Identifying Dislocations in Materials: Experimental Techniques

Overview

Dislocations are line defects in crystal lattices that govern plastic deformation. Their characterization requires specialized techniques.

Key Methods

1. Transmission Electron Microscopy (TEM)

• Gold standard for direct imaging.
• Thin foil specimen; high-energy electrons.
• Dislocations appear as dark lines (strain contrast) or via weak-beam dark-field for Burgers vector analysis.
• Resolution: atomic scale.

2. Scanning Electron Microscopy (SEM)

• Surface-sensitive; detects dislocation outcrops as etch pits or steps after chemical etching.
• Lower resolution than TEM but larger field of view.

3. X-ray Diffraction (XRD)

• Non-destructive; bulk-sensitive.
• Peak broadening (Williamson-Hall) → dislocation density.
• Diffuse scattering / X-ray topography for spatial distribution.

4. Etching Techniques

• Preferential chemical attack at dislocation cores → etch pits.
• Counted via optical/SEM; gives density and distribution.
• Material-specific etchants (e.g., Secco for Si, Murakami for steel).

5. Ultrasonic Methods

• Dislocation damping of ultrasonic waves → internal friction peaks (Bordoni peak).
• Measures mobile dislocation density.

6. Electrical Techniques

• Resistivity changes (dislocations scatter electrons).
• Deep-level transient spectroscopy (DLTS) for electrically active dislocations.

Selection Criteria

Choice depends on material, dislocation type (edge/screw), density range, spatial resolution needed, and whether bulk or surface information is required.