On the evolution of Cu-Ni-rich bridges of Alnico alloys with tempering

Tempering is a critical step in Alnico alloy processing, yet the effects of tempering on microstructure have not been well studied. Here we report these effects, and in particular the effects on the Cu-Ni bridges. Energy-dispersive X-ray spectroscopy (EDS) maps and line scans show that tempering changes the elemental distribution in the Cu-Ni bridges, but not the morphology and distribution of Cu-bridges. The Cu concentration in the Cu-Ni bridges increases after tempering while other element concentrations decrease, especially Ni and Al. Furthermore, tempering sharpens the Cu bridge boundaries. These effects are primarily related to the large 2C₄₄/(C₁₁−C₁₂) ratio for Cu, largest of all elements in Alnico. In addition, the Ni-Cu loops around the α₁ phases become inconspicuous with tempering. The diffusion of Fe and Co to the α₁ phase during tempering, which increases the difference of saturation magnetization between the α₁ and α₂ phases, is observed by EDS. In summary, α₁, α₂ and Cu-bridges are concentrated with their major elements during tempering which improves the magnetic properties. The formation of these features formed through elemental diffusion is discussed via energy theories.