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ME 230: Control Of Properties Of Materials > Diffusion > Flashcards

Diffusion Flashcards

1
Q

What are the natural tendencies of atoms

A

Mixed bonding : forming bonds with dissimilar atoms
Mixed atoms

So there is a natural tendency for diffusion and mixing to occur

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2
Q

What is diffusion majorly dependent on

A

Temperature- it is entropy driven

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3
Q

What does mixed bonding do in terms of thermodyanmics

A

Increases enthalpy-internal energies

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4
Q

What does mixing atoms do in terms of thermodyanmics

A

Increases entropy

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5
Q

What is flux

A

The rate of change of flow of heat, particles, energy or fluid through a plane with a unit area

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6
Q

What is another name for diffusion

A

Mass flux (J)

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7
Q

What are the key diffusion relationships

A

Proportional to mass
Inversely proportional to time
Inversely proportional to area

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8
Q

What speed up diffusion

A

Higher mass
Less diffusion time
Less plane area

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9
Q

What is a steady state

A

When the concentration profile does not change with time
Ficks first law applies

NOT TIME DEPENDENT

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10
Q

What situation is flicks first law used for

A

Steady state diffusion

Note: it sates flux is equal to diffusivity constant times concentration gradient

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11
Q

What is the diffusivity coefficient based on

A

The diffusion and host species material properties

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12
Q

(T/F) In steady state diffusion, the same amount of diffusion occurs in 10 s vs 1 million s

A

T

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13
Q

What are the two physical mechanisms for diffusion

A

Substitutional solutes
Interstitial solutes

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14
Q

What is a substitutional solute

A

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

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15
Q

What controls the speed of diffusion by the mechanism of substitutional solute

A

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

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16
Q

What are interstitial solutes

A

A mechanism for diffusion
Atoms enter the host species and fit between the host atoms - in interstitial sites

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17
Q

Which physical mechanism for diffusion is faster

A

Interstitial solutes since it does not require the host atoms to move first and thus is faster by alot

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18
Q

(T/F) Diffusion require activation energy to begin

A

T
Diffusion is a thermally activated process and there is an energy barrier (activation energy) for diffusion

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19
Q

Compare the activation energy of diffusion for substitutional solutes vs interstitial solutes

A

Sub&raquo_space;» interstitial

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20
Q

(T/F) The diffusivity constant is temperature independent

A

F
It changes with temperature - higher at higher tempretures

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21
Q

Why is diffusion increased at higher tempreture s

A

The activation energy can be overcome more often since there is a high kinetic energy

22
Q

What is diffusion dependent on

A

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)

23
Q

What is the space available in the host effected by

A

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

24
Q

What is the APF

A

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

25
Q

Which diffusion mechanism is more mobile

A

Interstitial solute

26
Q

What is self diffusion

A

Moving through self

27
Q

What must be true to compare diffusivity

A

Same temperature- greatly effects it

28
Q

When are atoms substitutional and when are the interstitial

A

Substitutional if >= size of host
Interstitial if < host

29
Q

Why is diffusion easier for open crystal structures

A

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

30
Q

What if any effect does GB have on diffusivity

A

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

31
Q

In terms of the parts of a crystal structure, where is the diffusion the highest

A

D surface > D dislocation > D GB > D lattice

32
Q

What is non steady state

A

The concentration gradient changes with time
Fick’s second law applies

33
Q

What is the concavity in terms of ficks second law

A

The second order derivative of the slope of the concentration of diffusing species as a function of distance graph

34
Q

How does concavity effect diffusion

A

More concavity = more diffusion

35
Q

How does rate of [] change per time effect diffusion

A

Increase RO [] change per time = increased diffusion

36
Q

What is a semi-infinite solid model

A

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

37
Q

What does Co mean in terms of the semi-infinite model

A

[] of host species at t = 0
So the initial concentration of host species in the solid we are looking at

38
Q

What is Cs in terms of the semi-infinite solid model

A

[] 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

39
Q

What is Cx in terms of the semi-infinite model

A

[] 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

40
Q

What are the conditions under which the semi-infinite model is valid

A

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

41
Q

(T/F) We must insure Co, Cs and Cx are in the same units for a to be acurate

A

T
Since z is unitless

42
Q

What is anodization

A

Passivation using a current (voltage supply) to create a thicker layer or protective film

43
Q

How does anodization effect diffusion

A

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

44
Q

Why does the most diffusion occur at the GB in terms of host species and diffusing species

A

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

45
Q

What is sanitization

A

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

46
Q

What causes the deformation mechanisms of creep

A

Stress induced

47
Q

How is creep related to diffusion

A

Most of the deformation mechanisms of creep are related to diffusion including self diffusion

48
Q

What are the deformation mechanisms of creep

A

Vacancy diffusion
Dislocation movement
Grain boundary diffusion
Grain boundary sliding

49
Q

How can we slow down creep

A

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

50
Q

When can diffusion actually cause permanent deformation

A

When stress < stress yield and T > 0.5 Tm
These condition can cause increased diffusion which can cause preferment deformation under these conditions

ME 230: Control Of Properties Of Materials (7 decks)

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