Micromechanics of Creep in Structural Steels for Fusion Power

Reduced activation ferritic-martensitic (RAFM) nuclear grade steels are widely used across the nuclear industry in a wide range of structural applications. Typically in these environments they are exposed for long time periods (up to 80-years in future civil reactors) to high temperatures and radiation damage. Future plant designs for both fusion and fission power will likely increase both thermal and irradiation loadings.

This can result in degradation of mechanical properties through formation of new precipitates and phases which are not seen at shorter time periods at lower temperatures or without radiation damage. New alloy compositions are being sought to mitigate the impact of increasingly harsh service conditions. Simulation of this is difficult and so-called thermal aging treatments are used to accelerate the formation of phases.

Whilst much is understood about the chemical and microstructural changes how this affects the mechanical properties is less well understood. This project will develop in-situ high temperature micro-mechanical methods to study the degradation induced in these samples and compare it to conventional macroscopic data, produced by industrial collaborators at UKAEA.

The proposed project involves the application of a range of micromechanical techniques to specific nuclear steels; the mechanical properties of most interest are yield strength, work hardening behaviour, hardness and creep. A range of materials of key interest will be studied in the datum condition and after irradiated by ion implantation or test reactor exposure as appropriate.

Testing will be carried out from room temperature to reactor specific temperatures in vacuum (likely 350oC). Thermal ageing post irradiation exposure will be explored for selected samples. For the samples which show the most change or interesting results additional in-situ tests will be performed in the SEM, using a newly purchased Pico Indenter.

This will allow direct observation and local strain measurements to be made by developing robust methods for performing digital image correlation on these tests. Extraction of materials properties will also necessitate appropriate finite element simulations and matching outputs to experimental data.

Typical specimen length scales are sub-micron to tens of microns and test geometries include tensile tests, compression and micro-bending. This small scale offers radically reduced difficulty and costs associated with testing irradiated materials. The small size of the tests, however, creates a size effect in the results that must be understood in detail to maximise the usefulness of the micromechanical testing results.

Related methods have been used in other projects involving industrial collaborators studying both fundamentals of deformation. This project will continue the extension of these methods into creep regimes at service relevant temperatures and apply to a range of steels in both legacy and new compositions.

The project aligns with the EPSRC Energy and Engineering themes, within the advanced materials thematic area, and contributes to the engineering net zero strategic priority.