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. 6 d), h)

 Such structure retains some “dendrite features”, therefore we entitled them as degenerate dendrites. Possible reasons for its appearance are discussed further. 

            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 reduction of anisotropy, transition processes and forced conditions.

 Reduction of anisotropy

             Formation of splitting patterns in absence of anisotropy was demonstrated widely in numerical modeling. As concerns Al-based alloy [1] declare the possibility of splitting pattern formation in consequence of alteration of preferential growth direction due to increase of Zn content for Al-Zn alloy. For verification such ability for Al-Si alloy, the experiments on crystal growth in concentration gradient were performed. Complex cylindrical sample from two parts – pure Al and Al-Si eutectic was fabricated. Sample in crucible was melted fully and then directional crystallization took place, so Al dendrites grow in eutectic region, Fig. 2.

Fig. 2.  Al dendrites grow in eutectic region. a) – Al region, b) – transitory region, c) – eutectic region  

             As it can be seen from the figure, dendrites continuously grow through the whole sample, and no changes of preferential growth direction or splitting were observed.

 Transition processes

             Relatively transition processes it must be noted that in real casting growing crystal is surrounded with neighboring, so transition processes from one stationary growth state to another can be not completed and structure of casting will fix just transition stage. Concerning possibility of splitting morphology formation as the result of such processes, we take into account two moments.

            At first, in [2] appearance of non-dendrite splitting patterns associates with two stage crystallization – formation of dendrites, which partially melted, and final structure is the result of loss of stability and further growth of fragments of different shares. To verify this hypothesis experiments with partial melting and further growth of camphene-10 wt.% salol alloy were carried out, Fig. 3

Fig. 3 Consequence stages of the growth of fragments. Row a) – seed crystal less than critical size, row b) – rounded seed crystal a little more than critical size. See the duplex dendrite structure. Row c) – small elongated seed crystal. See dumbbell-like structure. Row d) – large elongated seed crystal

         During the melting of initial dendrite different types of fragments were formed. Depending on  shape and size of fragments further growth formed different but regular dendrite structures.

            At second, one of the possible reason for formation of irregular splitting morphology can be deceleration of the  growth. Background for such suggestion are results of Trivedi [3]. It was pointed out that at least for directional solidification abrupt reduction of growth rate resulted in transient process accompanied with splitting of the pattern, see Fig 4 b), c).

Fig. 4 Changes of growth morphology due to reduction of growth rate. a) – d) – data from [1]. Directional solidification of SCN-based alloy. e) – h) – reduction of  supercooling from 2,2 ⁰C to 0,6 ⁰C.  Camphene-based alloy.

        We made experiments with reduction of growth rate by abrupt reduction of  supercooling from 2,2 ⁰ C to 0,6 ⁰C. As shown Fig. 4 e) – h), no splitting took place during transition. “Thickening” of dendrites took place by suppression of neighbors. Multiple dendrites are the result of large size of seed crystal, see Fig. 3 d).

 Forced conditions    

            Relatively forced conditions, “constrains” crystal to grow in non preferential direction, growth of succinonitrile – 2 wt. acetone alloy was studied in thin gap (20 µm) between parallel glass slides as well as its directional solidification with different orientation of seed crystal relatively direction of thermal gradient.

As shown Fig. 5 c), d) as dendrite as splitting patterns can exist depending on orientation of seed crystal relatively plane of the slide. The last pattern looks like rather as result of percolation than crystal growth.

Fig. 5 Various morphologies. a) unconstrained dendrite growth, b) dendrite pattern in the gap, c) splitting pattern in the gap.

 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. 6 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. 6 c), d) and Fig. 1 b), d) ).

Fig. 6 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.

References

 1. Orientation selection in dendrite evolution / T. Haxhimali, A. Karma, F. Gonzales, M. Rappaz // Nature Materials.-  5, 660-664 (9 July 2006) doi:10.1038/nmat1693 Article

2. Flemings M. C., Yurko J.A., Martines R.A.  Semi-solid forming – our understanding today and its implications for improved processes // Symposium in Honor of Wilfred Kurz, Charlotte, NC, USA, March 14-18, 2004.- Р. 3-14.

3. Somboonsuk K., Trivedi R. Dynamic studies of dendritic growth // Acta metallurgicaю- 1985.- Vol. 33.-  № 6.- Р. 1051-1060.

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