VEC's Modelling Themes
Modelling of materials and processes are performed at different length scales:
As a part of design: Design optimisation; New products; New processesFor research: Understanding, Predicting, Complementing experimentsFor training: Skill development; The “flight simulator” functionBelow please find further information on the VEC's Modelling Themes.
This topic is concerned with the critical assessment of thermodynamic and phase diagram data. Particular attention is being paid to the modelling of ternary and quaternary intermetallic phases.
These critically assessed data, when used with appropriate software, provide the means of calculating important properties of alloys during solidification such as the liquidus and solidus temperatures, the enthalpy change on solidification, the solid phases predicted to form and the segregation of elements between phases. The data will be essential to model undercooling necessary to crystallise specific phases and the segregation of elements on the surface of inoculants and grain refiners. This is particularly important for recycling of aluminium, where various intermetallic phases as a result of impurities such as iron (figure).
An isothermal section of the Al-Fe-Mn system at 800 K, showing three types of phase region; a blank white region where one single phase is stable; these single phase regions are separated from each other by two phase regions where sets of tie lines represent the compositions of the single phases in equilibrium; finally three two-phase regions meet together to form three phase regions, which are lightly shaded in the diagrams.
Atomic / Atomistic Simulation
The ab initio structural optimizations produce information about interface structural relaxation, local chemical bonding, etc. The result helps us to understand effects of structural factors on nucleation and the final products, as well as to obtain an insight into dislocation formation (top figure).
Molecular dynamics (MD) simulations are used to study the structure of liquid near the substrate / liquid interface during different stages of heterogeneous nucleation. MD simulations show that during prenucleation the liquid exhibits atomic ordering at the interface, in the form of both atomic layering and in-plane ordering, even at temperatures above the liquidus (bottom figure).
The electron density distribution (yellow is high, white/red is low/medium density) along the (1000) orientation of the interface of compressed Al atoms (silvery spheres) on an Al substrate (bottom), simulated by using an ab initio density functional theory (DFT) method.
Simulation of heterogeneous nucleation. The blue and yellow balls represent substrate and Al atoms, respectively. In this example, heterogeneous nucleation starts at ~340 ps and ends ~380 ps for a system with a misfit of -8% at 760 K.
Microstructure simulation is used to understand the correlation between the solidification process parameters on the one hand, and microstructural features, such as the grain structure, segregation pattern and intermetallic phase distribution, on the other hand. This is pursued by developing physically-based models to predict and better understand the formation of microstructures and defects in casting (with a strong accent on shape casting) and heat treatment processes. Examples of microstructure modelling are shown in the figures below.
Process simulation aims to shed light on the fundamentals of metallurgical processes at the macroscopic scale (mm upwards). Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) are used to simulate and hence optimise processes such as casting and forging in a virtual environment. This is done by linking the relevant field variables, such as temperature, stress, and velocity fields, to the design parameters.
CFD simulation of melt conditioning