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.

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 http://polymer.bu.edu/ 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.

~ You see 1000 particles of two different types, red (A) and blue (B) patched in a dense triangular crystal of sixe 200 x 200. Each atom has a hard core distance r_AA = r_AB = r_BB = 6.6.


~ You can simulate different process using different initial configuration. You can simulate different processes using this initial configuration. You can modify the initial configuration by

1. Changing the SYSTEM SIZE to a larger value (You cannot make the system size smaller than 200).

2. Changing the masses of A and B in the field C.ATOM TYPES.
3. Changing the diameter of the visible diameter of the atoms on the screen (3.3).
4. Changing the interaction parameters r_AB, R_AB and eAB in the field of D.NON-ELASTIC COLLISIONS. Note you cannot increase r_AB but only decrase it. You cannot make R_AB < r_AB. Don't make R_AB too large ( >> 20) because the simulation will be too slow. There is no reason also to make the absolute value of e_AB too large than 10 or less than 0.1. All interesting phenomena happens in the range of temperatures T < |e_AB|_max
5. You can make the change in the composite of alloys by changing types of atoms in H.LIST OF ATOMS.


~ After you change the initial file Alloy.txt:


1. Open Universal Application. Press on Open Simulation and select Alloy.txt. Change the temperature to 10 to randomize the initial configuration and wait 100-1000 units time. save the text file as a new starting file


2. Quench the random initial configuration to low temperature 0.1, to do that Press Stop- Change Temperature, set new temperature in the field `temperature' and press Ok . Check Heat Batht. Click Heat Bath Options and make sure that the heat-bath is 0.1 and the heat exchange coefficient has value 1.


3. Start to record the movie of your simulation selecting Save Movie from the menu and averaging for simulations selecting Save Data. The time step of the movie should not be too small, otherwise you will fill up the disk space. The quenching time (before the potential energy of the system stops changing) is about 10000-20000 unit time so the time step of the movie should not be less than 10-20 steps (1000 frames of the movie of 1000 atoms will take 6 MB). The same is true for the Data fill. Moreover if you make time interval too small the data will fluctuate a lot. Averaging over time interval which is performed while saving data reduce the temporal fluctuations. You can perform time averaging while running your simulation for arbitrary time interval by selecting Averaging for simulations from option menu. Press reset in order to start averaging for a new time interval. The parameters that you might calculate are temperature, volume, pressure, mass, density, potential energy, total energy, N⁰ of type 1, N⁰ of type 2, Kine E of type 1, Kine E of type 2, density of type 1, density of type 2.


4. After quenching your system for certain time (the potential energy of amorphous stops to decrease) you may start to annheal it by raising the heat bath temperature to e.g 0.3 and setting the heat bath coefficient to e.g 0.001.


~ In such a way we prepare different movies we present here.


5. See our movies.