Using molecular dynamics (MD) and boundary element method (BEM), different aspects of solidification in the aluminum silicon system are studied. The angular embedding atom model (AEAM) was implemented on LAMMPS, and the necessary potentials are developed. Firstly, a modified version of the Stillinger-Weber (SW) interatomic potential for pure Si is proposed. The advantage of this potential is that, in contrast to the original SW form, the modified version allows one to grow diamond cubic crystal structures from the melt at high temperatures. Additionally, an Al-Si binary potential of the AEAM type is able to accurately predict the experimental enthalpy of mixing. It is also able to predict an Al-Si phase diagram with a eutectic concentration for the liquid that agrees with experiment within 4 at% and a eutectic temperature that differs from experiment by only 13 K.
Considering the importance of step mobility and step free energy on the solidification growth rate, chapters 3 and 4 are devoted to calculation of these concepts using MD simulations. In chapter 3 the step mobility, which is the proportionality constant between the velocity and driving force, was determined for the alloy with melt composition of Al-90%Si as a function of temperature and composition. It was found that mobility decreases fairly rapidly with the addition of Al solute. Also, from the variation with temperature, it appears the mobility is proportional to the interdiffusion coefficient in the liquid. It is observed that for the Al-60%Si alloy diffusion-controlled growth is the dominant scenario, even for a few degrees of undercooling. In chapter 4 equilibrium molecular dynamics (MD) simulations and the capillaryfluctuations method (CFM) are employed to calculate crystal-melt step free energies
at three different melt compositions. Anisotropy of steps are investigated by setting up the systems with different crystal orientations of steps on the high-symmetry interface plane, (111) in this case. A complete isotropy of step free energy is observed for Al-60%Si and Al-90%Si alloying systems, while CFM failed in determining step free energy in Al-30%Si due to lack of step roughness. In chapter 5 the BEM is utilized to numerically compute the concentration profile in a fluid phase in contact with an infinite array of equally spaced surface steps. In
addition, under the assumption that step motion is controlled by diffusion through the fluid phase, the growth rate is computed and the effect of step spacing, supersaturation
and boundary layer width is studied. BEM calculations were also used to study the phenomenon of step bunching during crystal growth and it is found that, in the absence of elastic strain energy, a sufficiently large perturbation in the position of a step from its regular spacing will lead to a step bunching instability. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/18358 |
| Date | 11 1900 |
| Creators | Saidi, Peyman |
| Contributors | Hoyt, J. Jeffrey, Materials Science and Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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