Prakt. Met. Sonderband 41 (2009) 281 
zrains increased 
Fig. 4: Microstructure in the region of direct contact of the melt with the wheel surface (a) and in the regions of dimples 
and canals (b). 
bbons. In our experiments, no metastable phase has been produced by the rapid solidification, other than 
the extended f.c.c. solid solution phase. At all ribbons this phase was formed in the fine equiaxed 
€ cross-sections zone and in the beginning of the columnar zone. It indicates the region where the interface growth 
three different rate V; was higher than the critical value V, needed for the segregation-free solidification (V, is 
coarse equiaxed threshold velocity for the absolute stability of the advancing solidification front). Small size of this 
pectively. With region, even at the thinnest ribbon, confirms the theory that in the alloys with the large freezing 
columnar zone range it is difficult to obtain the segregation-free solid. Namely, increase in the freezing range 
- on the ribbon results in higher V, and from some critical value solute trapping limits the formation of segregation 
ains ~ 1 pm in free solid. 
cles or eutectic The microstructural morphology evolving during solidification is dependent on the local conditions 
d in the regions at the solid liquid interface. At all ribbons nucleation occurred along the surface in contact with the 
Cross the ribbon quenching substrate (outer equiaxed zone). After these grains had nucleated, they grew towards the 
equiaxed grains top of the casting and this resulted in the so-called columnar zone. The transition from the outer 
5 the degree of equiaxed zone to the columnar zone, as well as the grain selection which occurs in the upper part of 
er interdendritic the equiaxed zone, can be understood in terms of anisotropic growth effects [8]. Competitive growth 
of crystals nucleated in the outer equiaxed zone into the liquid leads to their selection. The grains 
which have one of their <100> crystallographic orientations most closely aligned with the heat flow 
direction overgrow those which have a less favourable orientation. 
In a positive temperature gradient, the columnar-to-equiaxed transition depends on the undercooling 
before the solidification front. In the melt-spinning this is affected mainly by the thickness of the 
melt. In our experiment this happened in the upper part of the melt during the solidification of all 
ribbons. Columnar-to-equiaxed transition is not unusual in melt spun ribbons and mostly this is a 
regular feature at a Au-La alloy. This is consistent with the wide freezing range of the Au-0.5 wt. % 
La alloy (~200 K). The nucleation ahead of the columnar grains probably started by the detachment 
of existing dendrite arms due to convection (the melt intrusion into the mushy zone) or to thermal 
fluctuations. Mass-nucleation in the bulk liquid cannot be discounted, but this would imply a very 
high melt undercooling in front of the dendrite tips. Newly nucleated grains can stop the growth of 
the columnar front, thus resulting in a columnar-equiaxed transition. The competition between the 
| ribbons columnar and equiaxed grains is determined by the extent and degree of the undercooled liquid 
ahead of the columnar zone (i. e. by the relative growth rate).