Conc gradient

Question 1

what is concentration?

Answer 1

number of atoms of a substance per unit volume

Question 2

what is the symbol for flux?

Answer 2

Jx

Question 3

what is Jx?

Answer 3

net flow of atoms through unit area per second in positive x direction

Question 4

what is the equation for Jx?

Answer 4

= -D dC/dx
D = diffusion coefficient
dC/dx = concentration gradient

Question 5

what is diffusion coefficient like?

Answer 5

temperature dependent

Question 6

what is the equation for diffusion coefficient?

Answer 6

D = Do e^(-Ea,diff / RT)

Question 7

what effects can creep have?

Answer 7

strain accumulates with time
stress relaxes with time

Question 8

when does creep cause strain to accumulate with time?

Answer 8

at constant stress

Question 9

when does creep cause stress to relax with time?

Answer 9

at constant displacement

Question 10

what happens for a creep test?

Answer 10

a specimen of material at constant tensile load is placed in a furnace at constant temperature
extension is measured as a function of time

Question 11

where on a strain against time graph is primary creep?

Answer 11

end of linear apart

Question 12

where on a strain against time graph is secondary creep?

Answer 12

linear part after primary creep

Question 13

where on a strain against time graph is tertiary creep?

Answer 13

rising part after secondary creep

Question 14

how does the strain against time graph differ for different temperatures?

Answer 14

primary creep is higher for increasing temperature
all rest of graph is higher up for increasing temperature

Question 15

what is the equation for steady state creep rate?

Answer 15

εs dot = C σ^n e^(-Ea/RT)
C and n = constant

Question 16

what are the mechanisms for creep?

Answer 16

dislocation
diffusion

Question 17

what happens in dislocation creep?

Answer 17

dislocations can climb (up) and glide (to the side)

Question 18

what type of creep happens for high stress? what about low?

Answer 18

high = dislocation
low = diffusion

Question 19

what is diffusion creep due to?

Answer 19

mass transfer of atoms

Question 20

what are the types of diffusion creep?

Answer 20

bulk diffusion
fast diffusion

Question 21

what are the main types of bulk diffusion?

Answer 21

interstitial
substitutional

Question 22

what is hydrogen embrittlement caused by?

Answer 22

interstitial diffusion

Question 23

what happens in tertiary creep?

Answer 23

creep damage begins to occur

Question 24

what is a creep-rupture diagram?

Answer 24

graph of logσ (y) against log(time to failure) (x)

Question 25

how does a creep rupture diagram change for increasing temperature?

Answer 25

lines move closer towards origin

Question 26

how can we resist dislocation movement?

Answer 26

maximise obstructions to dislocation movement

Question 27

how can we describe flow of polymers and glasses?

Answer 27

viscous fluid flow

Question 28

what is the equation for viscous fluid flow?

Answer 28

τ = n γ dot n = dynamic viscosity γ dot = shear strain rate

Question 29

what are the three stages of creep?

Answer 29

primary secondary tertiary

Question 30

how does primary creep occur?

Answer 30

relatively quickly

Question 31

what does tertiary creep cause?

Answer 31

material damage