Effects of Co and Zr additions on microstructure and anisotropy of HDDR-treated NdFeB alloy powders

The microstructure and orientational anisotropy of HDDR-treated NdFeB alloy powders with Co and Zr additions were investigated. It was found that Co and Zr additions have significant effects on the microstructure and orientational anisotropy of HDDR-treated powders. Magnetic measurements show that the HDDR-treated NdFeCoZrB alloy powder is anisotropic, whereas the NdFeZrB alloy powder is basically isotropic. Microstructural observations reveal that a Zr-rich minor phase exists in the homogenized NdFeZrB alloy due to Zr addition, and remains unchanged during the HDDR process. However, Co addition depresses the formation of the Zr-rich phase, resulting not only in a reduced volume fraction but also in a decreased Zr content of this phase. The results strongly suggest that the optimization of the chemical composition of the starting NdFeB alloys to promote solution of alloying elements such as Zr in Nd2Fe14B is the key to the preparation of anisotropic HDDR powders.