What are the two primary diffusion mechanisms in crystalline solids?

• A. Vacancies and Interstitials
• B. Dislocations and Grain boundaries
• C. Surface diffusion and Grain boundary diffusion
• D. Interdiffusion and Self-diffusion

Answer: (A)

Diffusion in crystalline solids happens mainly through two elementary processes: vacancy diffusion and interstitial diffusion. In the vacancy mechanism, atoms move by jumping into neighboring vacant lattice sites, effectively exchanging places with a vacancy. The presence and distribution of these vacancies set how quickly atoms can migrate through the bulk of the crystal. In the interstitial mechanism, smaller atoms squeeze into the spaces between host atoms and hop from one interstitial site to another, often with lower energy barriers because they don’t have to displace a lattice atom. This route is especially important for light interstitial species like carbon in iron or hydrogen in metals, where diffusion can be quite rapid.

Other diffusion pathways, such as along grain boundaries, dislocations, or surfaces, can provide faster or more limited routes depending on the microstructure, but they are not the fundamental bulk mechanisms. Choices that describe diffusion along interfaces or different diffusion phenomena (like interdiffusion vs self-diffusion) don’t identify the primary bulk mechanisms themselves.