) compare interstitial and vacancy atomic mechanisms for diffusion. (b) cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion.
The Correct Answer and Explanation is:
(a) Comparison of Interstitial and Vacancy Atomic Mechanisms for Diffusion
In vacancy diffusion, atoms move by jumping into adjacent vacant lattice sites (vacancies). This mechanism is common in substitutional solid solutions, where atoms of the same or similar size switch places with neighboring vacancies. The rate of diffusion depends on the number of vacancies and the activation energy required for atoms to move.
In interstitial diffusion, smaller atoms move through the spaces (interstices) between larger host atoms in the crystal lattice. This mechanism is typically seen in systems where small atoms like hydrogen, carbon, or nitrogen diffuse through metals.
(b) Reasons Why Interstitial Diffusion Is Normally More Rapid Than Vacancy Diffusion
- Interstitial atoms are smaller and can move through the lattice more easily without requiring a vacancy.
- The activation energy for interstitial diffusion is typically much lower than that for vacancy diffusion.
Explanation
Diffusion is the process by which atoms move through a material, and it is essential in processes such as alloying, sintering, and phase transformations. The two primary atomic mechanisms for diffusion in solids are vacancy diffusion and interstitial diffusion.
In vacancy diffusion, an atom moves into a neighboring vacant site within the lattice. Since atoms in a crystal structure are normally tightly packed, diffusion via this mechanism depends heavily on the presence of vacancies. These vacancies are usually thermally generated and increase in number with temperature. Because the migrating atom must overcome a significant energy barrier to move into the vacancy, this mechanism is generally slower and requires higher activation energy.
In contrast, interstitial diffusion occurs when smaller atoms move through the interstitial spaces between the host atoms. These diffusing atoms are typically much smaller than the host atoms, such as hydrogen or carbon in iron. Because interstitial atoms are not restricted to hopping into vacancies, they can move more freely through the crystal. Additionally, the energy barrier for interstitial movement is lower because it does not involve the breaking of strong atomic bonds in the host lattice.
As a result, interstitial diffusion is usually faster than vacancy diffusion. The small size and high mobility of interstitial atoms enable them to diffuse more rapidly, especially at lower temperatures. This difference is critical in applications like carburization and nitriding, where rapid diffusion of small atoms is essential to modify material properties.
