202 Prakt. Met. Sonderband 30 (1999) 
In a positive temperature gradient, the rate at which columnar grains grow, depends on the 
undercooling before the solidification front. In the melt spinning this is affected mainly by the 
thickness of the melt. At appropriate low thickness, where the rate of growth is high enough, these 
grains propagate across the entire ribbon thickness and a columnar zone extends up to the free 
surface. This had been obtained at the ribbon thickness below 40 pm thick, cast at the wheel 3 
velocities higher than 22 ms. On the other hand, if the crystal growth rate is low, nucleation ahead 
of the columnar grains can occur. In our experiment this happened in the upper part of the melt 
during the solidification of thicker ribbons. Columnar-to-equiaxed transition is not unusual in melt 
spun ribbons and mostly this is a regular feature at a Cu-Yb alloy. This is consistent with the wide 
freezing range of the Cu-1.3 wt. % Yb alloy (~220 K). The nucleation ahead of the columnar grains 
probably started by the detachment of existing dendrite arms due to convection (the melt intrusion 4078 
into the mushy zone) or to thermal fluctuations. Homogeneous nucleation in the bulk liquid cannot Pc 
be discounted, but this would imply a very high melt undercooling in front of the dendrite tips. ALTE 
Newly nucleated grains can stop the growth of the columnar front, thus resulting in a columnar- . 
equiaxed transition. The competition between the columnar and equiaxed grains is determined by 
the extend and degree of the undercooled liquid ahead of the columnar zone (i. e. by the relative a 
growth rate). 
Conclusions 
Up to three morphological zones are identified in the melt-spun ribbons of a Cu-1.3wt.% Yb alloy 
produced with different wheel velocities. Their number, type and relative thickness depend on the 
ribbon thickness. € : 
F.c.c. supersaturated solid solution was the only metastable phase produced by the rapid 
solidification. Its small volume fraction in the melt-spun ribbons indicates the low possibility for © 
producing the microsegregation-free Cu-Yb alloy, as the best precursor for producing ODS cooper. = . 
The appearance of the columnar-equiaxed transition at lower wheel velocities strongly increases So 
the degree of microsegregation to the undesirable size range (100-300 nm). a 
Melt spinning processing at the wheel velocities higher than 22 ms” offers an opportunity for in 
reducing the intermetallic segregate size in the Cu-Yb alloy to the point that a reasonable Se 
distribution of oxide dispersoids could be produced by internal oxidation. 5 
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