Damage tolerance of Ni-based alloys at cryogenic temperatures

With a large global push to reduce emissions, there is a need to address aerospace fuel systems. In 2019, aviation accounted for 2.5% of global CO2 emissions [1]. This was predominantly due to the burning of fossil fuels within aircraft engines.

In an effort to combat these emissions, Rolls-Royce are exploring an alternative fuel: liquid hydrogen. While liquid hydrogen shows promising propulsion abilities, its storage within a fuel system prior to combustion is vastly different to that of fossil fuels, as it must be stored at cryogenic temperatures around -253C. For nickel-based alloys, materials often seen in fuel systems, material characteristics are well documented for high temperature applications, with excellent strength, corrosion and oxidation resistance making the materials ideal for hot engine components such as turbine blades.

However, there is limited literature on the properties of nickel-based alloys at cryogenic temperatures. This project aims to understand the mechanisms behind deformation and fracture behaviour in these alloys at cryogenic temperatures down to -253C, to aid Rolls-Royce in their full-scale liquid hydrogen fuelled engine ground test and evaluate whether current fuel system materials could withstand liquid hydrogen as an alternative fuel.

This project shall utilise a plethora of experimental techniques to build a novel comprehension of material behaviour. Initially, materials will be investigated at ambient and near-ambient temperatures, then test temperatures will be gradually lowered to cryogenic. It is expected that in situ (within its intended environment) neutron and X-ray diffraction experiments will be conducted at the Rutherford Appleton Laboratory - these will offer information on material structure and deformation mechanics.

Smaller scale lab testing will involve 'electron backscatter diffraction' (EBSD), which can map the strain in a material at the microscale, as well as micro-mechanical testing.

A large issue in the hydrogen industry is hydrogen embrittlement; as hydrogen is the smallest element by mass, it can easily penetrate materials used in its storage, causing the material to become brittle. This presents a major challenge for hydrogen's long-term storage, and it is anticipated that the latter years of the project will build upon the prior cryogenic testing to investigate embrittlement behaviour.

This project falls within the EPSRC 'Materials engineering - metals and alloys' research area, and is in collaboration with Rolls-Royce plc.

[1]: Hannah Ritchie (2024) - "What share of global CO2 emissions come from aviation?" Published online at OurWorldinData.org. Retrieved from: 'https://ourworldindata.org/global-aviation-emissions' [Online Resource]