JNTU B.Tech II THERMODYNAMICS OF MATERIALS Previous Question Paper (August/September - 2022 - R16)

Diffusion is the process of mass transport in solids by the stepwise movement of individual atoms. In crystalline materials, this occurs primarily through two fundamental mechanisms: Vacancy Diffusion and Interstitial Diffusion.

1. Vacancy Diffusion: This mechanism involves the exchange of an atom from a normal lattice site to an adjacent vacant lattice site (vacancy). For an atom to move, it must break its bonds with neighboring atoms and overcome a potential energy barrier. The rate of vacancy diffusion depends on the number of vacancies present and the activation energy required for the atom to jump into the vacancy. Since the number of vacancies increases with temperature, the rate of vacancy diffusion is highly temperature-dependent.

2. Interstitial Diffusion: This mechanism involves the movement of atoms from one interstitial site to an adjacent empty interstitial site. This type of diffusion typically occurs with smaller atoms (such as hydrogen, carbon, nitrogen, or oxygen) that can fit into the spaces between the larger host metal atoms. Because interstitial sites are more numerous than vacancies and the smaller atoms do not need to displace host atoms to move, interstitial diffusion is generally much faster than vacancy diffusion.

Comparison and Key Differences:

• Atomic Size: Vacancy diffusion involves host atoms or substitutional impurities of similar size, whereas interstitial diffusion involves significantly smaller solute atoms.
• Energy Requirements: Interstitial diffusion typically requires lower activation energy because the migrating atom does not need to distort the lattice as significantly as an atom moving into a vacancy.
• Lattice Sites: Vacancy diffusion is limited by the concentration of vacancies, while interstitial diffusion is limited by the availability of adjacent empty interstitial sites.

In summary, while both mechanisms rely on thermal activation, interstitial diffusion is faster due to the higher mobility of small atoms through the interstitial network compared to the slower, vacancy-dependent migration of larger substitutional atoms.