7. Crystal Structures II

Why do metals appear granular under a scanning electron microscope?

Because they are composed of grains (crystals) with different orientations, separated by grain boundaries.

What is anisotropy in materials?

Having different properties in different direction

What is isotropy in bulk materials?

When bulk material properties appear the same in all directions

What are crystallographic directions?

They are vectors defined in a crystal lattice. They are represented using indices in square brackets, e.g., [uvw][uvw][uvw].

How are crystallographic directions defined?

1. Draw a vector passing through the origin.

2. Determine its projections along the unit cell axes (a,b,c).

3. Reduce the projections to the smallest integers.

4. Represent the direction as [uvw].

What are crystallographic planes?

Flat surfaces within a crystal lattice, defined using Miller indices ((hkl)), which are derived from the intercepts of the plane with the crystal axes.

What is planar density?

The number of atoms on a specific crystallographic plane divided by the area of that plane.

Why do material properties depend on crystallographic directions and planes?

Mechanical properties like elasticity, ductility, and fracture behavior depend on atomic arrangements, which vary with direction and plane in the crystal structure.

What happens during the solidification of polycrystalline materials?

• Nucleation occurs at various points in the melt.

• Grains grow with different orientations.

• Grain boundaries form at imperfect fits between grains.

Why are grain boundaries important?

Grain boundaries are regions of atomic disorder and high energy. They are chemically active, affecting mechanical, thermal, and electrical properties.

Why do bulk polycrystalline materials appear isotropic?

The random orientation of grains in polycrystalline materials averages out directional properties, making bulk properties appear isotropic.

How do FCC and BCC structures differ in planar density?

FCC structures have higher planar density on close-packed planes (e.g., (111)) compared to BCC due to their higher packing efficiency.

What are grain boundaries, and how are they visualized?

They are regions of atomic mismatch between adjacent grains. They can be visualized using microscopy techniques.