The crystals shape (Part 2).

The accelerated growth of protruding dendrites (shoots) is due to several reasons. First, it can be associated with the features of packing atoms and placing defects on the surface of these areas. Secondly, the dendritic form of the crystal can be the result of uneven heat sink. The protruding sections of the crystal are in contact with the large volume of liquid per unit of their surface. This contributes to a better dissipation of the heat of crystallization released at the solidification front. With the growth of the crystal in the form of a tip, its apex always contacts the super cooled melt and advances more rapidly. Thirdly, impurities can contribute to the formation of the dendritic form of the crystals. Accumulating in the melt at the concave sections of the crystal, the impurity retards their growth. The growth of the sharp protrusions, which come into contact with the melt of the original composition, does not linger. The formation of new processes on the already grown branches begins with the appearance of protrusions. Growing up, they become new branches. The protrusions are formed primarily on those areas of the surface where there are defects that ensure the creation of spirals and rapid growth. Of the nascent protrusions on the branches, not all are grow.

Due to the thermal and concentration inhomogeneity of the crystallization front, some of them, falling into less favorable growth conditions, are wedged out. Growing ledges, ahead of neighbors, discard branches of the third order and muffle the growth of neighboring branches of the second order. Thus, the branches of a lower order are wedged out.

Depending on the conditions that form on each segment of the crystallization front, the branches branch out unevenly in different directions. In some cases, dendrites can consist only of trunks; in others, branches of higher orders are formed. The faster the melt cools, the more branching occurs.

With the dendritic growth, the total crystallization front and the number of surface defects are larger than in the case of unbranched crystals. Due to this, the volume rate of solidification, that is, the amount of the crystalline phase formed per unit time, is also greater. In this sense, dendritic crystallization is kinetically more advantageous. The thermodynamically same branched crystal is less stable than the unbranched crystal, which has a smaller thermodynamic potential.

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