6. Solid Solutions
Sometimes impurities are desirable, example, sterling silver, 7.5% Cu and 92.5% Ag.
The copper makes silver stronger and harder.
Zn is added to Cu in brass, which is stronger, harder and more ductile than copper.
Both alloys are solid solutions in which the original structures of the metals are changed.
Solid solutions consist of a solute and a solvent
Solute = small amount, solvent = large excess

7. Solid solutions consist of a solute and a solvent
Solute = small amount, solvent = large excess

8. Types of solid solution
Substitutional
Solute atoms replace solvent atoms.
It can be formed with limited solubility
e.g. Bronze (Sn in Cu)
or unlimited solubility e.g. Brass (Zn in Cu), in this case both atoms have nearly the same size.
2) Interstitial solid solutions
Small atoms located at the interstices between larger atoms. E.g. C in Fe
3) Ordered solid solutions
The two types of atoms arranged in a specific order.
Heating always destroys the ordering in the solution.

10. Interstitial solid solutions
Ordered solid solutions

11. Example 4-4.1
Bronze is a solid-solution alloy of Cu and Sn in which 3%, more or less, of Cu atoms are replaced by tin atoms. The fcc unit cell of copper is retained, but is expanded a bit, because the RSn ~ nm. (a) What is the weight percent in a 3 a/o tin bronze? (b) Assuming that the lattice constant increases linearly with the atomic fraction of tin, what is the density of this bronze?
Procedure Select a basis for calculation. 100 atoms (25 fcc unit cells) is a natural; thus, 97 Cu + 3 Sn. (a) Determine the masses. (b) Since  = m/V, determine ā for V= a3 and use the calculated mass.

15. Solid solutions in ceramic and metallic compounds
Solid solutions occur also in ionic phases, e.g.

16. Nonstoichiometric Compounds
Simple compounds are stoichiometric
Other compounds deviate from the stoichio-metric ratio, e.g. Cu2Al includes 31 a% to 37a% Al (16-20 w% Al), and not 33.5 a% Al
FeO is not 50% a% oxygen.
These compounds are present as solid solutions.

17. Defect Structures