In experiments connected with the study of rheocasting methods for Al-Si alloy (A356) besides “classic dendrites” (Fig. 1a) were fined morphologies demonstrating more or less splitting of structure elements (Fig. 1 b – d).

Fig.1.  a) – classic dendrite, b) - d) – splitting patterns. Dashed lines marks “dendrite stems”. Arrows on b) and d) indicate splitting elements for comparison with Fig. 2 d), h)

Such structure retains some “dendrite features”, therefore we entitled them as degenerate dendrites.    It is known that regular dendrites forms spontaneously by growth in preferential direction (<100> for cubic symmetry), that reflects crystallographic anisotropy. For growth in other directions anisotropy had be “overcome” due to some circumstances. It seems that such circumstances can be forced conditions of directional solidification.

For  growing of crystals in non preferential direction installation for directional solidification was used, that  permits to rotate cuvette with seed crystal relatively direction of imposed thermal gradient, which was 40 ⁰C/sm. It was found that for rate 6 µm/s, Fig. 2 e) – h), structure elements (dendrites) were growing in preferential direction <100> independently from orientation of seed crystal,  while for rate 3 µm/s structure elements tilted to thermal gradient direction and formed splitting patterns similar to those for degenerate dendrites (compare marked with red  arrows structures on Fig. 2 c), d) and Fig. 1 b), d) ).

Fig. 2 Directional solidification. G – direction of thermal gradient. Angle between G and  <100> direction is 0⁰ for a), e) ; 15⁰ for b), f); 30⁰ for c), g) and 45⁰ for d), h). Rate of growth is 3 µm/s  for a) – d) and 6 µm/s  for e) – h). Arrows on c) and d) indicates splitting element to compare with Fig. 1.

So analogy in morphologies put forward the question relatively analogy of “forced conditions”   in both cases. Back to the rheocasting process (producing of degenerate dendrites) it must be noted that its essential feature is intensive shearing of the melt. Thereby it seems that flow incoming on solid-liquid interface generate some gradient. Due to this locally crystal is growing along this gradient (not in preferential direction) and some “directional solidification” take place.