156 Prakt. Met. Sonderband 38 (2006) 
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Fig 1: Microstructure of rapidly solidified ribbon; a) The overall cross-section; b) Equiaxed 
grains at the top side of the ribbon; c) Fine columnar grains at the wheel side 
The microsegregation-free region contains fine grains about half micron in size (Fig. 1c) with 
some second phase particles visible at the grain boundaries. These particles were probably it ci 
formed by precipitation from the highly-supersaturated copper solid solution. The transition incre 
from microsegregation-free to the banded structure has occurred at all places very abruptly. MOS 
This structure consists of dark and light layers formed perpendicular to the growth direction, The 
with a band spacing of about 0.7 um (Fig. 2a). In the horizontal cross-section of the ribbons intel 
the banded structure shows spirale-type morphology (Fig. 2b). Carrard et al. [10] reported sub: 
that a banded structure forms at the transition from microsegregation-free to cellular the 
solidification at growth rates when the solidification front becomes periodically unstable. The 
Acceleration and deceleration of the solidification front result in light and dark bands which sele 
are microsegregation-free (light) and cellular (dark) regions. prac 
The microstructure at the top of the coarse columnar zone consists of a cellular structure met 
with cellular spacing of less than 1 um and with fine precipitates at the cell boundaries. As of tt 
described in the beginning, the coarse columnar zone transforms into an equiaxed zone that goo 
consists of coarse grains about 10 ym in size with fine precipitates inside the grains and sub! 
coarser ones at their boundaries. corr 
Second phase particles are clearly visible in all three regions. We believe that these with 
particles were formed during solidification in the interdendritic or intercellular region where 
the last melt rich with the alloying element solidified or by precipitation from the