Materials Science II
Diffusion, Nucleation and Growth
1. A concrete dam has a base thickness of 50m and an
upper thickness of 20m. The dam is used to retain water which is found to
leak through it by diffusion. Calculate the flux of water through the base
and top of the dam in litres/day/m2. Would you expect the leak
rates to be higher in summer or in winter? [Assume D for water in concrete
The thickness of a carburised layer is 1.5mm after 6h processing at
950oC. How thick would the layer be if the processing time is
raised to 20h?
Steel can be decarburised by holding in an oxidising atmosphere at elevated
temperature as often occurs during annealing. Calculate the thickness of
the decarburised layer if the steel is held at 900oC for 100h
in a homogenising process. For C in g-iron
A solution for non-steady state diffusion relevant to the carburising process
is given by the following equation. For a 0.25%C steel calculate the time
needed to raise the carbon concentration to 0.8% at a depth of 0.5mm in a
carburising process where the surface carbon content is maintained at 1.2%.
You may assume that the carburising process takes place at 950oC
and the diffusion parameters are as in question 3.
The change in free work, W, during solidification as a function of precipitate
radius, r, is given by the following equation.Derive expressions for the
critical radius of the nucleus required for stable growth, r* and the critical
The driving force for solidification is given by the following equation.Calculate
the critical nucleus size for aluminium (melting point 660oC)
if solidification occurs at (a) 500oC and (b) 300oC.
[The latent heat of melting for Al is 5x108Jm-3 and
its solid surface energy is 0.5Jm-2].