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Title: Efficient atomistic approaches to thermodynamic quantities for solid-liquid equilibria in alloys
Author: Angioletti-Uberti, Stefano
ISNI:       0000 0004 2691 1283
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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Two important thermodynamic quantities are bulk solid and liquid free-energy as a function of composition G(x) and the solid-liquid interfacial free-energy γsl. For both, an accurate determination is required for modelling crystallisation and melting processes. In this thesis, I combine statistical mechanics and atomistic simulations to develop new approaches to calculate G(x) and γsl. In the case of G(x), a method based on a Free Energy Perturbation (FEP) technique is proposed, which allows to achieve ab initio accuracy at a fraction of the cost of previously proposed techniques. A proof-of-principle of this approach is given using simple many-body potentials. The case of the melting point calculation for a pure element and that of the free-energy for a binary Ni-Al alloy are discussed. Based on simplified theoretical models, the reasons for the success of this approach and its limitations are explained and guidelines for future, full ab initio calculations are given. For the case of γsl, it is proposed to use the Metadynamics (MTD) technique to reconstruct the Free Energy Surface (FES) for the solidification/ melting process, from which γsl can be extracted. This approach is first presented and discussed using a model Lennard-Jones potential. The robustness of this method is demonstrated and its advantages over other techniques are discussed, together with its limitations and possible ways to extend its use to more complex energy descriptions. The method is then applied to the case of Pb as described with a more realistic Embedded Atom Model (EAM) potential, and the results are used to assess experimental data. Given the promising results shown by these novel techniques, their use to build the foundations of a multi-scale approach to solidification and their application with more realistic calculations and complex problems can be envisaged in the future.
Supervisor: Lee, Peter Sponsor: EPSRC ; Royal Academy of Engineering ; MACAN consortium
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral