Dendritic single crystals of Al-7wt%Si alloy have been directionally solidified at a thermal gradient of 40 K cm-1 over growth speeds, ranging from 5 to 85 to 31µm s-1, using aluminum single crystal seed aligned along [100] crystallographic orientation. Mushy zone morphology parameters, such as, primary dendrite nearest neighbor spacing, primary dendrite trunk diameter, side (secondary dendrite) branch length and side branch orientation, have been characterized in to investigate the transients introduced by step increase and step decrease in growth speed.
An increase in the growth speed shows a decrease in both nearest neighbor spacing and trunk diameter. A comparison between the two parameters suggests that the trunk-diameter may be a better metric to quantify differences due to small change in processing parameters, such as, growth in low-gravity in the absence of convection. The trend of decreasing trunk diameter with increasing growth speed is similar to the primary dendrite tip radii trend (predicted by theoretical models). Statistical analysis of the transverse microstructures after the step decrease in growth speed (from 85 to 31µm s-1) showed that primary dendrites which survive the transient are the ones with larger neighbor spacing. However, the predictive ability of nearest-neighbor distance (NNS-1), mean of four nearest neighbor distances (NNS-4) or the mean of six nearest neighbor distances (NNS-6) in terms of which dendrites are likely to dissolve-off appears to be similar. Average side-branch length of the surviving primary dendrites increases and those of the disappearing primary dendrites decreases after the growth speed decrease. Primary dendrites whose side-arms are not orthogonal are more likely to dissolve-off than those which are aligned closer to [100]. Porosity formation during directional solidification can lead to spurious grain formation which will seriously degrade the high temperature creep properties of directionally solidified components.