MAE 3324 Ch5 Flashcards
What is the general definition of diffusion?
Material transport by atomic motion.
Describe a diffusion couple.
A diffusion couple is formed by joining bars of two different metals together so that there is intimate contact between the two faces.
T or F
Interdiffusion is the same thing as impurity diffusion?
True
Define impurity diffusion.
The process by which atoms of one metal diffuse into another type of metal.
For an atom to move from its current position during a diffusion process, what two conditionals must be met?
- There must be an empty adjacent site for it to go.
2. It must have enough energy to break it’s interatomic bonds.
What is self-diffusion?
Diffusion occurring in pure metals but all atoms exchanging positions are of the same type. e.g. aluminum diffusing with aluminum.
What are the two main models for metallic diffusion?
- Vacancy
2. Interstitial
Briefly describe vacancy diffusion.
The interchange of an atom from a normal lattice position to an adjacent vacant lattice site.
Briefly describe interstitial diffusion.
Involves atoms that migrate from an interstitial position to a neighboring one that is empty.
Extra Notes: This is the primary diffusion method for impurities such as Hydrogen, Carbon, and Nitrogen. Host or substitutional impurity atoms rarely form interstitials and do not normally diffuse via this mechanism.
T or F
In most metal alloys, interstitial diffusion occurs much more rapidly than diffusion by the vacancy mode, because the interstitial atoms are smaller and thus more mobile. Furthermore, there is more empty interstitial position than vacancies; hence the probability of interstitial atomic movement is greater than for vacancy diffusion.
True
What is the definition of diffusion flux?
J = M/At J = diffusion flux [kg/m^2*s] M = mass or number of atoms A = Cross sectional area t = elapsed diffusion time
Diffusion is a ______ depependant process.
Time
Give Fick’s first law of steady-state diffusion in one direction.
J = -D*dC/dx dC/dx = concentration gradient D = diffusion coeficient [m^2/s]
Note: The negative sign indicates diffusion goes from a high concentration to a low concentration.
Define steady-state diffusion.
The diffusion flux does not change with time–that is, the mass of diffusing species entering the plate on the high-pressure side is equal to the mass exiting from the lower pressure surface–such that there is no net accumulation of diffusing species in the plate.
Define “driving force.”
The force that compels a certain reaction to occur.
When diffusion is occurring what is the driving force?
The concentration gradient.
The magnitude of the diffusion coefficient D is indicative of what?
The rate at which atoms diffuse.
What are the main 2 factors that influence diffusion?
Diffusing species and temperature.
Define “activation energy.” What are the units?
The energy required to produce the diffusive motion of one mole of atoms.
J/mol or eV/atom
A large activation energy will result in a relatively ___diffusion coefficient.
small
What are the principal mechanisms of diffusion?
- Brownian motion
2. Vacancy/interstitial diffusion in solids.
In terms of vacancy diffusion, the rate of diffusion depends on what two factors?
- The number of vacancies
2. The activation energy to exchange.
T or F
Interstitial diffusion is less rapid than vacancy diffusion?
False, Interstitial diffusion is MORE rapid than vacancy diffusion.
Give four examples of when diffusion occurs fast.
- Open crystal structures
- Materials with secondary bonding
- Smaller diffusing atoms
- Lower density materials
Give four examples of situations when diffusion occurs slow.
- Close-packed structures.
- Materials with covalent bonding.
- Larger diffusing atoms.
- Higher density materials.
