Growth of noncrystallographic dendrites
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First time experimental evidence is presented for the steady state growth of dendrites away from their normal crystallographic growth directions. The evidence is shown for transparent dendrites in the succinonitrile system, which are induced to grow along wetting surfaces. A method for the quantitative characterization of the tips of such dendrites is described and the differences in the secondary arm formation between off-axis and regular dendrites are recorded. The scientific and technological implications of this finding are examined.
Tree-like evolution of solid morphology, called the dendrite, is a common form of growth observed during solidification of isotropic or mildly anisotropic materials. Dendrites growing independently of a solid substrate always grow in well-defined crystallographic directions. The preferred growth direction manifests itself regardless of whether the dendrite is growing freely in a large melt or is forced to grow in a temperature gradient, which imposes the heat flow direction during solidification, a technique that is called directional solidification.1 For the material discussed in the letter, succinonitrile (SCN), a cubic plastic crystal, nonrestrained dendrites are known to grow in the cube direction (100).2 Although asymmetry of the form of the dendrite is possible because of a distortion in the thermal or solutal field, there has been no previous evidence of steady state dendritic growth along nonpreferred crystallographic directions. We show evidence, for the first time, that it is possible to grow noncrystallographic directional dendrites under certain conditions of restricted growth. Unidirectional solidification experiments were carried out in the SCN-acetone system. The technique3 and apparatus4 have been described elsewhere. The experiments consisted of the unidirectional withdrawal of a glass slide containing the alloy through a pre-set temperature gradient. Strips of Teflon were introduced longitudinally into the glass slides. These strips became the walls against which solidification was carried out. Two types of Tefions were used. One was the Teflon (TFE) that produced a wetting effect with the solidifying material. The TFE was 95% crystalline. The other was Teflon (FEP), 45% crystalline and made up the nonwetting wall. Directional dendrites may be characterized by the velocity of growth, the imposed temperature gradient, and the alloy composition. Figure 1 shows a photograph of the dendritic array during directional growth when no walls were present. The heat removal direction was parallel to the dendritic growth direction. It is assumed J. Mater. Res., Vol. 7, No. 8, Aug 1992
FIG. 1. Steady state dendritic morphology during unidirectional solidification of succinonitrile-1.4 wt. % acetone alloy. External velocity V = 0.01 mm/s. Thermal gradient G = 4 K/mm.
that the dendrites grew along one of the (100) axes and that this direction was in the plane of observation. When a wall of FEP Teflon was introduced parallel to the heat flow direction, the dendrites would leav
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