Spatio-temporal microstructure evolution in directional solidification processes

Experimental studies in Al-4.0wt%Cu alloys were conducted in samples of different sizes in which the effect of convection increases with increasing sample diameter. Detailed measurements of dendrite tip radii and the primary arm spacing have been carried out under both diffusive and convective growth conditions. The experimental results on tip radius and primary spacing show a good agreement with the microsolvability theory and Hunt-Lu model, respectively, for diffusive growth processes. In larger samples where convection effects are present, an inhomogeneous microstructure develops. A localized growth model is proposed to explain the spatio-temporal microstructure formation under convective growth conditions. The key parameter in this model is the effective diffusion coefficient which increases with the sample diameter and plays the decisive role in selecting the length scales of the solidification microstructures under convective growth conditions.