Page 211 - Fiber Fracture

196 H.U. Kunzi

Fig. IO. Production of amorphous wires by quenching a jet of a molten alloy ejected from a nozzle into
water that rotates with the turning wheel. On the left side a sample of a 100 )*.m diameter wire obtained
with this method. (From Baltzer and Kunzi, 1987.)

determines the ribbon thickness. Foils of up to 30 cm width for the fabrication of
transformer cores are now commonly produced by this method. Here we will discuss
only filamentary products that are smaller than 1 mm width and have a typical thickness
of 20 to 50 km. In order to get these small dimensions the sessile drop that is usually
held between the small orifice of the melting crucible (diameter 50-200 km) and the
rotating wheel has to be kept small. Typically, the velocity of the Cu-wheel surface
is about 120 km/h. This high velocity produces vibrations in the liquid droplet, and
bubbles are drawn along the liquid metal/Cu interface. Both types of perturbations,
vibrations and bubbles, give rise to characteristic surface defects that can affect the
tensile strength.
The bubbles moving with the solidifying metal give rise to a porous surface on
the ribbon side that was in contact with the wheel. The opposite side is usually quite
smooth, but thickness variations due to vibrations may occur. The characteristic serrated
edges in these ribbons are produced by rapid vibrations and probably also by air
inclusions that may escape from the ribbon edges during solidification. Melt spinning in
a vacuum chamber gives much better surface and edge qualities. The tensile strength of
ribbons with serrated edges and irregular surfaces due to air inclusions is far inferior to
those that are either produced under vacuum or have polished edges and surfaces. Also,