Question

Yield point phenomenon observed in mild steel is due to the presence of

• A. Silicon
• B. Manganese
• C. Carbon
• D. Chromium

Hint:

The distinct yield drop in mild steel is a dislocation locking-unlocking mechanism driven by interstitial Carbon atoms.
This phenomenon is also associated with the formation of visible deformation bands called Lüders bands on the specimen surface.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the element responsible for the distinct yield point phenomenon (upper and lower yield points) observed during the tensile testing of mild steel.

Step 2: Detailed Explanation:

• The Yield Point Phenomenon:
When mild steel (a low-carbon steel) is tested in tension, the stress-strain curve exhibits a unique transition from elastic to plastic deformation characterized by an upper yield point, a sudden drop in stress to a lower yield point, followed by a plateau (yield point elongation).

• Cottrell Atmosphere Theory:
This behavior is explained in physical metallurgy by the interaction between dislocations and small interstitial solute atoms, primarily Carbon (C) and Nitrogen (N).
Because these solute atoms are small, they migrate to the strain fields surrounding edge dislocations to minimize the elastic strain energy of the crystal lattice. This clustering of solute atoms is called a "Cottrell atmosphere".

• Mechanism of Yielding:
The Cottrell atmosphere pins or locks the dislocations in place.
To initiate plastic deformation, a high stress (the upper yield point) must be applied to tear the dislocations free from these solute atmospheres.
Once the dislocations are unlocked, they can multiply and move through the lattice at a much lower stress level (the lower yield point).

• Carbon is the primary interstitial alloying element in mild steel responsible for this pinning effect.

Step 3: Final Answer:
The yield point phenomenon observed in mild steel is due to the presence of carbon.