There are 4 major ways to strengthen metals, and all
work because they make dislocation motion more
difficult. They also reduce the ductility:
1) Cold work (Strain Hardening)
2) Reduce grain size (Strengthening by Grain Size
3) Add other elements in solid solution (Solid Solution
4) Add second phase particles (Precipitation or Age
• These mechanisms may be combined.
• For example, the world’s strongest structural material (with some ductility) is steel piano wire. It combines all
four strengthening mechanisms, and can have a yield
strength of 500,000 psi. One wire, 0.1” in diameter, can
hold up a 4,000 lb Ford Explorer.
• Ductile material becomes harder and stronger as it is
• The dislocation density – expressed as total number
dislocation length per unit volume – mm/mm3
increases from 105 to 106 mm-2 for a heat treated
metal to 109 to 1010 mm-2 for a heavily deformed
– Dislocation strain field interactions
– Dislocation density increases with deformation or
– Dislocations are positioned closer together
– On average, dislocation-dislocation strain fields
STRENGTHENING BY GRAIN SIZE
• Dislocations cannot penetrate grain boundaries, because
the crystal planes are discontinuous at the grain
• Therefore, making a smaller grain size increases strength (more obstacles and shorter mean slip distance.)
SOLID SOLUTION STRENGTHENING
Impurity atoms that go into solid solution impose lattice strains on surrounding host atoms
Lattice strain field interactions between dislocations and
impurity atoms result in restriction of dislocation movement This is one of the most powerful reasons to make alloys,
which have higher strength than pure metals.
Small impurity atoms exert tensile strains (see figure below) Large impurity atoms exert compressive strains
Solute atoms tend to diffuse...