Chapter 5: In-class Notes

Question 1

the transfer of mass

Answer 1

Diffusion

Question 2

What equation this:
A
—. Units[m^2/s]
t
A= Known Distance
t= Measured Distance

Answer 2

Gaseous Diffusion: Calculated Diffusion Rate

Question 3

Diffusion between the two types of metals together

Answer 3

Interdiffusion

Question 4

An atom will diffuse if…

Answer 4

• it has somewhere to go
  And
• it has energy

Question 5

With diffusion what does moving to another spot need?

Answer 5

An empty adjacent site

Question 6

With diffusion what does having energy mean?

Answer 6

• sufficient energy to break bonds with its neighbor atoms
  As well as…
• cause some lattice distortion during the displacement

Question 7

What is atomic vibrational energy? And how does it work?

Answer 7

It’s the vibration of the atoms around the moving atom
This increases with the increase with temperature

Question 8

Diffusion Mechanism: Vacancy Diffusion
How does diffusion occur? Method 1: vacancy Diffusion

Answer 8

The motion of a host/substitutional atom to another spot

Question 9

What is vacancy mean?

Answer 9

Available open space

Question 10

Diffusion Mechanisms: Interstitial Diffusion
How does diffusion occur? Method 2: interstitial Diffusion

Answer 10

The positional of the interstitial atom slips through the atoms because it’s smaller than the standing attoms

Question 11

One of the sites in the atomic lattice that doesn’t already have an atom

Answer 11

Interstitial

Question 12

Which method of diffusion is faster and why?

Answer 12

Interstitial diffusing
Because the atom tends to be smaller and more mobile being able to slip past the atoms

Question 13

What is the rate of diffusion?

Answer 13

• it’s time dependent
• and how fast does diffusion occur
  -> Rate of mass transfers = diffusion Flux

Question 14

What is diffusion Flux’s equation?

Answer 14

J= M <— Mass, or number of atoms (kg/atoms)
—
At <— Cross-sectional area [m^2]*times[s]

J[kg/m^2s] or [atoms/m^2s]

Question 15

What is Flick’s First Law of Diffusion?

Answer 15

J=-D (dC/dx)

Where:
DC
— : Concentration gradient (change in concentration with change in dx position)

D: diffusion coefficient [m^2/s]
Diffusion occurs from high to low concentration

Question 16

Mass Transfer Equation

Answer 16

q=-D(dC/dx)

Where:
DC
— :Concentration profile; also the driving force for diffusion
dx

Question 17

Heat Flux Equation

Answer 17

q=-k(dT/dx)

Question 18

What’s the Concentration Gradient

Answer 18

dC Change C CA-CB
— = ————— = ———
dx Change x XA-XB

Question 19

What’s Compressive Strain?

Answer 19

When the central atom is the same size as the surrounding atoms and putting strain on them

Question 20

What is Tensile Strain?

Answer 20

It’s when the central atom is smaller than the surrounding atoms; The surrounding atoms are putting strain onto the central atom

Question 21

What influences the rate of diffusion?

Answer 21

Atoms:
- diffusing atoms and the matrix atoms by:
—> atomic size
—> Diffusion Mechanism: Vacancy/Interstitial

Temperature:
—> higher the temperature the faster the diffusion rate is

Question 22

What’s the Dependence of the Diffusion Coefficient on Temperature Equation

Answer 22

D=D0 exp(-Qd/RT)

Where:
D0= a temperature-independent prexpoential (m^2/s)
Qd= the ACTIVATION ENERGY for diffusion (j/mol and eV/atom)
R= the gas constant, 8.31 J/molK or 8.62x10^-5 eV/atomK
T= absolute temperature (K)