Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy

Further property enhancement of alnico, an attractive near-term, non-rare-earth permanent magnet alloy system, primarily composed of Al, Ni, Co, and Fe, relies on improved morphology control and size refinement of its complex spinodally decomposed nanostructure that forms during heat treatment. Using a combination of transmission electron microscopy and atom probe tomography techniques, this study evaluates the magnetic properties and microstructures of an isotropic 32.4Fe-38.1Co-12.9Ni-7.3Al-6.4Ti-3.0Cu (wt.%) alloy in terms of processing parameters such as annealing temperature, annealing time, application of an external magnetic field, as well as low-temperature draw annealing. Optimal spinodal morphology and spacing is formed within a narrow temperature and time range (similar to 840 degrees C and 10 min) during thermal-magnetic annealing (MA). The ideal morphology is a mosaic structure consisting of periodically arrayed similar to 40 nm diameter (Fe-Co)-rich rods (alpha(1) phase) embedded in an (Al-Ni)-rich (alpha(2) phase) matrix. A Cu-enriched phase with a size of similar to 3-5 nm is located at the corners of two adjacent {110} facets of the alpha(1) phase. The MA process significantly increased remanence (B-r) (similar to 40-70%) of the alloy due to biased elongation of the alpha(1) phase along the (100) crystallographic direction, which is closest in orientation to the applied magnetic field. The optimum magnetic properties of the alloy with an intrinsic coercivity (H-cj of 1845 Oe and a maximum energy product (BHmax) of 5.9 MGOe were attributed to the uniformity of the mosaic structure. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.