Modelling microstructural and interfacial phenomena in welding and irradiation processes for Fe-based systems

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.