Diffusion Flashcards
What is Diffusion?
Diffusion is the time dependent, transfer of mass.
What is Interdiffusion?
Interdiffusion is the 1st type of diffusion where atoms of one type diffuse through a lattice of different atoms
What is Self-Diffusion?
Self-Diffusion is the 2nd type of diffusion where both the atom moving and the atoms on the lattice are the same type
What are the Mechanisms that each type of diffusion can occur by?
Both self-diffusion and impurity diffusion can occur by vacancy diffusion while interstitial diffusion is only in impurity diffusion, where the impurity is small compared to the lattice atoms.
What is the Arrhenius Equation?
For an atom to change location there needs to be both a site to go to and sufficient energy to break the bonds with its neighbours. This energy needed is given by the Arrhenius equation, where rate = Ce^(-q/kt).
What is Vacancy Diffusion?
In this mechanism, an atom moves from its normal lattice position to an adjacent vacant lattice site.
What is Interstitial Diffusion?
The interstitial impurity atoms move to adjacent interstitial sites (remember that interstitial sites a normally empty)
What is Fick’s 1st Law?
Fick’s 1st law gives how fast diffusion occurs expressed as, J= -D dC/dx. But Fick’s 1st law only applies to steady-state diffusion.
What is Fick’s 2nd Law?
We use Fick’s second law for non-steady state diffusion which is given by, ∂C/∂t = D ∂^2C/∂x^2.
What are Diffusion Coefficients Based on?
Based on the element and the temperature.
How is Diffusion related to creep?
The rate of the mechanisms of creep are controlled by diffusion.
What is Dislocation creep and how is it affected by diffusion?
The movement of dislocations causes plastic deformation which is resisted by intrinsic lattice resistance and obstructions such as solute atoms and precipitates. Diffusion ‘unlocks’ dislocations from obstacles but there is still a residual force resisting the deformation. However, if atoms at the base of the dislocation diffuse away then the dislocation can move (this is called climb). (Draw it)
What is Diffusion creep?
Diffusion creep occurs at lower stresses and can occur in two different ways, Bulk diffusion (where atoms diffuse through the body of the grain) and grain boundary diffusion (where atoms diffuse along the edge of the grain). This causes the elongation of the grains.
How do we design against creep?
For designing against dislocation creep
1. Use materials with high melting points
2. Maximise dislocation obstructions
3. If possible choose a material with a large lattice resistance
For designing against diffusion creep
1. Also use materials with high melting points
2. Have a large grain size
3. Have precipitates at grain boundaries
What is a phase?
