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 (∼ 840 °C and 10 min) during thermal-magnetic annealing (MA). The ideal morphology is a mosaic structure consisting of periodically arrayed ∼ 40 nm diameter (Fe-Co)-rich rods (α1α1 phase) embedded in an (Al-Ni)-rich (α2α2 phase) matrix. A Cu-enriched phase with a size of ∼ 3–5 nm is located at the corners of two adjacent {110}{110} facets of the α1α1 phase. The MA process significantly increased remanence (BrBr) (∼ 40–70%) of the alloy due to biased elongation of the α1α1 phase along the 〈100〉〈100〉 crystallographic direction, which is closest in orientation to the applied magnetic field. The optimum magnetic properties of the alloy with an intrinsic coercivity (HcjHcj) of 1845 Oe and a maximum energy product (BHmaxBHmax) of 5.9 MGOe were attributed to the uniformity of the mosaic structure.

Magnetic properties and morphology of spinodal decomposed phases in an isotropic 32.4Fe-38.1Co-12.9Ni-7.3Al-6.4Ti-3.0Cu (wt.%) alloy is sensitive to external magnetic field and annealing temperature. Optimal morphology was formed in a narrow temperature and time range during thermal-magnetic annealing (MA). The ideal morphology is a mosaic structure consisting of periodically arrayed ∼40nm diameter (Fe-Co)-rich rods (α1 phase) embedded in an (Al-Ni)-rich (α2 phase) matrix with a Cu-enriched phase at the corner of two adjacent {110} facets of the α1 phase.Figure optionsDownload high-quality image (271 K)Download as PowerPoint slide