Tip Splitting Dynamics in Degenerate Seaweed Patterns of Aluminum Alloys Induced by Anisotropic Surface Tension

The formation of degenerate seaweed in Al-4.5 wt % Cu, Al-3 wt % Mg, and commercial A356 alloys during quasi two-dimensional directional solidification is studied. By designing (100)[011]-and (-211)[111]-oriented planar-front seeds, the surface tension profiles are symmetrically arranged with respect to the temperature gradient direction. The results show that growth toward the surface tension maxima is preferred, and the tips tend to grow outward in both directions, leading to degenerate seaweed formation. The comparison of the degenerate seaweeds in the three Al alloys shows that the morphologic features are qualitatively similar. However, the anisotropic surface tension has a significant effect on the growth dynamics of the degenerate seaweed. This quantitatively indicates that Al alloys with a larger anisotropy tend to exhibit growth with a smaller tip spacing (lambda) and a higher splitting frequency (f).