Solidification of Metals

1)      A metal in molten condition possesses high energy.

2)      As the melt cools, it loses energy to form crystals.

3)      Since heat loss is more rapid near mold walls than any other place, the first submicroscopic metal crystallites called nuclei form here.

4)      Melt experience difficulty in starting to crystallize if nuclei in the form of impurities are present to start the crystallization.

5)      However under such conditions melt inter cools and thus nuclei or seed crystals form.

6)      Nuclei thus formed tend to glow at the second stage of the solidification.

7)      The crystal growth proceeds with release of energy at crystalment interface.

8)      The crystal growth occurs in dendrite manner.

9)      Dendrite growth takes place by the evolution of small arms on the branches of the individual dendrite.

a)      Slow cooling makes the dendrite to grow long whereas fast cooling causes short dendrite growth.

b)      Since, eventually dendrite becomes grains, slow cooling results in large grain structure and fast cooling in small grain structure in the solidified metal. This solid dendrite gives rise to grains.

10)  As solidification proceeds, more and more arm grow on an existing dendrite and also more and more dendrite form until the whole melt is crystallized.

11)  Matter in the crystallize state shows long range order and has regularity of atomic spacing. Metallic crystals posses’ internal symmetry.

12)  Dendrite arm grow because metal atom attach themselves to the solid dendrite.

13)  Atoms arrange themselves in a three dimensional pattern which is repeated many times during the crystal growth.

14)  This unit of repetition is called a unit cell.

15)  Unit cells arrange themselves in straight lines.

16)  Straight lines thus formed in geometric pattern at right angles to each other produce dendrite structure.

17)  Dendrite grow outward until they contact the neighboring dendrite and generate grain boundaries i.e. boundaries between crystals or grains.

18)  Quite likely that the dendrite arm becomes thickened and ultimately a solid crystal or grains may remain with no indication of no dendrite growth.

Unit cells are of types:

a)      Body structure cubic type (BCC)

b)      Face centered cubic structure (FCC)

c)      Hexagonal close pack structure (HCP)