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Magnetic materials

Grain-oriented electrical steel (Fe with about 3% Si) used for transformer cores has especially low loss and high permeability when magnetized along the rolling direction. Which reason is correct?

  1. AThe grains are aligned in the Goss orientation {110}<001>, so the easy axis <100> points along the rolling direction
  2. BThe texture is controlled by rolling and annealing so that <111>, the easy axis of iron, is aligned with the rolling direction
  3. CThe grains are refined and randomly oriented so that the sheet magnetizes equally easily in every direction, including the rolling direction
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Answer: A. The grains are aligned in the Goss orientation {110}<001>, so the easy axis <100> points along the rolling direction

Iron is cubic, and because of magnetocrystalline anisotropy the <100> directions are the easiest to magnetize (easy axes) and the <111> directions the hardest. Grain-oriented electrical steel is given coarse grains in the Goss orientation, with {110} planes parallel to the sheet surface and <001> along the rolling direction, by secondary recrystallization using inhibitors (fine MnS or AlN precipitates). Magnetized along the rolling direction, it therefore approaches saturation in a weak field and its loss is small. The option treating <111> as the easy axis confuses the easy and hard axes. Non-oriented electrical steel, with random orientations, is used for motor cores, where the direction of the flux rotates. Si is added to raise the electrical resistivity and reduce eddy-current loss.

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