Diffusion in Solid-Lec.3

Diffusion in binary substitutional materials (alloys): Kirkendall Effect and Darken’s equation
Today’s topics
• Understand the basic concepts of vacancy, substitutional diffusion, Kirkendall effect and its implication in alloy practice.
• What is the major difference between interstitial diffusion and substitutional diffusion in terms of “diffusion coefficient”.
• Understand Darken’s equation and interdiffusion coefficient, D? .
Short history: Although atomic diffusion in solids is far slower than that in gases and liquids, such diffusion does take place, and it is related to various processes such as recrystallization, precipitation, and oxidation (as we will learn throughout this course). The study of diffusion in solids was initiated just 100 years ago when Sir Roberts-Austen discovered the diffusion

Real crystals are never perfect: they always contain a considerable density of defects
and imperfections that affect their physical, chemical, mechanical and electronic
properties. The existence of defects also plays an important role in various
technological processes and phenomena such as annealing, precipitation, diffusion,
sintering, oxidation and others. It should be noted that defects do not necessarily have
adverse effects on the properties of materials. There are many situations in which a
judicious control of the types and amounts of imperfections can bring about specific
characteristics desired in a system. This can be achieved by proper processing
techniques. In fact, “defect engineering” is emerging as an important activity