A Moving Cracking Story: Designing against Hydrogen Embrittlement in Titanium

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The global cost of corrosion-related damage is estimated to be £1.9tn annually (3.4% of GDP) and corrosion costs the UK ~£80bn per annum. Hydrogen-associated stress corrosion embrittlement is an important class of environmental degradation. Titanium alloys were until the late 60s considered immune to stress corrosion embrittlement by reacting with water vapour, but subsequent experience has falsified this hypothesis. Therefore, substantial industrial and safety benefit to the UK can be obtained if H-associated degradation in Ti alloys can be understood and mitigated by material design. Because of its ubiquity in the world, hydrogen related cracking is a grand challenge in materials science; from ceramics to perovskite solar cells H-associated degradation mechanisms are critical to the in-service viability of many materials, including metals. Our strategy will be to provide H-tolerance to a material, either by limiting the ingress of embrittling species or by providing traps within the material, where such species can be somehow deactivated. Hydrogen is highly mobile and therefore can concentrate and embrittle critical micro- and nano-scopic features in materials, this can happen over the course of minutes or hours. A main challenge however has been the detection of H inside metallic systems. Lacking an electron shell to excite, H cannot be measured in electron microscopy and vacuum systems often contain H, and so even mass spectrometry techniques struggle to sensitively measure H in a sample. Therefore, our understanding of how hydrogen leads to cracking in different materials systems is much more limited than we might like to concede. We will develop new methods for atomic-scale experimental measurements to identify where Hydrogen locates within a material. Small samples will be prepared and handled at cryogenic temperatures to limit H mobility and elemental "atom-by-atom" mapping will be conducted to understand how the mobility of H changes by trapping at different material phases, interfaces and crystal defects.Some Ti alloys are more resistant to Hydrogen embrittlement and corrosion than others, but the physical mechanisms behind are not well understood. For instance, highly pure titanium is nearly immune to H, but its corrosion performance drastically changes if small impurities are present; some elements, such as Fe, are known to reduce corrosion performance, whereas others, including Mo and Pd, dramatically improve corrosion. We will then carefully examine the effect of typical alloy additions on the cracking propensity using bend tests under H exposure in alloys with different compositions. Detailed microscopic inspection at several length-scales will be conducted to understand the mechanisms of H-induced failure. The prediction of H mobility and H-related damage in engineering alloys is complicated, as these materials contain several phases, crystal defects and alloying elements, which all influence H behaviour. With so many interacting effects, the use of physically-faithful models and simulations will be vital to disentangling them fully from each other. Therefore, we will develop new computational models for hydrogen diffusion within a material to elucidate how different features affect local H transport and trapping. In addition, we will adopt and improve micro-mechanics modelling techniques, via incorporating equations for the newly-unravelled embrittlement mechanisms in Ti, and compare the mechanical performance of H-containing alloys against their H-free version. Based on these outcomes, we will develop optimal material guidelines for the alloy and process designer, highlighting what phase/alloy combinations are more resistant against H-induced failure. In addition, optimal materials will be designed, manufactured and tested in order to provide final validation of our concepts.