Question

The surface defects are two-dimensional defects, which have:
A. Grain boundaries
B. Tilt boundaries
C. Twin boundaries
D. Stacking boundaries
Choose the correct answer from the options given below:

• A. A, B and C only
• B. A, C and D only
• C. A, B, C and D
• D. B, C and D only

Hint:

Remember the dimensionality of common crystal defects:

0D (Point Defects): Vacancies, Interstitials, Substitutional atoms.
1D (Line Defects): Edge dislocations, Screw dislocations.
2D (Planar/Surface Defects): Grain boundaries, Twin boundaries, Stacking faults.
3D (Volume Defects): Voids, Pores, Cracks, Precipitates.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Crystal defects are classified by their dimension. Surface defects, also known as planar or two-dimensional (2D) defects, are imperfections that extend across a two-dimensional plane within the crystal. The question asks to identify which of the given options are 2D defects.
Step 2: Detailed Explanation:
A. Grain boundaries: These are interfaces where crystals of different orientations meet. Since they are interfaces separating 3D volumes (grains), the boundaries themselves are 2D defects.
B. Tilt boundaries: This is a specific type of low-angle grain boundary, which can be described as an array of edge dislocations. It represents a planar imperfection and is thus a 2D defect.
C. Twin boundaries: A twin boundary is a special type of grain boundary across which the crystal lattice is a mirror image of the other side. This mirror plane is a 2D defect.
D. Stacking boundaries (Stacking Faults): This defect occurs when there is an error in the sequence of stacking of crystal planes (e.g., an ABCABC... sequence in FCC becomes ABCABC...). This fault plane is a 2D defect.
Step 3: Final Answer:
All the listed items—Grain boundaries, Tilt boundaries, Twin boundaries, and Stacking boundaries—are examples of two-dimensional (surface) defects in crystals. Therefore, all statements A, B, C, and D are correct.