Ch 8
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Ch. 8. Deformation and Strengthening Mechanisms
3/11/2009
Motion of the Edge Dislocation
3/11/2009
The Formation of Steps by the Motion of Dislocations
Slip plane
3/11/2009
Dislocation Density The total dislocation length per unit volume OR The number of dislocations that intersect a unit area of a random section. Unit: mm-2 • Heavily deformed metals: 109 – 1010 mm-2 • Heat treatment of the deformed metals: 105 – 106 mm-2 • Typical ceramic materials: 102 – 104 mm-2 • Silicon single crystal: 0.1 – 1 mm-2
3/11/2009
Strain around a Dislocation
3/11/2009
Interactions between Dislocations
3/11/2009
Source of Dislocations •
During plastic deformation, the number of dislocations increases dramatically.
•
Dislocation sources: 1. Existing dislocations -> multiply 2. Grain boundaries 3. Internal defects 4. Surface irregularities (scratches)
3/11/2009
Slip Systems • Dislocations move on the preferred planes and preferred directions. => Slip planes and slip directions (slip systems: combination of slip planes and slip directions) • Slip plane: has most dense atomic packing. • Slip direction: direction with most closely packed with atoms
FCC (111)[110] slip system 3/11/2009
Slip Systems
3/11/2009
Slip in Single Crystals
Slips in a zinc single crystal
3/11/2009
Plastic Deformation of Polycrystalline Metals
Before Slip lines after plastic deformation 3/11/2009
After plastic deformation
Deformation by Twinning
3/11/2009
Mechanisms of Strengthening in Metals
1. Grain size reduction 2. Solid-solution strengthening 3. Strain hardening One principle for all strengthening techniques: Restricting or hindering dislocation motion renders a material harder and stronger.
3/11/2009
Strengthening by Grain Size Reduction
1. A dislocation passing into a different grain has to change its direction of motion. 2. Discontinuity of slip planes within a grain boundary. 3/11/2009
Strengthening by Grain Size Reduction
σy = σ0 + kyd-1/2 3/11/2009
Solid-Solution Strengthening
3/11/2009
Solid-Solution Strengthening
Smaller impurity atom
Larger impurity atom
3/11/2009
Solid-Solution Strengthening
1. Separation of dislocation from the impurity increases the overall lattice strain. 2. Strain Interactions between impurity atoms and dislocations.
3/11/2009
Strain Hardening (Work Hardening)
%CW = 100 × (A0-Ad)/A0
3/11/2009
Strain Hardening (Work Hardening) The influence of cold work on the stress-strain behavior for a low-carbon steel
3/11/2009
Strain Hardening (Work Hardening) Mechanism 1. Cold work (deformation) increases the dislocation density due to dislocation multiplication or the formation of new dislocations. 2. The average distance between dislocations decreases. 3. Since dislocation-dislocation strain interactions are repulsive, the motion of a dislocation is hindered by the presence of other dislocations.
3/11/2009
Recovery, Recrystallization, and Grain Growth • Some fraction of the energy expended in deformation is stored in the metal as strain energy: tensile, compressive, and shear zones around the dislocations • By appropriate heat treatment, the deformed metal can revert back to the precold-worked states. 1.
Recovery: During heating, some of the stored internal strain energy is relieved by dislocation motion (by atomic diffusion) =>reduction of dislocation density => lower strain energy.
2.
Recrystallization: • Formation of a new set of strain-free and equiaxed grains. • Driving force: the difference in internal energy between the strained and unstrained material. • Nucleation and then the nuclei grow until they completely consume the parent material (involving short-range diffusion).
3.
