Metals and alloys

Which pair of solute elements forms interstitial solid solutions in iron?

ANi and Cr
BMn and Si
CC and N

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Answer: C. C and N

Carbon (C) and nitrogen (N) have atomic radii much smaller than iron, so instead of replacing iron atoms on lattice sites they enter the gaps (interstitial sites) and form interstitial solid solutions. In BCC ferrite these sites distort the lattice asymmetrically, so even small amounts strengthen strongly; they also gather around dislocations and pin them, causing the distinct yield point and strain aging of mild steel. Ni, Cr, Mn and Si are close to iron in atomic size and dissolve substitutionally, replacing iron atoms on lattice sites. A rough guide for extensive substitutional solubility is that the atomic radii differ by no more than about 15%, among other conditions (the Hume-Rothery rules).

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