Question

1. Briefly explain the difference between self-diffusion and interdiffusion.

2. (a) Compare interstitial and vacancy atomic mechanisms for diffusion.
(b) Cite two reasons why interstitial diffusion is normally more rapid than
vacancy diffusion.

3. A sheet of BCC iron 2 mm thick was exposed to a carburizing gas atmosphere on
one side and a decarburizing atmosphere on the other side at 675�C. After
reaching steady state, the iron was quickly cooled to room temperature. The
carbon concentrations at the two surfaces of the sheet were determined to be
0.015 and 0.0068 wt%, respectively. Compute the diffusion coefficient if the
diffusion flux is 7.36 x 10-9 kg/m2-s.

(conversion from weight percent to mass per unit volume) to convert the
concentration from weight percent to kilograms of carbon per cubic meter of iron.
Density of iron = 7.87 g/cm3).

Answer 1

1. Self-diffusion is atomic migration in pure metals—i.e., when all atoms exchanging positions are of the same type. Interdiffusion is diffusion of atoms of one metal into another metal.

According to IUPAC definition, self-diffusion coefficient is the diffusion coefficient Di* of species i when the chemical potential gradient equals zero. It is linked to the diffusion coefficient Di by the equation:

Here, ai is the activity of the species i in the solution and ci is the concentration of i. This term is commonly assumed to be equal to the tracer diffusion determined by watching the movement of an isotope in the material of interest.

Interdiffusion is the mutual diffusion of two fluids, especially through a membrane.