Question

d. What is the maximum size of an impurity atom (not necessarily that of carbon) that might be comfortably hosted in an FCC octahedral interstitial position whilst being on the verge of, yet not effectively, triggering lattice strain in the immediate vicinity? i r=0.075 mm. Hence, we might still accommodate an atom slightly larger than that of carbon while maintaining complete solid solubility

Answer 1

In FCC lattice, Octahedral interstitial site is taken by an impurity atom.

2r = a – 2R [ R = Atomic radius, a = Edge Length, 2r = Diameter of an atom]

In FCC a = 2R root 2

On solving r = a - 2R/2 = 2R root2 - 2R/2 = 0.41R

Whereas, For BCC r = 0.291R

Hence, interstitial site size for FCC is approx. 1.4 times that of BCC