In this thesis, different modelling techniques, including physically based molecular
dynamics simulations and empirical models using neural network architectures have
been used to address particular problems in the understanding of microstructural
development in iron-based systems. The two main areas of investigation are concerned
with the prediction of composition and mechanical properties of steel welds
and the effect of irradiation on the grain boundary microstructure of a-iron, both
very important industrial issues.
Microstructural evolution models in steel welds require weld metal composition as
their starting point. Extensive analyses have been carried out concerned particularly
with the prediction of weld metal chemistry, and also complex mechanical properties
such as toughness, using neural network techniques and a database developed for
one pass per side submerged arc welds typical of those used in the manufacture of
linepipe. The neural network techniques used were based on a Bayesian framework,
implemented using Markov chain Monte Carlo methods. The results showed a significant
advantage in the use of neural network models for prediction of toughness
compared with simpler regression analyses.
In order to study the effects of irradiation on the structure of Fe-based systems,
a molecular dynamics methodology was initially set up to study the equilibrium
relaxed atomic configuration of symmetric tilt and twist grain boundaries in a-iron.
These structures have been classified in terms of both the energy and width of the
grain boundary region and the atomic arrangement has been also analysed for the tilt
models in terms of structuraJ units. Radiation damage has then been studied near the
relaxed structures of a symmetrical tilt and a symmetrical twist boundary in a-iron.
Collision cascades have been initiated.inthe structure by imparting an initial energy
of 1 keY to a single Fe atom, i.e. a primary knock-on atom (PKA). The subsequent
interaction of the cascade with the grain boundary has been studied using molecular
dynamics simulations. As a result of radiation, reordering is produced in the atomic
structure of the boundaries, the damage being more pronounced in the twist model
studied. Clusters of interstitial atoms are produced at the boundary. Changes in the
properties of the interfaces after irradiation are discussed.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.