Defect dynamics in energy materials

609d471d-0a7b-4e57-9f56-b93293c97c08

Closed

EPSRC
EPSRC
EPSRC
EPSRC
EPSRC

£4,082,950

March 31, 2018

Sept. 30, 2023

Advanced materials form the cornerstone of many emerging technologies, from next-generation energy production, transport and defence, to prosthetics and targeted drug delivery. Some of these eg. fusion energy require materials that do not yet exist, because the operating environment is so ferocious: high temperatures, corrosive environments and intense radiation mean no currently available material can be used. Theoretical modelling and predictive computer simulations are crucial steps in the development of new materials, since they can provide deeper understanding of the complex processes at work, and reduce lead times in product development. All modelling and simulation techniques are based on approximations, which limit their range of applicability. Whilst they have served well in the past, the extreme conditions mentioned above mean that some of these simplifying approximations no longer apply, and new techniques are required. The aims of this project are to develop new modelling approaches and simulation methods that are capable of handling the conditions, and apply them to unsolved problems in nuclear materials science.

The most precise simulation methods currently available track every atom in the system. Although they can be very accurate, the computer power required to run them means they can only model a few cubic nanometres of material for a few nanoseconds. This cannot capture the large-scale, long-time processes that control material performance, and eventually decide, for example, how many years a nuclear reactor can be safely run before it needs to be replaced. At the other end of the scale, computer-aided design programs simulate reactor-sized components, but base this on simple rules on how materials behave. Ideally, these would be derived from microscopic simulations, but there is a huge gap in length and time-scales between them. The mesoscale simulations that this project will develop aim to bridge that gap.

Over the last 60 years, particle physicists have developed powerful mathematical tools to understand quantum fluctuations. These tools can be modified to treat thermal fluctuations instead, and this will form the foundations of the new simulation methods. Instead of following every atom in the system, the new techniques will identify only the degrees of freedom that play important roles in the evolution of the material over time. These are the defects: impurity atoms, vacancies and self-interstitials (formed when atoms are knocked out of place in the regular lattice of eg. a metal) and dislocations (defect lines whose motion controls deformation).

Though the new methods will be widely applicable, this project will focus on 3 case studies. This will answer technologically important questions, as well as testing the new techniques. The first case study concerns the clustering of Re atoms in W. Under the intense radiation of a fusion reactor, up to 5% of W atoms will transmute into Re. According to currently available modelling, the Re atoms should disperse through the W, yet experiments show clusters form. These clusters cause the material to become brittle, limiting its useful lifetime. The first case study will apply the new simulations to understand this. The second concerns the behaviour of dislocations under irradiation. This can be very different from their usual behaviour, and will strongly affect the mechanical properties of reactor materials. Current simulation methods ignore the single-atom defects, but these are crucial for understanding radiation effects. The new methods will track both kinds of defect, and help provide the understanding needed to mitigate and control them. The final case study will investigate the interaction of C atoms with dislocations. This is the process that makes iron into steel, and its importance can hardly be overstated. Although identified decades ago, important unanswered questions remain, and the new tools this project aims to develop will answer them.

Potential Impact:
The research performed during this fellowship will deliver far-reaching impact across academic and industrial science and engineering.

Academic: The principal academic impacts will be twofold. Firstly, the project will deliver fully quantitative, predictive simulations of the complex, out-of-equilibrium processes that control microstructural evolution in materials under irradiation, and their effects on mechanical properties. This will represent a major increase in our fundamental understanding of the behaviour of structural and functional materials under nuclear reactor conditions. Secondly, the mathematical and computational tools that will be developed to enable the simulations will be applicable to a wide range of material systems beyond the nuclear-focussed case studies that will be investigated in detail. These innovative tools, and their implementation in hybrid simulations, have transformative potential for mesoscale materials simulations.

Industrial and societal: Reliable predictive modelling benefits industry by accelerating product development and reducing the time delay between fundamental science and technological implementation. Computer simulations allow for fast screening of alloy compositions, explanations of experimental data, and optimization of experimental campaigns to test new materials. The mesoscale tools developed during the fellowship will be particularly important, because they will bridge the gap in quantitative understanding between accurate yet limited atomistic methods, and large scale computer-aided design. Letters of support from Rolls Royce, Culham Centre for Fusion Energy, and Materials Design demonstrate that the proposed research is of great interest and relevance to a range of commercial and industrial science and engineering organizations.

The benefits to wider society of advanced nuclear energy generation are clear, and the innovative simulation techniques this project creates will form an important step on the road to its delivery. Furthermore, graphical outputs from the simulations, such as animations depicting microstructural processes, are a powerful tool for visualization and research communication.

Wider academic: The implementation of the new simulation methods first requires a number of conceptual and technical barriers to be surmounted. This will involve deriving new transition rate formulae which take into account system memory and inertia, optimizing numerical methods for handling diffusion through periodic crystals, and developing algorithms capable of accurately simulating the stochastic motion of extended objects. Though they will initially be applied to crystal defect motion relevant to materials for nuclear energy, these advances will eventually be applicable to a wide range of physical and biological systems.