FORMATION OF ALLOYS : The mechanical properties of metal are improved by alloying. In general, however, a useful alloy is formed only when the metals in question are mutually soluble in the liquid state.

When such a liquid solution solidifies, it will be found that one of the following conditions obtains :

a)                  The Eutectic - When two metals which are completely soluble in the luquid state
but completely insoluble in the solid state solidify, they do so by crystallising out as
alternate layers of the two pure metals. Thus a laminated type of structure, something
like plywood, is obtained and is termed as "eutectic".'

b)                   The solid solution - The solubility prevailing in the liquid state may be retained
either completely or partially in the solid state, giving rise to what is called a "solid
solution".

c)                   Intermetallic Compound-The two metals react chemically as solidification proceeds
and form an "rntermetallic compound".

Sometimes a solid solution which has already formed in an alloy, transforms at a lower temperature to a eutectic type of structure. When this happens, the eutectic type of structure produced is called a eutectoid, since it was not formed from a liquid solution like a eutectic, but from a solid solution. The transformation of solid solution austentite, to the eutectoid, pearlite, is an example of this type of change.

If an alloy is water-quenched from a temperature above that at which the eutectoid begins to form, the solid solution (or possibly a modified form of it, such as martensite in steels is often retained. This demonstrates that the eutectoid had not formed from the liquid but had been produced at a .later stage.

A solid solution is formed when two metals which are mututally soluble in the liquid state remain dissolved in each other after crystallisation. In order that one metal may dissolve in another to form a solid solution, its atoms must fit in some way into the crystal lattice of the other metal. This may be achieved by the formation of either a "substitutional" or an "interstitial" solid solution. Interstitial solid solutions can be formed only when the atoms of the added element are very small compared with these of the parent metal. Thus enabling them to fit into the interstices or spaces in the crystal lattice of the parent metal. An example of this type of solid solution is exhibited by carbon when it dissolves in face-centred cubic iron to form the solid solution austentite. Nitrogen is also able to dissolve interstitially in steel, making the nitriding process possible.

In a substitutional solid solution the atoms of the parent metal replaced in the crystal lattice by atoms of the added metal. This substitution may be either "ordered" or "disordered". Solid solutions of the ordered type tend to be hard and brittle, whereas those of the disordered type one both tough and ductile, and therefore most useful of metallurgical phases.

Intermetallic compounds are hard wearing and of low frictional properties whereas, the solid solutions provides the necessary tough matrix capable of withstanding mechanical shock and compressive forces.

The solid solutions are strong, tough and ductile whereas intermetallic compounds are brittle, hard and weak.

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