Thermal treatment induced nano-scale spinodal decomposition for enhanced hard-magnetic properties in laser additively fabricated Alnico-8H

The nano-scale microstructural evolution in Alnico-8H alloys additively manufactured (AM) from blended elemental powders using laser-directed energy deposition (LDED) technique and the consequential development in hard magnetic properties were explored in the present study. A single-step thermal annealing, (830 degrees C for 10 minutes followed by furnace-cooling), in absence of the magnetic field of as-LDED processed Alnico-8H alloy significantly enhanced intrinsic-coercivity from 30 Oe to 935 Oe. The superior hard magnetic properties of annealed Alnico-8H stemmed from the spinodal decomposition of single-phase bcc alpha into nano-scale Fe-Co-rich alpha 1 phase having B2 crystal structure and Ni-Al-Ti-rich alpha 2 phase with L21 structure and presence of Cu-rich fcc precipitates, as revealed from the complementary transmission electron microscopy and atom probe tomography investigations. This fundamental understanding of the different constituents of the nano-scale spinodally decomposed microstructure, and its influence in determining the hard magnetic properties, paves the way for the development of next generation AM processed Alnico permanent magnets.