Diffusion

Question 1

What are the natural tendencies of atoms

Answer 1

Mixed bonding : forming bonds with dissimilar atoms
Mixed atoms

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

Question 2

What is diffusion majorly dependent on

Answer 2

Temperature- it is entropy driven

Question 3

What does mixed bonding do in terms of thermodyanmics

Answer 3

Increases enthalpy-internal energies

Question 4

What does mixing atoms do in terms of thermodyanmics

Answer 4

Increases entropy

Question 5

What is flux

Answer 5

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

Question 6

What is another name for diffusion

Answer 6

Mass flux (J)

Question 7

What are the key diffusion relationships

Answer 7

Proportional to mass
Inversely proportional to time
Inversely proportional to area

Question 8

What speed up diffusion

Answer 8

Higher mass
Less diffusion time
Less plane area

Question 9

What is a steady state

Answer 9

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

NOT TIME DEPENDENT

Question 10

What situation is flicks first law used for

Answer 10

Steady state diffusion

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

Question 11

What is the diffusivity coefficient based on

Answer 11

The diffusion and host species material properties

Question 12

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

Answer 12

T

Question 13

What are the two physical mechanisms for diffusion

Answer 13

Substitutional solutes
Interstitial solutes

Question 14

What is a substitutional solute

Answer 14

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

Question 15

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

Answer 15

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

Question 16

What are interstitial solutes

Answer 16

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

Question 17

Which physical mechanism for diffusion is faster

Answer 17

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

Question 18

(T/F) Diffusion require activation energy to begin

Answer 18

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

Question 19

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

Answer 19

Sub&raquo_space;» interstitial

Question 20

(T/F) The diffusivity constant is temperature independent

Answer 20

F
It changes with temperature - higher at higher tempretures

Question 21

Why is diffusion increased at higher tempreture s

Answer 21

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

Question 22

What is diffusion dependent on

Answer 22

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)

Question 23

What is the space available in the host effected by

Answer 23

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

Question 24

What is the APF

Answer 24

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

Question 25

Which diffusion mechanism is more mobile

Answer 25

Interstitial solute

Question 26

What is self diffusion

Answer 26

Moving through self

Question 27

What must be true to compare diffusivity

Answer 27

Same temperature- greatly effects it

Question 28

When are atoms substitutional and when are the interstitial

Answer 28

Substitutional if >= size of host Interstitial if < host

Question 29

Why is diffusion easier for open crystal structures

Answer 29

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

Question 30

What if any effect does GB have on diffusivity

Answer 30

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

Question 31

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

Answer 31

D surface > D dislocation > D GB > D lattice

Question 32

What is non steady state

Answer 32

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

Question 33

What is the concavity in terms of ficks second law

Answer 33

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

Question 34

How does concavity effect diffusion

Answer 34

More concavity = more diffusion

Question 35

How does rate of [] change per time effect diffusion

Answer 35

Increase RO [] change per time = increased diffusion

Question 36

What is a semi-infinite solid model

Answer 36

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

Question 37

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

Answer 37

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

Question 38

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

Answer 38

[] 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

Question 39

What is Cx in terms of the semi-infinite model

Answer 39

[] 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

Question 40

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

Answer 40

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

Question 41

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

Answer 41

T Since z is unitless

Question 42

What is anodization

Answer 42

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

Question 43

How does anodization effect diffusion

Answer 43

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

Question 44

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

Answer 44

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

Question 45

What is sanitization

Answer 45

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

Question 46

What causes the deformation mechanisms of creep

Answer 46

Stress induced

Question 47

How is creep related to diffusion

Answer 47

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

Question 48

What are the deformation mechanisms of creep

Answer 48

Vacancy diffusion Dislocation movement Grain boundary diffusion Grain boundary sliding

Question 49

How can we slow down creep

Answer 49

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

Question 50

When can diffusion actually cause permanent deformation

Answer 50

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