Chapter 3 The structure and properties of alnico permanent magnet alloys

Abstract

The alnicos are an important group of permanent magnet alloys. They contain Fe, Co, Ni, and A1 with minor additions of Cu and Ti. This chapter discusses the structure and properties of alnico permanent magnet alloys. The relatively high coercivities of the alnicos are due to the shape anisotropy of Fe or Fe–Co rich single-domain particles, which are precipitated in a weakly ferromagnetic or nonferromagnetic Ni–Al rich matrix. After cooling from about 1200 °C at a controlled rate ≈30 °Cs –1 , the isotropic alnicos 1–4 are subsequently tempered for several hours at about 600–650 °C. The phase separation takes place by spinodal decomposition at 800–850 °C as the alloys are cooled. The final shapes and sizes of the particles are determined in the very early stages of the spinodal decomposition and microstructural investigations have shown that the Fe or Fe–Co rich particles are elongated parallel to the (100) directions in the matrix. The anisotropic alnicos 5, 6, 7, and the grain-oriented alnico 5DG are produced by controlled cooling from 1250 °C in a saturating magnetic field ≈200–300 kAm –1 and then tempered for several hours at about 600 °C. The high coercivity alnicos 8 and 9 are also cooled from about 1250 °C and then annealed isothermally in a saturating magnetic field ≈200–300 kAm –1 for a few minutes at about 820 °C after which they are given a two-stage tempering treatment usually for several hours at about 650 °C and then at 550 °C.