Lecture 5 Diffusion in Solids

Question 1

Define Carburizing

Answer 1

The addition of carbon to the surface of low-carbon steels at temperatures ranging from 849C to 949C

Question 2

What is Diffusion

Answer 2

Is the phenomenon of material transport by atomic motion

Its is a consequence of the constant thermal motion of atoms ,molecules and particles that results in materials moving from high concentration to areas of low concentration

Question 3

Diffusion Mechanisms :1.Diffusion in liquid or gas

Answer 3

Question 4

how can diffusion be used in a solid ?

Answer 4

Through the use of a diffussion couple.
a DIFFUSION COUPLE is formed by joining bars of two different metals together so that there is intimate contact btwn the two faces

Question 5

interdiffusion (impurity diffusion)

Answer 5

Usuallly occurs in an alloy
* it is whereby atoms tend to migrate from regions of high concentration to regions of low concentration

Question 6

Diffusion mechanisms

Answer 6

Atoms in solid materials are in constant motion , rapidly changing position
For an atom to move ,2 conditions mus be met
1.There mus be an empty adjacent site and
2.
2. The atom must** have sufficient (vibrational) energy to break bonds with its
neighboring atoms** and then cause lattice distortion during the displacement.
At a specific temperature, only a small fraction of

Question 7

what are the 2 dominant models for mettallic diffusion

Answer 7

1.Vacancy &
2.Interstitial diffusion

Question 8

Vacancy Diffusion

Answer 8

Its a mechanism involving the intercharge of an atom from a normal lattice position to an adjacent vacant lattice site/vacancy.

Question 9

interstitial diffusion

Answer 9

It refers to atoms that migrate from an interstitial position to a neighboring one that is empty

This mechanism is found for interdiffusion of impurities with small atoms (H, C, O, N) to diffuse between atoms (i.e., interstitial positions)

Question 10

Process of interstitial diffusion

Answer 10

• Interstitial diffusion does not require presence of vacancy. * Activation energy is much less– More rapid than vacancy diffusion due to more mobile small atoms and more empty interstitial sites than vacancies.

Question 11

Steady-state diffusion across a thin plate

Answer 11

steady-state diffusion across a thin plate * If the diffusion flux does not change with time, a steady-state condition exists. * Steady-state diffusion: the rate of diffusion is independent of time. * One common example of steady-state diffusion is the through diffusion of atoms of a gas a plate of metal for which the concentrations (or pressures) of the diffusing species on both surfaces of the plate are held constant