Abstract due to its superior properties. The major

Abstract

Tungsten carbide is currently being investigated for use in high temperature applications. This review covers the crystal structure and thermodynamic properties of tungsten carbide, focussing on tungsten monocarbide due to its superior properties. The major deformation mechanisms that are expected to occur in the expected applications are also discussed. The current state of the literature is then reviewed and gaps in the understanding of high temperature mechanical properties are noted. Techniques that could be used to further this understanding are then described and finally, conclusions are drawn on the experimental work required in order to further the understanding of the high temperature behaviour of tungsten monocarbide.

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Introduction

The discovery of fossil fuels and their use as an energy source brought about the beginning of the industrial revolution 1. It completely changed the way in which we live today and allowed for vastly improved living standards. Since then, energy consumption has risen at a prolific rate. It is becoming increasingly obvious that our current means of energy production are completely unsustainable. Burning of fossil fuels has led to the production of greenhouse gasses, driving climate change and polluting air and water. Furthermore, fuel supplies are finite, and in the short-term often lack security of supply. With no end in sight to the rise in our energy consumption we must now look to other energy sources if we are to provide a sustainable future.

Nuclear fusion, if harnessed, could provide almost limitless carbon free energy. It could also resolve issues of proliferation and long-lived radioactive waste that have prevented nuclear fission from becoming more wide spread. In order for nuclear fusion to avoid the production of long-lived radioisotopes only materials with low activation properties can be used in components that will undergo neutron irradiation. This greatly limits materials choices, providing some unique challenges for materials scientists. One such challenge is to find a material that is suitable for shielding the superconducting magnetic components from neutrons produced in the reactor. It is crucial that these components are not irradiated by neutrons as neutron-induced heating or irradiation damage would seriously impact reactor efficiency and lifetime. A shielding material is therefore required that has excellent neutron stopping properties whilst not becoming highly active. It must also be mechanically stable at elevated temperatures and withstand large amounts of helium production. One material that has recently shown promise for this application is tungsten carbide, although its post-irradiation mechanical properties are little studied 2. Whilst the wide use of cemented tungsten carbides has led to extensive research in these materials, the properties of binderless tungsten carbide remain less understood. Tungsten carbide without a metal matrix has often been overlooked due to its lower fracture toughness leading to its poor wear resistance 3. This makes it an undesirable material in many engineering applications. It does, however, have superior chemical and thermal stability as well as hardness 3, 4. These properties may be more useful in a neutron shielding application than wear resistance.

The overall objective of this thesis work is therefore to examine the changes in properties caused by irradiation. Before this can be achieved, however, its mechanical behaviour in the un-irradiated state must be properly understood. This project will aim to explore this goal, with a focus on elevated temperatures. The rest of this document is therefore composed as follows: chapter 2 will cover structure and thermodynamic properties; chapter 3 will explain the different creep mechanisms; chapter 4 will then move into the mechanical properties, both at ambient and high temperatures; and chapter 5 will cover the techniques that have so far been employed in the literature to investigate these properties. The document will conclude by defining some experimental goals for the project in light of what is already understood.

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