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Title: Investigation of the nucleation of aluminium by atomic simulation
Author: Wearing, David
ISNI:       0000 0004 6423 5079
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The aim of this project is to investigate the nucleation of aluminium off the substrate titanium diboride, by using surface and interfacial energies derived from Density Functional Theory (DFT) calculations, and bulk energies derived from thermodynamic data & theory, to calculate the total Gibbs energy change according to classical nucleation theory. It has been known for a long time that TiB2 particles are essential for the nucleation of aluminium in industry. However, the detailed mechanism of how TiB2 aids nucleation is still not known. In particular, it is not known why there needs to be excess dissolved Ti in the melt. Ground state DFT calculations are performed to calculate the interfacial energies of four different systems at 0K: TiB2(0001)//Al(111) and TiB2(0001)//Al3Ti(112), for both Ti and B terminations of TiB2. The DFT calculations for the TiB2//Al interfacial energies are improved over the existing literature results, due to the consideration of strain at the interface, while the TiB2//Al3Ti interfacial energies have not been calculated anywhere else. The DFT results are augmented with thermodynamic data and classical nucleation theory to compare the Gibbs energy of formation of solid on the TiB2 substrate occurring via each of these 4 systems, as a function of titanium concentration in the melt. Thus an evaluation of the comparative likelihoods of the four candidate mechanisms for the nucleation of aluminium is made. Finally, the surface and interfacial energies of the TiB2(0001)//Al(111) systems are investigated at finite temperature, using Density Functional Perturbation Theory under the harmonic approximation, to take account of the vibrational phonon energies occurring at finite temperature. The temperature dependent interfacial energies of TiB2(0001)//Al(111) are estimated, and used to refine the evaluation of the nucleation mechanism of aluminium.
Supervisor: Horsfield, Andrew ; Lee, Peter Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral