The aim of this experiment is to:
     1 . Study the phenomenon of Solid State Diffusion, with its different types.
     2 . Recognize the factors that influence solid state diffusion.
     3 . Observing and understanding the effect of temperature and heating time on he extent of diffusivity of Al in Cu.

Diffusion is defined as a mechanism of transfer of matter through matter. Such a phenomenon can happen in either solid, liquid or gaseous states. Yet, for both liquid and gaseous states, diffusion is rapid in comparison to the solid state diffusion due to the lower inter-atomic bonds and the openness of their structures. In solids, the situation is different due to the high packing of the atoms in the crystal structure and the strength of bonds between the atoms.

Still, Solid State Diffusion (SSD) can be induced through thermal agitation of atoms, thus increasing their vibration in their equilibrium positions. These induced vibrations allow some atoms to displace from their equilibrium positions to occupy other favorable positions. Examples of SSD are the precipitation of secondary phases in solid solutions, nucleation and growth of new grains in the re-crystallization of a cold worked metal, and case hardening using carbon (carburizing) and nitrogen (nitriding) diffusion.

There are two basic solid state diffusion mechanisms: vacancy (substitutional diffusion) and interstitial diffusion. Vacancy diffusion happens when atoms move from one position to another if the energy barrier between the two positions is recovered, which is termed "Activation Energy". This energy is provided through thermal agitation which allows the atom to move to other vacant positions created by vacancy defects in the lattice.

As shown in the schematic diagram figure (1), atom A can move between the two positions in case of recovering the activation energy barrier between the two positions. This example of vacancy diffusion is called "Self-Diffusion", because it happens within the same material, not due to the diffusion of an alloying atom within the lattice. It should be noted that the activation energy barrier equals to the amount of energy required to form a vacancy, plus the energy to move a vacancy. It could be inferred that the activation energy increases as the melting point increases since melting point is a reference to bond strength. Such a diffusion mechanism can still happen in solid solution, where the activation energy henceforth depends on the respective bond strength and melting point of the other atoms.

Figure (1) Activation Energy in Vacancy SSD After W. F. Smith 1996

In general there are five basic factors that affect the nature, type and the intensity of diffusion, which can be summarized as:

1. Type of diffusion mechanism: whether interstitial or vacancy/substitutional: interstitial diffusion favors the diffusion of small atoms (hydrogen, carbon, nitrogen, oxygen) , and it is possible to diffuse between interstitial sites with less energy to break the energy difference. For substitutional diffusion, it happens as the atomic radii sizes of the solvent and the solute are similar.
2. The temperature at which diffusion takes place: higher temperature indicates higher diffusion rates. This is because higher temperature increase the thermal vibration of the atoms about their equilibrium sites, thus allowing more diffusion to happen. Yet, this depends in general on the melting temperature of the solute and the solvent.
3. Type of crystal structure of the solvent lattice: BCC have less packing factor (0.68), which indicates possibility of higher diffusion under same conditions when compared to FCC (0.74).
4 . Type of crystalline imperfections: for example, grain boundaries which are locations of atomic mismatch are open zones for diffusion. In addition, increase in vacancies increases the possibility of substitutional diffusion.
5 . Concentration of diffusing species: the higher the concentration of the species inside the atom makes it more difficult for more atoms to penetrate inside it.

In fact, diffusion of atoms increases the material strength by creating highly stressed regions, which impede the motion of dislocation. Yet, the accumulation of these diffused atoms in a less mannered way could cause diffusion and movement of vacancies, thus creating cracks or weak stress zones in the structure. Therefore, further understanding for the factors should be employed to make the best use of the phenomenon of diffusion.

1. 81 mm long strips (3.25x20 mm²) were used, made of pure aluminum and copper.
2. The surfaces were ground using 400 and 600 grits, and polishing with alumina powder.
3. Each two strips were joined together with ground sides touching each other, with tight fastening using steel wires.
4. Specimens were heated at 550C for 4 and 8 hours, followed by water quenching.
5. After heat treatment, the specimens are prepared and mounted for microstructural investigation.

Optical Microscope

Results/Discussion

--The following sites and books contain useful information and images for several articles that we have encountered in this experiment. Make sure to check them.

Kalpakjian, Serope. Manufacturing Process for Engineering Materials. Addison Wesley, 3^rd Ed., 1997.

Smith, William F. . Principles of Materials Science and Engineering. McGraw Hill, 3^rd Ed., 1996. (p. 128-132)