Crystallization, Nucleation, Nuclei Growth

Solidification is a calssical example of a nucleation and growth process. In the general case of freezing within the bulk of pure molten metal, minute crystalline nuclei form independently at random points.After this homogeneous form of nucleation, continued removal of thermal energy from the system causes these small crystalline regions to grow independently at the expense of the surrounding melt. Throught the freezing process, there is a tendency for bombardment by melt atoms to destroy embryonic crystals; only nuclei which exceed a critical size are able to survive.Rapid cooling of a pure molten metal reduces the time available for nuclei formation and delays the onset of freezing by a temperature interval of ΔT. This thermal undercooling (or supercooling) varies in extent depending upon the metal and conditions, but can be as much as 0.1-0.3 T_m. Where T_m is the absolute melting point.
However, commecial melts usually contain suspended insoluble particles of foreign matter (e.g. from the refractory crucible or hearth) which act as seeding nuclei for so-called heterogeneous nucleation.
The growing crystals steadily consume the melt and eventually impinge upon each other to form a structure of equiaxed (equal-sized) grains. Heterogeneous nucleation, by providing a large popularity of nuclei, produces a smaller final grain size than homogeneous nucleation.

Production of metallic single crystals:
Bridgman:
A pure metal sample is loaded in a vertical mould of smooth graphite, tapered to a point at the bottom end. The mould is lowered slowly down a tubular funace which produces a narrow melt zone. The crystal grows from the point of the mould.
Czochralski:
This method ofter is often referred to as 'crystal pulling'. A seed crystal is withdrawn slowly from the surface of a molten metal, enabling the melt to solidify with the same orientation as the seed. Rotation of the crystal as it is withdrawn produces a cylindrical crystal. This technique is used for the preparation, in cacuo, of Si and Ge crystals.

Phase:
The term 'phase' refers to a seperate and identifiable state of matter in which a given substance may exist.
When refering to a particular phase in the structure of a material, we imply a region comprising a large number of atoms (or ions or molecules) and the existence of a bounding surface which seperates it from contiguous phases.

Eutectic Reaction:
Liq⇔α+β
Eutectoid Reaction:
γ⇔α+β
Monotectic Reaction:
Liq₁⇔α+Liq₂
Peritectic Reaction:
Whereas eutectic system often occur when the melting points of the two components are fairly similar, the second important type of invariant three-phase condition, the peritectic reaction, is often found when the components have a large difference in melting points.
α+Liq⇔β
Peritectoid Reaction:
α+β⇔γ
Syntectic Reaction:
Liq₁+Liq₂⇔α
Size-factor compounds:
r(interstitial)/r(metal)<0.41:>0.59, the distortion becomes appreciable, more complicated crystal structures are formed.

Kirkendall Effect:
In a solid solution, each of the two atomic forms can move with a different velocity. The Kirkendall effect can be taken as a confirmation of the vacancy mechanism of diffusion. The vacancy mechanism is generally conceded to the the correct mechanism for diffusion in faced-centered cubic metals.