Module 6

## Module 6: Dislocations and Strengthening Mechanisms

### Contents
- **Dislocations & Plastic Deformation**
- Mechanisms of plastic deformation in metals.
- **Strengthening Mechanisms in Metals.**
- **Recovery, Recrystallization, and Grain Growth.**

### Plastic Deformation – Dislocations
- Permanent plastic deformation is due to:
- Shear processes where atoms change their neighbors.
- Inter-atomic forces and crystal structure play significant roles.
- Cumulative movement of dislocations leads to gross plastic deformation.
- Edge dislocations move by slip and climb.
- Screw dislocations move by slip and cross-slip.
- Dislocation interactions are complex due to multiple dislocations moving across different slip systems.

### Dislocation Interactions
- Dislocations moving on parallel planes can annihilate each other, leading to:
- Vacancies or interstitials.
- Dislocations on non-parallel planes hinder each other's movement, causing sharp breaks:
- **Jog**: Break out of slip plane.
- **Kink**: Break in slip plane.
- Other hindrances to dislocation motion include:
- Interstitial and substitutional atoms.
- Foreign particles and grain boundaries.
- External grain surfaces and phase change structures.
- Material strength can be increased by arresting dislocation motion.

### Plastic Deformation Mechanisms – Slip
- Two main mechanisms: **Slip** and **Twinning**.
- Slip is more prominent, involving sliding of crystal blocks along slip planes.
- Occurs when shear stress surpasses a critical value.
- Slip occurs in specific directions (slip directions) on certain crystallographic planes.
- A combination of a slip plane and a slip direction forms a **Slip System**.
- During slip, each atom moves an integral number of atomic distances along the slip plane.

### Factors Affecting Slip
- Extent of slip is influenced by:
- External load and resulting shear stress.
- Crystal structure and slip planes’ orientation relative to shear stress directions.
- Critical shear stress defined for single crystals by Schmid.

### Slip in Polycrystalline Materials
- In polycrystalline aggregates:
- Individual grains mutually constrain each other, preventing plastic deformation at lower stresses.
- Slip involves the generation, movement, and rearrangement of dislocations, maintaining mechanical integrity at grain boundaries.
- Need at least five independent slip systems for ductility and grain boundary integrity (von Mises).
- Twinning also contributes to crystal deformation.

### Slip Systems Overview
| **Crystal Type** | **Slip Planes** | **Slip Directions** |
|------------------|--------------------|--------------------|
| FCC | {111} |