Refining Alnico 8 magnets with composition optimization of matrix phase, directional solidification and magnetic field annealing

Abstract

Optimized shape anisotropy of the Fe-Co rich ferromagnetic (α1) phase and the effective separation of the α1 and α2 phases with maximum magnetization difference—where α2 represents a paramagnetic magnetic phase—are the key parameters for maximizing the magnetic properties of Alnico magnets. In this manuscript, efforts have been undertaken to optimize the shape anisotropy of the α1 phase and increase the difference in magnetization and separation between α1 and α2 phases by fine tuning Al and Ni contents, directional solidification and magnetic field annealing methods. At first, Alnico magnets were fabricated by adjusting the content of α2 phase followed by homogenization, magnetic field annealing at high temperature, and low-temperature tempering. Intrinsic coercivity (Hcj) of ⁓146.0 kA/m, remanence (Br) of ⁓0.90 T and maximum energy density ((BH)max) of 46.02 kJ/m3 are achieved in the magnet with 8.2 wt% Al and 12.8 wt% Ni treated in field of 1.5 T for 10 minutes. At the second stage, the magnet with optimal magnetic properties is directionally solidified to develop columnar grains, and then the α1 phase is grown in the direction of the columnar grains by magnetic field annealing to enhance the (BH)max to 79.8 kJ/m3. The axial ratio of the α1 phase increased significantly with the increase of the grain size, particularly in the directionally solidified magnets. The Curie temperature of the α1 phase is noted near 1150 K, while the Curie temperature of the α2 phase is below 400 K. The magnets demonstrated excellent temperature stability, with magnetic properties decreasing by only 10–12 % at 800 K. These results may pave the way for further development of Alnico magnets specifically for high temperature applications.