SimuLab 6: Properties of Alloys at Nanoscales

The mechanical, electrical and magnetic properties of the material depends on the way of its preparation. From the old days smiths know that in order to make steel hard it should be quenched. If instead it is anhealed i.e. cooled down at slow rate it becomes plastic. Today it is possible to study the alloys under electron microscope. Israeli scientistic Yossi Lereah looked at the thin films of amorphous alloy of Ge and Al under high-power electron microscope (See Fig .2.9). When he heated his sample to 4000 he saw the beautifull flowers (See Fig .2.9) of Ge start to grow within the patches of Al that appeared all over the place. This process is know as phase segregation.

figures/alloyd1.jpg figures/alloyd2.jpg
Figure 2.9: .

But in the his process, phase segregation happens together with crystallization of both compounds Al and Ge. The Al crystal start to grow from the amorphous phase as soon as the temperature goes above the glass transition temperature, from little crystals of pure Al that are always present in the amorphous phases. The problem is that Al crystals cannot grow if the concentration of Ge exeeds certain solubility limit. What is amazing in Lereah experiment is that Ge can diffuse inside the crystal and precipitate inside to form its own crystallite aggregate. This process is known as diffusion limited aggregation which results in branchy aggregates, sometimes ressembling trees or bacteria colonies. Our Universal program allows you to simulate formation of amorphous substances in quenching the crystallization together with phase segregation during diffusion limited aggregation. in sergey/yossi.html you will be able to see a simulation of this process. Initially the lattice is filled with green and red pixels of ceratin concentration, representing Al and Ge atoms respectively. At the top of the layer of Al crystal (white) and Ge aggregate (pink) the Al crystal start to grow from the amorphous alloy boundary and occasionaly Ge crystals if the concentration of Ge inside Al is larger than certain solubility limit. No growth at this point of the interface can occur until Ge atoms diffuse throw the Al crystal and precipitate at the ge aggregate inside. See the pink Ge branches growing inside the white Al crystal.


You will be able to:

a) Construct your own experiment to simulate the Growth of Al-Ge crystals from the amorphous thin film.