Question

Stacking faults are commonly observed in

• A. BCC metals
• B. FCC metals
• C. Ionic crystals
• D. Polymers

Hint:

Stacking faults are highly prevalent in FCC metals with low stacking fault energy (like Copper or Austenitic Stainless Steel), where full dislocations easily dissociate into partials.

Answer 1

The Correct Option is B

Concept: A stacking fault is a planar planar defect found in crystalline structures where the regular, periodic stacking sequence of close-packed atomic planes is locally interrupted. In close-packed structures, atoms are nestled efficiently into gaps. The two major close-packed configurations are Face-Centered Cubic (FCC) and Hexagonal Close-Packed (HCP).

Step 1: Structural differences and stacking mechanisms.
Let us analyze the normal sequence of planes along the close-packed directions:
• Normal FCC Stacking: The close-packed planes are the \(\{111\}\) family. The standard repetitive sequence repeats every three layers, represented as: \[ \dots A B C A B C A B C \dots \]
• Normal HCP Stacking: The close-packed planes are the \((0001)\) basal planes. The sequence repeats every two layers, given as: \[ \dots A B A B A B \dots \] When a dislocation passes through an FCC lattice, a full dislocation often splits into two partial dislocations (Shockley partials) to minimize its overall strain energy. The region bounded between these two partial dislocations possesses a disrupted stacking order. For example, missing a \(C\) layer converts the local region into: \[ \dots A B C A B \, [A B A B] \, C A B C \dots \] Notice that the sequence inside the brackets \(\dots A B A B \dots\) matches an HCP configuration locally. Because the close-packed plane spacing and atom densities match perfectly between FCC and HCP, the energy penalty required to create this fault—known as the Stacking Fault Energy (SFE)—is quite low in many FCC metals (e.g., copper, silver, brass).

Step 2: Comparison with other materials.

• BCC metals: Body-Centered Cubic structures do not have close-packed planes. Therefore, standard planar atomic stacking faults of this type are structurally unfavorable and possess high energy.
• Ionic crystals: Altering the plane sequence would bring ions of identical charge into immediate proximity, causing severe electrostatic repulsion.
• Polymers: These consist of long, disordered, or semi-crystalline molecular chains rather than organized close-packed atomic layers. Hence, stacking faults are highly stable and regularly observed in FCC metals.