Chp.4 Diffusion

Question 1

What is Diffusion?

Answer 1

Diffusion is defined as mass transport by the movement of atoms.

Question 2

What is interdiffusion?

Answer 2

Interdiffusion is defined as motion of atoms of one metal into another metal and occurs at high temperatures below the melting temperatures of both metals.

Question 3

What is self-diffusion?

Answer 3

Self-diffusion is the motion of atoms in a pure metal. Self-diffusion is not easily observable because atoms are all of the same element.

Question 4

What is vacancy diffusion?

Answer 4

The diffusion mechanism in which net atomic migration is from a lattice site to an adjacent vacancy.

Question 5

What is interstitial diffusion?

Answer 5

This type of diffusion involves atoms that migrate from an interstitial position to a neighboring one that is empty. (Done with small impurity atoms).

Question 6

Diffusion of flux

Answer 6

J = M/(A*t)
J = diffusion flux
M = mass or N = # of atoms
A = Area across where diffusion is occurring
t = elapsed diffusion time

Question 7

Fick’s first law

Answer 7

J = -D(dC/dx)
J = Diffusion flux
- = Indicates direction of gradient is down (high to low)
dC/dx = concentration gradient (slope of concentration profile at a specific point)

Question 8

What is diffusion flux J?

Answer 8

The diffusion flux (J) is equal to the mass diffusing through and perpendicular to a unit cross-sectional area of material per unit time. The diffusion flux is in units of kg/m^2*s.

Question 9

What is the diffusion coefficient?

Answer 9

The constant of proportionality between the diffusion flux and the concentration gradient in Fick’s first law. Its magnitude is indicative of the rate of atomic diffusion.

Question 10

What is Fick’s first law?

Answer 10

The diffusion flux is proportional to the concentration gradient. This relationship is used for steady-state diffusion situations.

Question 11

What is a steady-state diffusion?

Answer 11

In steady-state diffusion there is no net accumulation or depletion of diffusing species and the diffusion flux is independent of time.

Question 12

What is a nonsteady-state diffusion?

Answer 12

In nonsteady-state diffusion there is some net accumulation or depletion of diffusing species and the diffusion flux depends on time.

Question 13

What is a concentration profile?

Answer 13

The curve that results when the concentration of a chemical species is plotted versus position in a material.

Question 14

What is the driving force?

Answer 14

The impetus behind a reaction, such as diffusion, grain growth, or a phase transformation. Usually attendant to the reaction is a reduction in some type of energy (e.g., free energy).