SOLID SOLUTIONS

When homogeneous mixture of two or more kinds of atoms occur in the solid state having definite crystalline structure, they are known as solid-solutions.

Those atoms present in large amount is called solvent and those present in small amount is called solute.

Solid solutions occur in either of two distinct types.
1) Interstitial solid solution
2) Substitutional solid solution

1) Interstitial Solid Solutions

In interstitial solid solutions, size of solute atoms is smaller than that of the solvent atoms. Here, solute atoms occupy the interstitial sites in the crystal structure of solvent metal.

H, B, N and C form interstitial solid solutions with a number of metals.
Eg. Carbon in solution of α-iron and γ-iron form interstitial solid solution known as ferrite and austenite, respectively.



2) Substitutional Solid Solutions


In substitutional solid solutions, there is direct substitution of solvent atoms by those of the solute atoms. Eg. Cu-Ni solution is a typical example of substitutional solid solutions in which, Ni atoms substitute for the Cu atoms and form substitutional solid solution of copper called 'Cupronickel'.




From the videos given above, we can understand why it is necessary to make alloys and why they contains better properties than pure metals. All metals contains dislocations which reduces strength due to generation of stress fields at the end of dislocations. Alloying eliminates/decreases those stress fields, resulting in increase in strength.

These solute atoms (impurities) can be of any type (metallic/non-metallic/metalloid) but the solvent atoms must be of metallic type, then & only then we can call them alloys.

Note : In solid solutions, crystal structure of solvent atoms is retained but lattice parameters change.


Hume-Rothery Rules

The rules governing the formation of substitutional solid solutions are called Hume-Rothery rules.
According to these rules, the following factors affect solid solubility :

  1. Size : Formation of substitutional solid solution is favoured if the size difference between solute atoms and solvent atoms does not exceed a value of 15%. The smaller the size difference, the smaller will be the lattice strains and the greater will be the possibility of formation of solid solutions.
  2. Crystal Structure : If two elements have same crystal structure, then and only then they will try to form substitutional solid solution. Eg. In Cu-Ni system, both Cu and Ni have FCC structure.
  3. Valency : A metal of higher valency has tendency to dissolve to a large extent in a metal of lower valency. Eg. Cu ( valency +1 ) and Zn ( valency +2), that is why solubility of  Zn in Cu is about 40% wheres solubility of Cu in Zn is negligible.
  4. Electrochemical Factor : Metals are arranged in electomotive series. Metals occupying higher positions in the series are electropositive elements & metals occupying lower positions in the series are electronegative elements with respect to those occupying higher positions. Electropositive and electonegative elements have strong chemical affinity and are more likely to form compounds instead of solid solutions.
    Eg. Mg is a group-2 element and Si, Sn, Pb are group-4 elements, will form Mg2Si, Mg2Sn & Mg2Pb instead of forming solid solutions.