Question

The two-dimensional defect is

• A. Grain boundary defect
• B. Point defect
• C. Line defect
• D. Volume defect

Hint:

Remember: Grain boundaries are surface defects and hence classified as two-dimensional defects.

Answer 1

The Correct Option is A

Defects are sorted by how many directions they extend in: a point has none, a line has one, a surface has two, and a volume has three.

1. Grain boundary defect: spreads across an interface area between grains, so it extends in two directions, a surface (2D) defect.
2. Point defect: confined to a single site, 0D.
3. Line defect: runs along one direction only, 1D.
4. Volume defect: occupies a 3D region such as a void.

So the two-dimensional defect among these is the grain boundary defect.

Answer 2

A quick way to confirm the dimensionality of each defect is to picture a concrete real-world example of it.

1. Grain boundary defect: picture the boundary between two adjacent metal grains under a microscope, it looks like a thin wandering surface separating two differently oriented crystals, an area-like feature, a two-dimensional defect.
2. Point defect: picture a single missing atom (a vacancy) in an otherwise perfect lattice, confined to one lattice point, a zero-dimensional defect.
3. Line defect: picture an extra half-plane of atoms inserted into the lattice, forming an edge dislocation that runs along a line, a one-dimensional defect.
4. Volume defect: picture a cavity or inclusion trapped inside the material, occupying a finite three-dimensional region, a three-dimensional defect.

Matching these real examples to the dimensionality scale confirms that the grain boundary is the two-dimensional defect.

Therefore, the correct answer is Grain boundary defect.