Hydrogen Embrittlement Mitigation by Engineering Grain Boundary Composition (HEnGB)

When exposed to water vapour, aqueous solutions or hydrogen gas, metallic components absorb hydrogen. The mobility of hydrogen and its interaction with the inner structural features of metals cause a significant decrease in their resistance to fracture and fatigue. This phenomenon, known as hydrogen embrittlement, has been responsible for numerous sudden catastrophic failures in metallic infrastructure. Moreover, this embrittlement effect poses major challenges to developing a safe and economical large-scale hydrogen infrastructure, which is critical to achieving net zero carbon emissions. This New Investigator Award, HEnGB, aims to deliver a new path for manufacturing alloys resistant to hydrogen. The structure of metals is formed by grains, the intersections of which are higher energy interfaces, called grain boundaries (GBs). The accumulation or segregation of hydrogen atoms at GBs can cause cracking of these interfaces in the most relevant engineering metal systems, e.g., iron and steel, nickel, and aluminium. This is seen macroscopically as a sudden brittle fracture of components. Increasing the resistance of GBs to hydrogen would thus enable a breakthrough in alloy engineering for hydrogen applications. Previous studies have shown that changes in the character of GBs result in improvements in resistance to hydrogen embrittlement. However, this strategy has scaling problems, due to the complexity, cost and energy consumption of the thermomechanical processes needed to change GB character. Recent atomistic simulations have revealed that the strength of GBs to hydrogen can be modified depending on the amount and types of other elements that also accumulate at them. Exploiting these interactions of elements at the GBs, known as co-segregation, has the advantage of being controlled simply by heating at moderate temperatures, a process that requires less energy/CO2 and is cheaper than the one required to change GB character. HEnGB aims to prove that co-segregation is a viable route to manufacture alloys that are less weakened by hydrogen, through a state-of-the-art methodology that will bring fundamental new insights into hydrogen-metal interactions. The research plan is designed to provide lab-scale validation of this co-segregation strategy, using a combination of cutting-edge experimental and modelling techniques. It will focus on lab-cast steel and nickel alloy samples with varying concentrations of the most promising elements for GB strengthening. The composition of GBs will be modified with tailored heat treatments. State-of-the-art microscopic characterisation of the grain structures produced will be performed. This will be followed by quantification of the co-segregation of hydrogen and other elements at GBs, a challenging milestone, using the world-leading cryogenic microscopic facility recently acquired by Imperial. Finally, new multiscale hydrogen embrittlement tests and models will be developed to identify the best alloy candidates with enhanced hydrogen embrittlement resistance. The successful lab demonstration of the co-segregation strategy will bring new science and a scalable engineering solution to the hydrogen embrittlement problem. The project will result in fundamental new knowledge in segregation kinetics, grain-boundary cohesion, hydrogen cryo-microscopy and micro-mechanical testing. This will benefit academics from alloy manufacturing, physical metallurgy, materials science, hydrogen embrittlement, and fracture mechanics research communities. Moreover, HEnGB’s vision can be scaled up from the nanometric to the macro scale and therefore have a short-term impact. This is because the GB analyses will be related to the compositional-thermal history of alloys, which can be implemented in the next industrial phase in collaboration with the project’s industrial partners.