Microstructural and material property changes in severely deformed Eurofer-97

TL;DR

This study investigates how high-pressure torsion severely deforms Eurofer-97 steel, significantly refining its microstructure and affecting its thermal and irradiation properties, with implications for nuclear reactor applications.

Contribution

It provides detailed microstructural and property analysis of Eurofer-97 after severe plastic deformation using HPT, including effects on grain size, dislocation density, and irradiation response.

Findings

Grain size reduced by a factor of ~30 after HPT

Dislocation density increased by over an order of magnitude

Irradiation caused a 20% reduction in dislocation density

Abstract

Severe plastic deformation changes the microstructure and properties of steels, which may be favourable for their use in structural components of nuclear reactors. In this study, high-pressure torsion (HPT) was used to refine the grain structure of Eurofer-97, a ferritic/ martensitic steel. Electron microscopy and X-ray diffraction were used to characterise the microstructural changes. Following HPT, the average grain size reduced by a factor of $\sim$ 30, with a marked increase in high-angle grain boundaries. Dislocation density also increased by more than one order of magnitude. The thermal stability of the deformed material was investigated via in-situ annealing during synchrotron X-ray diffraction. This revealed substantial recovery between 450 K - 800 K. Irradiation with 20 MeV Fe-ions to $\sim$ 0.1 dpa caused a 20% reduction in dislocation density compared to the as-deformed material. However, HPT deformation prior to irradiation did not have a significant effect in mitigating the irradiation-induced reductions in thermal diffusivity and surface acoustic wave velocity of the material. These results provide a multi-faceted understanding of the changes in ferritic/martensitic steels due to severe plastic deformation, and how these changes can be used to alter material properties.