Phase and Elemental Distributions in Alnico Magnetic Materials

Insight into microstructural features in commercial alnico alloys is revealed through electron microscopy. Significant differences were found in the chemical make-up of the primary phases and their distributions for the different classes of the alloys. Changes in the Co + Fe : Al + Ni and minor alloying elements shift the matrix phase from an ordered B2 phase found in alnico 5-7 to a L2₁ phase found in alnico 8 and 9. The question whether the FeCo-rich phase has a B2 or body-centered cubic structure remains open. The grain-aligned 5-7 alloy shows a well ordered array of prismatic cells of the FeCo-rich phase that is 20-40 nm in width, 100´s nm in length and epitaxial with the B2 matrix. Cu is uniformly distributed in the matrix phase for alnico 5-7 whereas for alnico 8 and 9, pure Cu precipitates locate along the phase boundaries. Ti-rich precipitates and non-spinodal FeCo-rich precipitates are observed.