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