On the Formation and Evolution of Cu-Ni-Rich Bridges of Alnico Alloys with Thermomagnetic Treatment

Despite decades of research and development of Alnico alloys, significant uncertainties in the underlying structure-property relationships remain. Here, we report on the effects of Ti on the Alnico microstructure and nanostructure, and the corresponding influence on magnetic properties. We show that Ti fosters the conditions resulting in the formation of Cu-Ni-rich bridges in the α₁ phases between the α-{2} phases. For Alnico containing Ti, a typical chessboardlike morphology with Cu-Ni-rich bridges is observed, whereas in the absence of Ti, the α1 phases connect to each other readily, especially with a high Co concentration, and a mazelike morphology with Cu-rich white-plate precipitates rather than Cu-rich bridges is observed. Furthermore, in Alnico containing Ti, an inhomogeneous distribution of Ni is found in the α ₂ phases, including loops with high Ni concentration surrounding the α ₁ phase and high concentrations in the bridges as well. An increase in the Cu concentration is also observed in the loops around the α 1 phases (Ni-Cu loops), and direct contact between the Cu-Ni-rich bridges and the Ni-Cu loops is observed in lieu of direct contact between the bridges and the α 1 phases. We also observe that the bridges are not perfectly round but ellipsoidal, with the long axis along the connection of two adjacent α 1 phases. The energy-dispersive X-ray spectroscopy line scans of the bridges shows that two types of Cu-Ni-rich bridges exist: those with more Cu than Ni and those with more Ni than Cu. A 3-D model is presented that explains the conditions and process of bridge formation, consistent with the observed composition distributions.