Crystal defects and diffusion
In α-iron at the same temperature, the diffusion coefficient of carbon is many orders of magnitude larger than that of nickel. Which is the main reason?
- ACarbon moves between interstitial sites, so it can jump even without a neighboring vacancy
- BCarbon diffuses by the vacancy mechanism, and vacancies tend to gather around carbon atoms
- CCarbon has a smaller atomic mass than nickel, so its thermal vibrations are faster
Show answer and explanation
Answer: A. Carbon moves between interstitial sites, so it can jump even without a neighboring vacancy
Small atoms such as carbon sit in the gaps between iron atoms (interstitial sites) and can jump one after another into neighboring empty interstitial sites. Since almost all interstitial sites are empty, jump opportunities are plentiful and the activation energy is small. Substitutional atoms such as nickel diffuse by the vacancy mechanism and can move only when a vacancy arrives next to them; the vacancy formation energy is added to the activation energy, making diffusion far slower. The difference in vibration frequency due to atomic mass plays a minor role, but only the difference in diffusion mechanism can explain a gap of many orders of magnitude.