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

Step 1: Classification of crystal defects.
Crystal defects are categorized based on their dimensionality: zero-dimensional (point defects), one-dimensional (line defects), two-dimensional (surface defects), and three-dimensional (volume defects).
Step 2: Understanding grain boundaries.
Grain boundaries are interfaces between two crystals of different orientations. Since they extend over an area, they are considered two-dimensional defects.
Step 3: Analysis of other options.
(B) Point defect: Zero-dimensional defects such as vacancies.
(C) Line defect: One-dimensional defects like dislocations.
(D) Volume defect: Three-dimensional defects such as voids and inclusions.
Step 4: Conclusion.
Therefore, grain boundary defect is a two-dimensional defect.

Answer 2

Crystal defects are grouped by dimensionality, that is, by how many directions the defect extends through the lattice. Let's check each option against that rule.

1. Grain boundary defect: A grain boundary is the interface where two crystal grains of different orientation meet. It spreads out over a surface, extending in two directions while staying thin in the third, exactly the signature of a two-dimensional (surface) defect.
2. Point defect: Vacancies, interstitials, and substitutional atoms are localized to a single lattice site. They have no extension in any direction, making them zero-dimensional defects.
3. Line defect: Dislocations (edge or screw) run along a line through the crystal. They extend in only one direction, making them one-dimensional defects, not two-dimensional.
4. Volume defect: Voids, cracks, and inclusions occupy a three-dimensional region of the crystal, extending in all three directions, placing them at the opposite end of the dimensionality scale from a two-dimensional defect.

Of the four, only the grain boundary spreads across a surface without significant thickness, matching the definition of a two-dimensional defect.

Therefore, the correct answer is Grain boundary defect.