Strengthening of Metals
STRENGTHENING OF METALS
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
Reduction)
3) Add other elements in solid solution (Solid Solution
Strengthening)
4) Add second phase particles (Precipitation or Age
Hardening)
• 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.
STRAIN HARDENING
• Ductile material becomes harder and stronger as it is plastically deformed
• 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 metal. – Dislocation strain field interactions
– Dislocation density increases with deformation or cold working
– Dislocations are positioned closer together
– On average, dislocation-dislocation strain fields are repulsive
STRENGTHENING BY GRAIN SIZE
REDUCTION
• Dislocations cannot penetrate grain boundaries, because the crystal planes are discontinuous at the grain boundaries. • 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
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
Reduction)
3) Add other elements in solid solution (Solid Solution
Strengthening)
4) Add second phase particles (Precipitation or Age
Hardening)
• 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.
STRAIN HARDENING
• Ductile material becomes harder and stronger as it is plastically deformed
• 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 metal. – Dislocation strain field interactions
– Dislocation density increases with deformation or cold working
– Dislocations are positioned closer together
– On average, dislocation-dislocation strain fields are repulsive
STRENGTHENING BY GRAIN SIZE
REDUCTION
• Dislocations cannot penetrate grain boundaries, because the crystal planes are discontinuous at the grain boundaries. • 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