Question

Cast iron typically shows

• A. High ductility
• B. Yield point
• C. Brittle fracture
• D. Large plastic deformation

Hint:

High carbon concentration in ferrous alloys generally increases hardness but dramatically compromises toughness and ductility, tilting the failure mode towards a sudden, catastrophic brittle fracture.

Answer 1

The Correct Option is C

Concept: Cast irons are iron-carbon alloys with a carbon content typically greater than \(2%\) (commonly up to \(4%\)), along with substantial amounts of silicon (usually \(1%\) to \(3%\)). Due to this high carbon and silicon composition, cast irons often develop microstructures filled with brittle phases, such as large networks of cementite (\(\text{Fe}_3\text{C}\)) or flaky graphite shapes that serve as internal sharp microcracks.

Step 1: Evaluation of structural behavior under mechanical load.
When a mechanical tensile force is applied to typical gray cast iron, the stress distribution across the cross-section becomes highly non-uniform:
• Stress Concentration: The tips of graphite flakes act as critical stress concentrators. The localized stress at these sharp features easily exceeds the theoretical cohesive strength of the matrix material.
• Lack of Plastic Yielding: In ductile materials, stress concentrations are blunted out by localized plastic deformation (dislocation movement). However, in cast iron, the surrounding matrix is heavily constrained, or contains brittle cementite, hindering widespread dislocation movement.
• Crack Propagation: Instead of deforming plastically, microcracks nucleate immediately at the graphite flake tips and propagate rapidly through the material with minimal absorption of energy. This swift propagation across the fracture planes results in a classic brittle failure showing little to no necking or macroscopic warning.

Step 2: Checking alternative parameters.

• High ductility Large plastic deformation: These properties require smooth, unobstructed dislocation glide over macroscopic distances, characteristic of low-carbon steels or FCC metals, completely opposite to the behavior of cast iron.
• Yield point: A prominent distinct yield point (such as upper and lower yield points) is typically observed in low-carbon mild steels due to interstitial solute atom interaction (Cottrell atmospheres), which is absent in cast iron stress-strain graphs. Hence, cast iron characteristically undergoes brittle fracture.