DENDRITIC SOLIDIFICATION

          The crystals which form in the solidification of a metal may have many different structures (denrite, lamellar, needle-type or acicular etc.) depending on the rate of cooling, and the type and amount of admixtures or impurities in the melt.

       Perfect crystals of proper external shape can be obtained only if crystallisation develops under conditions when the degree of super cooling is very slight and the metal has a very high purity.

          In great majority of cases, branched tree like crystals are obtained, which are called dendrities. A freely growing crystal acquires a dendritic (tree like) shape (shown in Fig.). It has been established that crystals grow with the highest rate along the planes and directions where atoms are packed most closely. Thus, long branches grow first, which are called the first-order dendritic axes. Then second-order axes branch off from them and third-order axes from the second-order ones, and so on. Finally, the metal remains between dendritic axes solidifies.

          Dendrites grow until they interfere with one another. After that the interaxial spaces are filled in and the dendrites turn into continues crystals of an irregular external shape. Such crystals are called grain or crystallites. If there is enough liquid metal to fill in interaxial spaces (for instance, at the opened end of a mould where a shrinkage cavity forms), some crystals may retain the dendritic shape. On solidification, impurities concentrate between dendritic axes at grain boundaries; further these are the most probable places for the formation of voids, because of shrinkage and impaired access of liquid metal to the solidification front.

ALLOTROPY OF METALS

          The existence of a given metal in two or more stable but different crystal structures is known as allotropy. The essence of allotropic transformations is that the atoms of a crystalline solid are converted from one crystalline form to another, i.e., then form a new crystal lattice. Modifications, stable at lower temperatures, are designated by the Greek letter α (alpha); β (beta) designate second form of the same material that is stable at some higher temperatures: γ (gamma) at still higher temperature. 

          Allotropy of iron is of special importance. Pure iron is relatively soft and ductile and its melting point is 1539°C . Iron exist in two allotropic forms: α-iron with a body centred cubic lattice, stable at temperature up to 910°c and γ-iron, with a face-centred cubic lattice, stable in the range from 910°c to 1400°c.

          In the case of oxygen, there are two forms: 'normal' dioxygen (O2) and ozone, or trioxygen (O3). These two allotropes have different molecular configuations. More commonly, allotropy occurs because of different crystal structures in the solid, and is particularly prevalent in groups 14, 15, and 16 of the periodic table.

          In some cases, the allotropes are stable over a temperature range, with a definite transition point at which one changes into the other. For instance, Tin has two allotropes:  White (metallic) Tin is stable above 13.2°C and grey (non-metallic) Tin is stable below 13.2°C. This form of allotropy is called Enantiotropy.

          Carbon also has two allotropes - Diamond and Graphite - although graphite is the stable form at all temperatures. This form of allotropy, in which there is no transition temperature at which the two are in equilibrium, is called monotropy.

Allotropy of carbon are shown below: