Origin of anisotropy in the HDDR process of Nd2Fe14 B-based alloys

To clarify the origin of magnetic anisotropy induced in Nd₂ Fe₁₄B-based magnet powders during hydrogenation, disproportionation, desorption, and recombination (HDDR) process, the microstructure of hydrogenated Nd_12.5Fe_70-xCo₁ Ga_xB₆ powders (0⩽x⩽5) are investigated. As the hydrogenation temperature is increased over 1148 K, a small amount of Nd ₂Fe₁₄B phase is observed to remain undecomposed in the Nd_12.5Fe₆₉Co_11.5Ga₁B₆ powder, even after the hydrogenation for 3.6 ks. This undecomposition becomes more evident when increasing the temperature up to 1198 K. The size of the undecomposed Nd₂Fe₁₄B particles in the powder hydrogenated at 1148 K is about 0.3 μm. The powder with the undecomposed Nd₂Fe₁₄B exhibits magnetic anisotropy after the subsequent desorption process at 1123 K. The increase of the amount of Ga content also decelerates the decomposition. The origin of the anisotropy could therefore be the finely dispersed undecomposed Nd₂Fe₁₄B particles, which can be formed under the controlled hydrogenation condition and with the additives to decelerate the decomposition. Such dispersed particles should act as nuclei for a growth or recrystallization of the Nd₂Fe₁₄B phase during the desorption process, with the preferred orientation, i.e., the orientation of the original Nd₂Fe₁₄B phase