280 Prakt. Met. Sonderband 41 (2009) 
Consequently the solidification velocity in this region was lowered and the size of grains increased 
(Fig. 4b). 
Fig. 2: Topography on the substrate side surface (a) and on the free surface (b) of rapidly solidified ribbons. 
Still more transparently the solidification history can be recognised in the transverse cross-sections 
(Fig. 5a). At practically all ribbon thicknesses the microstructure consists of three different 
morphological zones - outer fine equiaxed zone, columnar crystals and zone with coarse equiaxed 
grains - which formed from the bottom to the upper surface of the ribbon respectively. With 
decreasing thickness the fraction of coarse equiaxed zone decreases and the columnar zone 
increases. The thickness of the outer equiaxed zone is practically independent on the ribbon 
thickness and extends to the height of about 10 pm. This zone contains small grains ~ 1 um in 
diameter with fine precipitates inside the matrix and coarser second phase particles or eutectic 
(Aut+AugLa) at the grain boundaries (Fig. 5b). No fine equiaxed grains are observed in the regions 
above dimples and canals. In this case, two morphological zones are distinguished across the ribbon 
section: zone with columnar grains at the bottom of the ribbon and zone with coarse equiaxed grains 
at the upper side of the ribbon. The columnar to equiaxed transition increases the degree of 
microsegregation. Coarser intermetallic particles (100 - 300 nm in size) and broader interdendritic 
space appeared in this zone 
Fie. 3: Microstructure on the substrate side surface (a) and on the free surface (b) of rapidly solidified ribbons