Question

Which material typically exhibits a distinct upper and lower yield point in its stress-strain curve?

• A. Pure Aluminum
• B. Mild Steel
• C. Copper
• D. Gray Cast Iron

Hint:

The distinct upper and lower yield points seen in mild steel are caused by carbon atoms locking dislocations in place. Once the dislocations break free from these "Cottrell atmospheres," the stress drops instantly, creating a visible step on the stress-strain graph.

Answer 1

The Correct Option is B

Concept: Certain metals display a sharp discontinuity at the onset of plastic deformation, characterized by an upper yield point followed by a sudden drop to a lower yield point. This distinct behavior is caused by pinned dislocations interacting with small interstitial solute atoms, a phenomenon known as the Cottrell atmosphere effect.

Step 1: The physics of dislocation pinning in Mild Steel.
Mild steel is a low-carbon steel containing small amounts of interstitial carbon and nitrogen solute atoms:
• These small solute atoms naturally migrate via diffusion toward the elastic strain fields located directly beneath the extra half-planes of edge dislocations, minimizing the lattice strain energy. This concentration of solute atoms forms a Cottrell atmosphere.
• When a tensile test begins, these atmospheres lock the dislocations firmly in place. To initiate plastic deformation, the applied stress must be raised to a high value just to break the dislocations free from their solute pins. This peak value is the upper yield point.
• Once the dislocations break free, they can move through the lattice at a much lower stress because they are no longer restricted by the solute clouds. The stress required to sustain deformation drops immediately to a lower plateau, known as the lower yield point.

Step 2: Checking alternative options.

• Pure Aluminum Copper: These face-centered cubic (FCC) metals have highly symmetric structures and lack the specific interstitial solute interactions required to firmly lock dislocations. Consequently, they exhibit smooth, continuous yielding curves.
• Gray Cast Iron: This is a brittle material that fractures suddenly during the elastic regime under tension, showing no macroscopic yield point or plastic deformation. Therefore, Mild Steel is the classic material that displays distinct upper and lower yield points, matching option (B).