Mechanical Engineering Final Exam Review

Mechanism of Diffusion

• Vacancy diffusion: Occurs when an atom jumps into an adjacent vacant lattice site, resulting in a net movement of atoms.
• Interstitial diffusion: Takes place when an atom moves into an interstitial site, which is a space between lattice sites, without displacing any other atoms.

Fick's Laws

• Fick's First Law: Describes the rate of diffusion in a concentration gradient, stating that the flux of atoms is proportional to the concentration gradient and the diffusion coefficient.
• Fick's Second Law: Describes the change in concentration over time, stating that the rate of change of concentration is proportional to the second derivative of concentration with respect to position.

Characteristics of Diffusion

• Steady-state diffusion: Implies that the concentration gradient remains constant over time, resulting in a constant flux of atoms.
• Temperature effect: Diffusion rate increases with increasing temperature, as higher temperatures provide more energy for atoms to overcome their activation energy and diffuse.

Important Concepts

• Diffusion coefficient: A measure of the ease of diffusion, dependent on the material's properties and temperature.
• Concentration gradient: The change in concentration of atoms or molecules over a given distance.
• Flux: The rate of flow of atoms or molecules across a given area.