The introduction of solute atoms into solid solution in the
solvent atom lattice invariably produces an alloy which is stronger than the
pure metal. There are two types of solid solutions. If the solute and solvent atoms
are roughly similar, the solute atoms will occupy lattice points in the
crystal
lattice of the solvent atoms. This is called substitutional solid solution. If
the solute atoms are much smaller than the solvent atoms, they occupy
interstitial positions in the solvent lattice. If the sizes of the two atoms,
as approximately indicated by the lattice parameter, differ by less than 15 per
cent, the size factor is favorable for solid-solution formation. When the size
factor is greater than 15 per cent, the extent of solid solubility is usually
restricted to less than 1 per cent. Metals which do not have a strong chemical affinity
for each other tend to form solid solutions, while metals which are far apart
on the electromotive series tend to form intermetallic compounds. The relative
valence of the solute and solvent is also important. The solubility of a metal
with higher valence in a solvent of lower valence is more extensive than for
the reverse situation. For complete solid solubility over the entire range of
composition the solute and solvent atoms must have the same crystal structure. Some
of the following mechanisms will occur on solid solution strengthening.
- Elastic interaction
- Modulus interaction
- Electrical interaction
- Stacking fault
- Short range order
- Long range order
The distribution of solute atoms in a solvent lattice is not
usually completely random. There is growing evidence that solute atoms group
preferentially at dislocations, stacking faults, low-angle boundaries, and
grain boundaries. However, even in a perfect lattice the atoms would not be
completely random. For a solid solution of A and B atoms, if B atoms tend to
group themselves preferentially around other B atoms, the situation is called
clustering. However, if a given B atom is preferentially surrounded by A atoms,
the solid solution exhibits short range order. The tendency for clustering or
short-range order increases with increasing solute additions. It is likely that
solid solution hardening is not simply the result of internal stresses due to the
local lattice disturbance from randomly dispersed solute atoms.
A number of types of solute-atom interaction must be
considered in explaining solid-solution strengthening. Cottrell locking due to
elastic interaction between the solute atoms and the dislocations is certainly
an important factor in solid solution strengthening. In view of the valency
effects observed in solid solutions, electrical interaction must also be
considered. However, estimates show that electrical interaction is only about
one third to one seventh as strong as elastic interaction. Thermodynamic
reasoning shows that the concentration of solute atoms at a stacking fault will
be greater than the average bulk concentration. Thus, there is a "chemical
interaction" between these regions and dislocations. In a binary alloy
with long-range order each of the constituent atoms occupies special sites in
the lattice. In effect, this results in a super lattice with a larger unit cell
and a new crystal structure. The interaction of dislocations with long-range
order results in a strengthening effect.
