Surface crystallization-A limitation on the magnetic properties of amorphous metal ribbons

Iron-base amorphous metal ribbons are exceedingly stress sensitive since their magnetostriction is comparable to that of crystalline alloys while the anisotropy induced by annealing in a magnetic field is small. Because of this stress sensitivity, the magnetic properties are strongly affected by surface crystallization, which causes a compressive stress in the interior of the ribbon. The occurrence and effects of surface crystallization were examined in 1.27 cm wide ribbons cast from five Fe-B-Si alloys and one Fe-B-Si-C alloy. Surface crystallization of the as-cast ribbon is most pronounced on the top side, and while the extent is affected by casting parameters and metalloid content, other elements, unidentified, may trigger surface crystallization as does aluminum. During annealing, surface crystallization preceeds bulk crystallization, and is most pronounced on the wheel side. It does not appear to be related to surface defects or composition gradients in the as-cast ribbon, since removing an appreciable fraction of the thickness by etching does not change the behavior.