Grain Growth: • The average size of grain increase, the number of grains decrease. • Driving force: reduction in total energy (decrease of the area of the grain boundaries)
3/11/2009
Cold-worked (33%CW)
3/11/2009
Recrystallizaton after heating 3s at 580 C
Partial recrystallization 4s at 580C
3/11/2009
Complete recrystallization 8s at 580C
Grain growth after 15 min at 580C
3/11/2009
Grain growth after 10 min at 700C
3/11/2009
3/11/2009
Grain Growth
dn –don = Kt n≥2
Driving force: reduction in total energy
3/11/2009
Grain Growth
3/11/2009
3/11/2009
Motion of the Edge Dislocation
3/11/2009
The Formation of Steps by the Motion of Dislocations
Slip plane
3/11/2009
Dislocation Density The total dislocation length per unit volume OR The number of dislocations that intersect a unit area of a random section. Unit: mm-2 • Heavily deformed metals: 109 – 1010 mm-2 • Heat treatment of the deformed metals: 105 – 106 mm-2 • Typical ceramic materials: 102 – 104 mm-2 • Silicon single crystal: 0.1 – 1 mm-2
3/11/2009
Strain around a Dislocation
3/11/2009
Interactions between Dislocations
3/11/2009
Source of Dislocations •
During plastic deformation, the number of dislocations increases dramatically.
•
Dislocation sources: 1. Existing dislocations -> multiply 2. Grain boundaries 3. Internal defects 4. Surface irregularities (scratches)
3/11/2009
Slip Systems • Dislocations move on the preferred planes and preferred directions. => Slip planes and slip directions (slip systems: combination of slip planes and slip directions) • Slip plane: has most dense atomic packing. • Slip direction: direction with most closely packed with atoms
FCC (111)[110] slip system 3/11/2009
Slip Systems
3/11/2009
Slip in Single Crystals
Slips in a zinc single crystal
3/11/2009
Plastic Deformation of Polycrystalline Metals
Before Slip lines after plastic deformation 3/11/2009
After plastic deformation
Deformation by Twinning
3/11/2009
Mechanisms of Strengthening in Metals
1. Grain size reduction 2. Solid-solution strengthening 3. Strain hardening One principle for all strengthening techniques: Restricting or hindering dislocation motion renders a material harder and stronger.
3/11/2009
Strengthening by Grain Size Reduction
1. A dislocation passing into a different grain has to change its direction of motion. 2. Discontinuity of slip planes within a grain boundary. 3/11/2009
Strengthening by Grain Size Reduction
σy = σ0 + kyd-1/2 3/11/2009
Solid-Solution Strengthening
3/11/2009
Solid-Solution Strengthening
Smaller impurity atom
Larger impurity atom
3/11/2009
Solid-Solution Strengthening
1. Separation of dislocation from the impurity increases the overall lattice strain. 2. Strain Interactions between impurity atoms and dislocations.
3/11/2009
Strain Hardening (Work Hardening)
%CW = 100 × (A0-Ad)/A0
3/11/2009
Strain Hardening (Work Hardening) The influence of cold work on the stress-strain behavior for a low-carbon steel
3/11/2009
Strain Hardening (Work Hardening) Mechanism 1. Cold work (deformation) increases the dislocation density due to dislocation multiplication or the formation of new dislocations. 2. The average distance between dislocations decreases. 3. Since dislocation-dislocation strain interactions are repulsive, the motion of a dislocation is hindered by the presence of other dislocations.
3/11/2009
Recovery, Recrystallization, and Grain Growth • Some fraction of the energy expended in deformation is stored in the metal as strain energy: tensile, compressive, and shear zones around the dislocations • By appropriate heat treatment, the deformed metal can revert back to the precold-worked states. 1.
Recovery: During heating, some of the stored internal strain energy is relieved by dislocation motion (by atomic diffusion) =>reduction of dislocation density => lower strain energy.
2.
Recrystallization: • Formation of a new set of strain-free and equiaxed grains. • Driving force: the difference in internal energy between the strained and unstrained material. • Nucleation and then the nuclei grow until they completely consume the parent material (involving short-range diffusion).
3.
Grain Growth: • The average size of grain increase, the number of grains decrease. • Driving force: reduction in total energy (decrease of the area of the grain boundaries)
3/11/2009
Cold-worked (33%CW)
3/11/2009
Recrystallizaton after heating 3s at 580 C
Partial recrystallization 4s at 580C
3/11/2009
Complete recrystallization 8s at 580C
Grain growth after 15 min at 580C
3/11/2009
Grain growth after 10 min at 700C
3/11/2009
3/11/2009
Grain Growth
dn –don = Kt n≥2
Driving force: reduction in total energy
3/11/2009
Grain Growth
3/11/2009
