2(Fe, Co)(14)B.