thickness) and put into a quartz tube. A high frequency 
sample induction furnace was used to heat the upper parts of the 
; sample to temperatures well above the liquidus tempera- 
heating ; : ture of the alloy (see fig. 1). The temperature gradient 
coil was generated by placing the tip of the sample in a water 
bath. Temperature gradients of up to [2K/mm were 
A . achieved. After a holding time of one hour, the sample 
was quenched, cut along the cylinder axes and ground 
and polished. The concentration along the cylinder axis 
was then measured by EDX in a scanning electron 
microscope. 
Fig. 1: Experimental set-up 
Melting/Resolidification Mechanism | 
The melting/resolidification process occurs similarly for both initial microsturctures. It is illustrated 
here using the columnar microstructure as an example. Immediately after reaching a stationary 
temperature profile, the mean concentration at all location is the initial concentration co. In the 
temperature range between liquidus and solidus temperature, a gradient of solid fraction forms (fig. 
2a). At the interface, the concentrations in the solid and liquid phase are imposed by the equilibrium 
phase diagram (fig. 3), i.e. higher interface concentrations at lower temperatures. Thus, at the 
interface a concentration gradient is established that gives rise to mass transport by diffusion and 
convection along the gradient toward regions with higher temperatures. By this, the solute content 
in the mushy zone decreases continuously, entailing gradual resolidification from regions of high 
solid fraction gradient == == time Pp concentration gradient 
| re c(X) =c¢q c(x)=c, 
Tey) | 
Ce. [wt%] 
Results 
c(x) =c, c(x)<c Sign 
> 91 already 
remain 
De . 
/ Ts(eg) © 
obo A Gl ) - 
Fig.2: Evolution of the concentration gradient for the initially columnar microstructure To 
water 
314