Modelling Microstructural Evolution in Materials Science

Abstract

<p>Continuum atomistic and mesoscopic models are developed and utilized in the context of studying microstructural evolution and phase selection in materials systems. Numerous phenomena are examined, ranging from defect-solute interaction in solid state systems to microstructural evolution under external driving conditions. Emphasis is placed on the derivation and development of models capable of self consistently describing the intricate mechanisms at work in the systems undergoing these phenomena.</p> <p>Namely, grain growth dynamics are studied in nanocrystalline systems under external driving conditions using a newly developed phase-field-crystal model, which couples an additional free energy source term to the standard phase-field-crystal model. Such external driving can be attributed to incident energetic particles. The nanocrystalline system is found to be susceptible to enhanced grain growth as a function of the intensity/flux associated with the external driving and the energy of driving. Static kinetic phase diagram calculations also seem to confirm that systems under external driving conditions can be forced into long metastable states.</p> <p>Early stage solute clustering and precipitation in Al alloys is also examined with a variant of the phase-field-crystal method, so-called structural phase-field-crystal models for multi-component alloys developed as part of this thesis. We find that clustering is aided by quenched-in defects (dislocations), whereby the nucleation barrier is reduced and at times eliminated, a mechanism proposed by Cahn for a single dislocation for spinodal systems. In a three-component system, we predict a multi-step mechanism for clustering, where the nature and amount of the third species plays an important role in relieving stresses caused by the quenched-in dislocations before clustering, i.e., segregation as predicted by the equilibrium phase diagram, can occur.</p> <p>Finally, we present a new coarse-graining procedure for generating complex amplitude models, i.e., complex order-parameter phase-field models, derived from phase-field-crystal models. They retain many salient atomistic features and behaviours of the original phase-field-crystal model, however is now capable of describing mesoscopic length scales like the phase-field model. We demonstrate the scheme by generating an amplitude model of the two-dimensional structural phase-fieldcrystal model, which allows multiple crystal structures to be stable in equilibrium, a crucial aspect of proper multi-scale modelling of materials systems. The dynamics are demonstrated by examining solidification and coarsening, peritectic growth, along with grain growth and the emergence of secondary phases.</p>