Physical Structure and Magnetic Anisotropy of Alnico 5. Part I

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Abstract
It is concluded from electron metallographic results that the high coercive force and anisotropy of Alnico 5 are caused by a very finely divided precipitate produced by the permanent magnet heat treatment. This precipitate is a transition structure rich in cobalt and is face‐centered cubic with a0 = 10A and appears as rods growing along the [100] directions of the matrix crystal when no magnetic field is applied during heat treatment. The size of the precipitate rods at optimum properties is approximately 75–100A by 400A long. The spacing between rows of rods is about 200A. The rods are not distinctly resolved in the electron images unless they are grown by aging at 800°C. Their orientation and structure is clearly evident in the electron diffraction patterns at all stages of growth. The precipitate responds to a magnetic field applied during heat‐treatment both by suppression of nuclei making an angle greater than about 70° with the field and by the forcing of the rods off the [100] direction into that of the field. The precipitate rods tend to scatter in direction about the field vector when the field is off the [100] but are aligned accurately when the field is along [100].