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Title: Computer modelling of microstructure and microsegregation in multicomponent aluminium alloys
Author: Jarvis, D. J.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2001
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A novel 3D computer model has been developed that simulates the microstructural evolution and microsegregation patterns of multicomponent aluminium alloys. The model operates within a microscopic domain and under non-equilibrium conditions. As a demonstration the model has been applied to Al-Cu-Si and Al-Cu-Mg ternary alloys. The three essential components of the numerical model are a cellular automation (CA) model for microstructural growth, a finite difference (FD) model for solute diffusion and coupling with a thermodynamic software package (ThermoCalc) for phase transformation data. The main objective of this research programme was to numerically simulate solute transport in a solidifying multicomponent alloy, in order to improve understanding of microsegregation. This information is critical to materials engineering, since microsegregation has implications for the mechanical, chemical and physical properties of alloys. The formation of brittle non-equilibrium phases can have deleterious effects on tensile strength and fatigue life, while coring can seriously affect corrosion resistance and the response to post-solidification heat treatment. The 3D CAFD model has been successful in simulating the multicomponent multiphase microstructures observed in real ternary aluminium alloys. Realistic branched dendritic structures have been generated using the model, both for columnar and equiaxed dendritic growth. Typically, the predicted solidification sequence was dendritic α(A1) growth, followed by binary eutectic growth, followed by ternary eutectic growth. Predicted results of constituent proportions and secondary dendrite arm spacings, over a range of cooling rates, are generally consistent with experimental data from the literature. However, discrepancies were found between predicted and experimentally-obtained copper profiles in the α(A1) for an A1-3.95Cu-0.8Mg alloy, which could be attributed to experimental inaccuracies, over-simplifications in the model and inaccurate diffusion coefficients and/or equilibrium phase diagram data. From the predicted results, it has been demonstrated that microsegregation in multicomponent aluminium alloys is primarily influenced by alloy composition, solidification conditions, back-diffusion of solute during primary and monovariant eutectic freezing, thermophysical data (i.e. diffusion coefficients and equilibrium phase diagram data), dendrite geometry and the degree of solute mixing in the liquid. It is hoped that the 3D CAFD model, which is at an advanced stage of development, will also be used to quantitatively examine other transport phenomena that occur during dendritic growth. These phenomena include the permeability of consolidated equiaxed dendrites and the thermal conductivity of two-phase solid/liquid mixtures. At present, the experimental measurement of these phenomena, as a function of time and varying fraction solid within a continuously evolving dendrite structure, is not feasible. The 3D CAFD model, therefore, provides a viable alternative to experimentation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available