Dislocations and Strengthening Mechanisms

Introduction

• Plastic deformation occurs via dislocation motion.
• Dislocations are line defects within the crystal structure.
• Understanding dislocations helps in understanding metal strengthening processes.

Basic Concepts

• Edge Dislocation: Extra half-plane of atoms.
• Screw Dislocation: Spiral planar ramp resulting from shear stress.
• Mixed Dislocation: Combination of edge and screw dislocations.
• Plastic deformation corresponds to dislocation motion.

Dislocation Motion

• Dislocations move under stress causing slip.
• Slip: Dislocation movement leading to deformation.
• Dislocation density affects plastic deformation.

Strain Fields

• Dislocations create regions of compressive, tensile, and shear strain.
• Strain fields interact, affecting dislocation motion.
• Dislocation annihilation occurs when opposite dislocations meet.

Strengthening Mechanisms

Grain Size Reduction

• Smaller grains create more grain boundaries, impeding dislocation motion.
• Hall-Petch equation relates grain size to yield strength.

Solid Solution Strengthening

• Impurity atoms impose lattice strain, hindering dislocation movement.
• Substitutional and interstitial solid solutions can strengthen materials.

Strain Hardening (Cold Working)

• Increase in dislocation density with deformation strengthens material.
• Common in metals to increase yield strength through mechanical working.

Precipitation Hardening

• Formation of small particles within the metal matrix hinders dislocation motion.
• Commonly used in aluminum alloys.

Grain Boundary Strengthening

• Grain boundaries act as barriers to dislocation movement.
• Smaller grains result in stronger metals due to more boundaries.

Recovery, Recrystallization, and Grain Growth

Recovery

• Reduction in dislocation density and internal strains.

Recrystallization

• Formation of new grains within a deformed metal.
• Reduces dislocation density and restores mechanical properties.

Grain Growth

• Occurs at elevated temperatures, grains increase in size over time.
• Driven by reduction in total grain boundary area.

Twinning

• Alternative to slip in some metals and conditions.
• Atomic displacements are more significant than in normal slip.

Conclusion

• Understanding dislocation and strengthening mechanisms is crucial for designing materials with desired mechanical properties.