How much carbon can fit between iron atoms in a BCC space lattice?

How much carbon can fit between iron atoms in a BCC space lattice?

• A. 0.0005%
• B. 0.0015%
• C. 0.0025%
• D. 0.005%

Answer: (C)

Carbon in iron occupies interstitial spaces between the iron atoms. In a body-centered cubic lattice, those interstitial sites are very small and can hold only a tiny amount of carbon before the lattice becomes too strained or carbon starts to prefer forming carbides. Because of this, the solid solution limit of carbon in ferrite is extremely low, on the order of a few thousandths of a percent by weight. The value 0.0025% is the practical solubility limit for carbon in BCC iron under typical conditions, which is why that option is the best fit. The lattice can’t accommodate much more carbon without causing instability or driving the system toward carbide phases.

Carbon in iron occupies interstitial spaces between the iron atoms. In a body-centered cubic lattice, those interstitial sites are very small and can hold only a tiny amount of carbon before the lattice becomes too strained or carbon starts to prefer forming carbides. Because of this, the solid solution limit of carbon in ferrite is extremely low, on the order of a few thousandths of a percent by weight. The value 0.0025% is the practical solubility limit for carbon in BCC iron under typical conditions, which is why that option is the best fit. The lattice can’t accommodate much more carbon without causing instability or driving the system toward carbide phases.