High-temperature microscale creep deformation behaviour of irradiated advanced structural steels (Materials Strand Project)

Supervisor/s – Angus Wilkinson and David Armstrong (University of Oxford). Viacheslav Kuksenko (UKAEA)

Availability of materials capable of withstanding the severe operational conditions of fusion reactors is among the main technological challenges for delivering fusion energy. Combination of extreme temperatures, mechanical stresses and bombardment by neutrons will leads to degradation of irradiated components including dimensional and volumetric changes that may compromise structural integrity of the fusion reactor. UKAEA is developing a new generation of reduced-activation structural steels, Castable Nanostructured Alloys (CNA) and Oxide Dispersion Strengthened Alloys (ODS), which should enable higher operating temperatures improving efficiency and allowing optimisation of reactor design for higher economic viability.

Stress-induced dimensional change intensified by mobile radiation-induced defects, so-called irradiation creep, is among the main factors which define the maximum allowable operating dose for structural steels for most operating scenarios at intermediate temperatures (up to about 700°C), therefore creep measurements will provide crucial information for qualification of the future materials for STEP and DEMO. Due to the luck of the capabilities of reproducing the extremes of neutron dose, temperature, neutron energy spectra, and extreme environment that materials will exhibit in fusion reactors, the surrogate such as ion irradiation is used to replicate irradiation damage in materials.

Typical sizes of irradiated layers are up to few tens of micrometres that makes impossible meaningful usage of the standardised creep testing with macroscopic specimen geometry for assessment of radiation damage effects on creep properties. Nano- and microscopic testing is demonstrated to be a useful tool for studying these effects on a local scale, allowing a detailed localized analysis of the deformation processes within the irradiated layers.

The proposed PhD will explore capabilities of the sub-sized creep experiments for assessment of the high-temperature properties of the advanced nanostructured steels and will consist of two main blocks – i) experimental microscopic creep measurements and validation with the standard size samples and ii) numeric modelling of microscale creep behaviours.

Part 1. Creep testing of irradiated samples.
In comparison to the room-temperature micromechanical mechanical, much less work has been carried out at elevated temperatures, where creep is among the major materials plastic deformation mechanisms. In order to study creep properties of the irradiated materials above room temperature, high-temperature spherical nano-indentation and micro-pillar compression testing will be used. Work in Oxford has demonstrated that these testing methods can be applied austenitic steels such as 316SS at room temperature. The systems at Oxford are able to conduct experiments in vacuum or other inert atmospheres at up to 950°C (well above current – or near future – reactor operating temperatures for structural materials such CNA or ODS). Micro-pillar testing will be used for studying changes in creep properties in a simple geometry with well-defined boundary conditions. Testing will be made using a range of tip sizes allowing for a study of the effect of ion irradiation on creep mechanisms.

For validation of the obtained results, the micro-scale creep measurement data will be compared to the data from the standard size samples available from the industrial and academic partners. Data from the ion-irradiated samples will be compared to the data from the neutron irradiated samples. The in-situ high-temperature stage integrated into a scanning electron microscope will be available in the MRF (UKAEA) to performed test on radioactive samples of ODS steel from the MARIA campaign provided by Oxford.

Part 2: Modelling of microscale creep behaviour
Creep of real fusion energy materials as steels is a very complex phenomenon controlled by several competing mechanisms including dislocation glide, climb, cross-slip, point defect diffusion, dislocation–particle interactions etc., therefore numerical simulation is vital for interpretation of the experimental results and understanding of the underlying mechanisms. Recent studies of the Oxford University led by Prof Tarleton successfully demonstrated that the three-dimensional discrete dislocation dynamics simulations provide an effective model to predict the creep behaviour of particle-strengthened materials at high temperature. This project intends to extend applicability of this approach toward irradiated materials and provide an excellent opportunity to maximise the project output.
It is anticipated that the student who works on this project will be based at the University of Oxford, however, will spend significant amount of time at UKAEA to conduct relevant experiments, such as testing of neutron irradiated samples.

This project is offered by The University of Oxford. For further information please contact: David.armstrong@materials.ox.ac.uk

This project is not compatible with part time study.