Role of the Applied Magnetic Field on the Microstructural Evolution in Alnico 8 Alloys

Abstract

Improving performance of permanent magnet (PM) alloys is a complex optimization of microstructure through control of chemistry and processing. Alnico alloys represent a class of chemically complex alloys whose functionality as a PM is dependent on the nano-structuring which develops during thermal heat treatments in an applied magnetic field. Initial phase separation during rapid quenching of a solid solution from 1523 K (1250 °C) results in a network structure consisting of interpenetrating continuous Fe-Co (α 1) and AlNi-based (α 2) phases. Annealing polycrystalline alnico with a fixed external magnetic field (H) at 1073 K (800 °C) causes isolation, coarsening, and faceting of the α 1 phase with increasing time. Also, the volume fraction and the Al, Ni, Ti, and Cu contents of the α 1 phase decreased, while the Fe and Co contents increased with increasing field annealing time. A Cu-rich phase forms along the adjacent corners of two α 1 facets in the α 2 phase during the phase separation and coarsening process. The phase distribution and micro-magnetic domain structure is closely related to the relative orientation between H and the principal axes of each grain. The α 1 rods are elongated in each grain along the 〈100〉 direction with projected H, where the preferentially extended length of the α 1 rods along the selected 〈100〉 is proportional to the magnitude of projected H.