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Peyman Saidi Winter 2013

Molecular Dynamics Study of Solidification in the Aluminum-Silicon System Supervisor: Dr. Jeffrey J Hoyt. Peyman Saidi Winter 2013. Motivation (Importance of Aluminum Alloys). Aluminum – Mineral Commodity Summary 2011. Motivation (Quench Modified Aluminum Silicon Alloy).

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Peyman Saidi Winter 2013

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  1. Molecular Dynamics Study of Solidification in the Aluminum-Silicon SystemSupervisor: Dr. Jeffrey J Hoyt Peyman Saidi Winter 2013

  2. Motivation (Importance of Aluminum Alloys) Aluminum – Mineral Commodity Summary 2011

  3. Motivation (Quench Modified Aluminum Silicon Alloy) Al-Si Eutectic 20μm/s Al-Si Eutectic 250μm/s Al-Si Eutectic 950μm/s T. Hosch, et al. Material science and engineering A 528 (2010) 226–232.

  4. Advacancy Growth of Silicon crystals Kink Temperature Step Melting Point

  5. Twin Plane Reentrant Edge Mechanism (TPRE) Twin Planes Twin Planes Twin Planes Twin Planes D. R. Hamilton and R. G. SeidenstickerJ. Appl. Phys. 31, 1165 (1960) • Propagation mechanism in Silicon dendrites is based on interaction of twinning. • Quench modified fibrous silicon is twin free. What is the growth mechanism of quench modified silicon in Aluminum-Silicon alloy? What is the critical condition for the transition from anisotropic to isotropic growth for Silicon dendrites? What is the magnitude and anisotropy of step kinetic coefficient, free energy and the stiffness of the interface? What is the effect of twins on kinetic parameters at different undercooling and compositions?

  6. Molecular Dynamics Method: • Newton’s equations of motion: • Where are we? Stability of System • What do we need to run a MD simulation? G=H-TS A model describing all interactions in the system H: Heat Content S: Randomness H=U+PV Implementing model in MD code U: Internal Energy U=K+E Potential energies for all interactions E: Potential Energy Interaction of atoms Cutoff distance K: Kinetic Energy Vibration Rotation Translation Aluminum Silicon Al-Si

  7. Molecular Dynamics models: Aluminum (Embedding Atom Method) • Aluminum • FCC Crystal structure, metallic bond M. S. Daw and M. I. BaskesPHYSICAL REVIEW B 29, 6443 (1984)

  8. Molecular Dynamics models: Silicon (Stillinger-Weber) • Silicon • Diamond crystal structure, covalent bonds F. H. Stillinger and T. A. Weber PHYSICAL REVIEW B 31, 5262 (1985)

  9. Molecular Dynamics models: Aluminum Silicon Interactions (AEAM) • Idea: • Reformulation of Embedding Atom Method by extracting a three body term from the Embedding functional in order to make these two methods (EAM and SW) compatible with each other. A. M. Dongare et al. PHYSICAL REVIEW B 80, 184106, 2009. Implementing A-EAM Aluminum Potential Silicon Potential Al-Si Potential

  10. Silicon Potential (Stillinger Weber)

  11. Silicon Potential (Modified Stillinger-Weber) • The difference between cohesive energy of diamond and wurtzite structures should be the same as the value calculated with DFT. • Modification should not affect the characteristics of the Stillinger-Weber potential which accurately predicts silicon properties. Diamond structure was stabilized by taking into account interaction with the third neighbor.

  12. Silicon Potential (Modified Stillinger-Weber)

  13. Aluminum-Silicon Potential Target: Aluminum-Silicon Potential should predict phase diagram accurately. Pure Components: Gibbs-Helmholtz relation Alloy: The alloy’s free energy as a function of composition Semi Grand Canonical Monte Carlo (SGCMC) simulation: random Si atom is switched to Al Atom Switches back T, C, P=0 U1 U2-U1 P. Sindzingre, D. Frenkel. Chem. Phys. Lett. (1987) 35–41.

  14. Results and Future Work • So far • Angular Embedding atom method was implemented on Molecular Dynamics code • A modified potential for Silicon was proposed • Current work • Making an accurate potential for Al-Si interactions • Studying and modeling nucleation and solidification growth • Studying the crystalline anisotropies of the interfacial free energy. • Characterizing the magnitude and anisotropy of the step kinetic coefficient. • Examining a faceted to non-faceted transition of the interface by changing undercooling. • Developing a model for the nucleation and growth phenomena. • Studying the effect of twinning on crystal growth • Studying growth rate as a function of undercooling on twinned Si crystals in contact with liquid. • Studying growth rate as a function of composition on twinned Si crystals in contact with liquid.

  15. Thank You

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