200 Prakt. Met. Sonderband 30 (1999) 
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Fig. 6: Banded structure in transverse (a) and horizontal (b) cross-section view at Cu-1.3 wt.% Yb. 
In our system, no metastable phase has been produced by the rapid solidification, other than the 
supersaturated f. c.c. solid solution phase. At all ribbons this phase was formed both in the fine 
equiaxed zone and at the beginning of the columnar zone as well. It indicates the region where the 
interface growth rate V; was higher than the critical value V, needed for the segregation-free 
solidification (V, is the threshold velocity for the absolute stability of the advancing solidification 
front). Small size of this region, even at the thinnest ribbon (< 20 pm), confirms the theory that in 
the alloys with the large freezing range it is difficult to obtain the segregation-free solid. Namely, 
increasing the freezing range results in higher V, and above a critical concentration value the 
velocity of solute trapping V: limits the formation of segregation free solid. 
Second phase particles are also visible in the microsegregation free region. Their volume fraction is 
small in agreement with the low solubility of Yb in Cu. We believe that these particles were formed 
by precipitation from the supersaturated solid solution. Diffraction patterns obtained from the larger 
intermetallic segregate particles could not be indexed conclusively to any of the Cu-Yb 
intermetallics reported in the literature. The compositions of particles has been analysed also by 
energy-dispersive spectroscopy and has been found close to the structure CusYb. which is the 
equilibrium second phase for alloys with < 17 at. pct. Yb.