BeginnerTopic 7 of 100Materials & Chemical
Diffusion
Atoms migrate in solids; rate follows Arrhenius temperature dependence and Fick's laws.
In depth
Atoms move through solids by vacancy or interstitial mechanisms, thermally activated: D=D₀exp(-Q/RT). Fick's first law J=-D dC/dx and second law ∂C/∂t=D∂²C/∂x². Applications: carburising, doping silicon, sintering, creep.
Key points
- Activation energy Q
- Interstitial diffusion faster than substitutional
- Penetration depth ∝ √(Dt)
ExampleCarburising steel at 925 °C diffuses carbon into the surface to harden it.
Diffusion explains heat treatment, corrosion, sintering and doping, so it is central to how microstructures change with time.
What is Diffusion?
Atoms migrate in solids; rate follows Arrhenius temperature dependence and Fick's laws.
Why is it important to understand diffusion?
Diffusion explains heat treatment, corrosion, sintering and doping, so it is central to how microstructures change with time.
What is Arrhenius dependence in diffusion?
The diffusion coefficient rises exponentially with absolute temperature, so a small temperature rise can speed diffusion greatly.
What drives diffusion?
A concentration gradient, with atoms moving from high to low concentration over time, as described by Fick's laws.
How is Diffusion used in practice?
Typical applications include carburising a steel gear to harden its surface, estimating how long a coating takes to diffuse into a substrate and predicting the growth of an oxide layer at high temperature.
What should I learn before Diffusion?
Study "Crystal defects" first (topic 6 of 100 in Materials & Chemical). It covers the groundwork this topic builds on.
What are the key points of Diffusion?
Activation energy Q. Interstitial diffusion faster than substitutional. Penetration depth ∝ √(Dt).
Can you give an example of Diffusion?
Carburising steel at 925 °C diffuses carbon into the surface to harden it.
What comes after Diffusion?
The next topic is "Mechanical properties", which builds directly on this one.
How difficult is Diffusion?
It is classed as Beginner in Materials & Chemical (topic 7 of 100). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.