Diffusion Flashcards
What are the natural tendencies of atoms
Mixed bonding : forming bonds with dissimilar atoms
Mixed atoms
So there is a natural tendency for diffusion and mixing to occur
What is diffusion majorly dependent on
Temperature- it is entropy driven
What does mixed bonding do in terms of thermodyanmics
Increases enthalpy-internal energies
What does mixing atoms do in terms of thermodyanmics
Increases entropy
What is flux
The rate of change of flow of heat, particles, energy or fluid through a plane with a unit area
What is another name for diffusion
Mass flux (J)
What are the key diffusion relationships
Proportional to mass
Inversely proportional to time
Inversely proportional to area
What speed up diffusion
Higher mass
Less diffusion time
Less plane area
What is a steady state
When the concentration profile does not change with time
Ficks first law applies
NOT TIME DEPENDENT
What situation is flicks first law used for
Steady state diffusion
Note: it sates flux is equal to diffusivity constant times concentration gradient
What is the diffusivity coefficient based on
The diffusion and host species material properties
(T/F) In steady state diffusion, the same amount of diffusion occurs in 10 s vs 1 million s
T
What are the two physical mechanisms for diffusion
Substitutional solutes
Interstitial solutes
What is a substitutional solute
A physical mechanism of diffusion
Flux of atoms is accompanied by a flux of vacancies in the opposite direction
Thus the new atoms come in and the host atoms vacate to another spot
What controls the speed of diffusion by the mechanism of substitutional solute
The flow of vacancies as there must be an open spot for atoms to move into and thus the flow of vacancies and the flow of substitutional atoms must be the same
What are interstitial solutes
A mechanism for diffusion
Atoms enter the host species and fit between the host atoms - in interstitial sites
Which physical mechanism for diffusion is faster
Interstitial solutes since it does not require the host atoms to move first and thus is faster by alot
(T/F) Diffusion require activation energy to begin
T
Diffusion is a thermally activated process and there is an energy barrier (activation energy) for diffusion
Compare the activation energy of diffusion for substitutional solutes vs interstitial solutes
Sub»_space;» interstitial
(T/F) The diffusivity constant is temperature independent
F
It changes with temperature - higher at higher tempretures
Why is diffusion increased at higher tempreture s
The activation energy can be overcome more often since there is a high kinetic energy
What is diffusion dependent on
Temperature
density of atoms in host determines how fast atoms can move around and thus diffusion since it is a space issue
Solute is substitutional or interstitial
Space available in host (saturation limit)
What is the space available in the host effected by
The crystal structure of host species
A higher APF means there is less space and thus less diffusion and less flux
Thus an APF of 0.74 means that there is also 26% volume available
BCC has more volume abatable than FCC
What is the APF
Atomic Packing Factor
How close packed the storms are too each other
FCC is the closest packed a cubic can get and HCP is very closed packed
Which diffusion mechanism is more mobile
Interstitial solute
What is self diffusion
Moving through self
What must be true to compare diffusivity
Same temperature- greatly effects it
When are atoms substitutional and when are the interstitial
Substitutional if >= size of host
Interstitial if < host
Why is diffusion easier for open crystal structures
Open structures refers to less closely packed structures and structures with impurities or defects
There structures have more space and thus more places for atoms to go into, especially interstitial impurities
This effect also applies to GB
What if any effect does GB have on diffusivity
Increases
GB have a lot of open space which means they are places atoms of a diffusing species can fit into
More space = more diffusion
Diffusivity species in GB tends to be > then in grain
In terms of the parts of a crystal structure, where is the diffusion the highest
D surface > D dislocation > D GB > D lattice
What is non steady state
The concentration gradient changes with time
Fick’s second law applies
What is the concavity in terms of ficks second law
The second order derivative of the slope of the concentration of diffusing species as a function of distance graph
How does concavity effect diffusion
More concavity = more diffusion
How does rate of [] change per time effect diffusion
Increase RO [] change per time = increased diffusion
What is a semi-infinite solid model
A model which models diffusion
A model which models the host species as a infinite solid and allows us to find the diffusivity as a function of the [] of the diffusing species by distance from the surface
What does Co mean in terms of the semi-infinite model
[] of host species at t = 0
So the initial concentration of host species in the solid we are looking at
What is Cs in terms of the semi-infinite solid model
[] of diffusing species at x = 0
So after diffusion has started, the surface ( where the diffusing species enters from) of the solid will theoretically have the maximum [] of the diffusing species which makes sense since that’s where its being supplied from
What is Cx in terms of the semi-infinite model
[] of the diffusing species at x depth at a time t
So at a given time we want the [] of the diffusing species at a specific depth
The time is accounted for in the equation
What are the conditions under which the semi-infinite model is valid
1) @ t = 0, Cx = Co
- the initial [] throughout the host of the host species must be constant (uniform)
2) @ t = t , C(x=0) = Cs
- at the surface of the solid (at x = 0), all the [] is diffusing species at the time we want to evaluate at
3) @ t = t, C (x = infinity) = Co
- at an infinite length, no diffusivity species is found, diffusion only occurs to a certain depth, not infinitly
(T/F) We must insure Co, Cs and Cx are in the same units for a to be acurate
T
Since z is unitless
What is anodization
Passivation using a current (voltage supply) to create a thicker layer or protective film
How does anodization effect diffusion
Diffusion is slow though a passive layer as there is more distance to get through
Thus is is difficult to diffuse through a thick passive layer
This restrict corrosion too since electron transfer also becomes difficult, to get to the anode, diffusion must occur through the passive film
Why does the most diffusion occur at the GB in terms of host species and diffusing species
Other then the fact that there is more space
Since the GB is where the host species meets most of the diffusing species (as this is where they collect since there is the most space here), greatest amounts of diffusion occurs at GB
What is sanitization
Occurs in steel
Where Cr(x)C (Cr in iron forms a carbide with the diffusing c atoms) at the GB leads to multi-phase corrosion at the GB
This occurs at GB since Gb have the most amount of diffusion as this is where diffusing species tend to enter the structure and thus meet the host species since this is where most of them are
What causes the deformation mechanisms of creep
Stress induced
How is creep related to diffusion
Most of the deformation mechanisms of creep are related to diffusion including self diffusion
What are the deformation mechanisms of creep
Vacancy diffusion
Dislocation movement
Grain boundary diffusion
Grain boundary sliding
How can we slow down creep
Since the deformation mechanisms of creep are mostly related to diffusion, we can slow down creep by limiting diffusion
Thus we can remove the highways (GB) to slow down the creep rate - this is the reason single grained structures are so beneficial
When can diffusion actually cause permanent deformation
When stress < stress yield and T > 0.5 Tm
These condition can cause increased diffusion which can cause preferment deformation under these conditions
