|If two pieces of different metal are joined together as shown here - for example, copper and nickel, and they are then heated for a long time (but below their melting points), the atoms from the metals migrate, or diffuse into the other.|
|Concentrations of both metals vary with position, as shown in the graph. This process is known as interdiffusion, and is a time - dependent process. Click below to see this |
Mechanisms of Diffusion
|At an atomic level, atoms are arranged in a lattice pattern, e.g. as shown simply in the diagram. Diffusion is just the stepwise migration of atoms from lattice site to lattice site.|
|One type of diffusion involves the exchange of an atom from it's normal lattice position, to an adjacent vacant lattice site or vacancy. This is known as substitutional or vacancy diffusion. Of course, this process requires the presence of vacancies, and vacancy diffusion depends on the extent of vacancies in the material. It is represented in this animation. |
|The second type of diffusion involves atoms that migrate from an "interstitial" or "in - between" position, to a neighbouring one that is empty. This occurs with the infusion of impurities such as Hydrogen or Carbon, which have atoms that are small enough to fit into the interstitial positions. This process is called, as you might expect, interstitial diffusion. |
x is the distance from the interface you're looking at, and=0 at the surface or interface of the material.
The instant before diffusion starts, time is taken as zero, and
Before diffusion starts, all the atoms that will be diffusing are evenly distributed.
|t500=||(Dt)600||= (5.3 x 10-13 m2/s) (10 hours)||= 110.4 hours|
|D500||4.8 x 10-14 m2/s|
|D = A exp-Q/RT|
|In D = In A -Q/RT|