The GATE Grind

GATE 2025 MT – Question 43

Physical Metallurgy · Diffusion in solids · 2 marks · Multiple choice

The following are the activation energies for diffusion of carbon and iron at 773 K in polycrystalline BCC iron: P = Activation energy for diffusion of carbon in BCC iron through the lattice Q = Activation energy for diffusion of iron in BCC iron through the lattice R = Activation energy for diffusion of iron in BCC iron along the grain boundary Which one of the following statements is CORRECT?

  1. R < P < Q
  2. R < Q < P
  3. Q < P < R
  4. P < R < Q

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (D) P < R < Q

Explanation

Interstitial carbon diffusion has a lower barrier than substitutional iron self-diffusion. Iron diffusion along a disordered grain boundary requires less activation than through the perfect lattice. The resulting ordering is **P<R<Q, D**: carbon lattice, iron grain boundary, iron lattice.