A Moving Cracking Story: Designing against Hydrogen Embrittlement in Titanium

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.

The work will substantially impact the UK aeroengine industry by providing novel solutions for H-associated stress corrosion embrittlement. More efficient engines will operate at higher stresses driven by higher compression ratios that drive fuel efficiency. Such conditions can accelerate hot salt stress corrosion in existing Ti alloys, e.g. in IP disc bolt holes, and new material/microstructure design strategies will be required for its mitigation. Consequently, the need for accurate understanding of environmental degradation in engineering alloys and their successful implementation into material design and in-service maintenance is paramount for the UK to remain technologically competitive in a highly-demanding industry. Our coupling of microstructure-oriented modelling with advanced characterisation capabilities will bring a unique approach to developing new Ti alloys from concept to final product. The microstructural design strategy will be useful for material designers to understand which alloys are more resistant against hydrogen-associated stress corrosion embrittlement, whilst achieving optimal mechanical performance in safety-critical components.
Significant value will be added to the UK engineering industry by providing tailored solutions and tools to global problems. The cost of corrosion-related damage is estimated to be 3% of GWP and corrosion costs the UK ~£80bn per annum. Up to 35% of this cost can be reduced by adequate measures against corrosion, including effective in-service material inspection. The work will help to define accurate descriptions and guidelines for material lifing under H-rich environments, which will be useful for effective anticipation and planning of part inspection and repairs. Smart material applications to corrosion management such as those developed here will provide insight applicable more broadly to auto manufacturing, where corrosion and H embrittlement are limiting for advanced high strength steels; to oil and gas where both marine environments and the products themselves are corroding; to shipping and naval applications where the British Navy alone spends >£300m/yr managing corrosion and where due to Lloyds the UK has a strong marine services presence globally.

The proposed research activities are designed for generic applicability across other safe-critical industries and materials, such as in the Nuclear and Chemical Processing plants. Hydrogen embrittlement is a major concern in the Nuclear industry and it not only affects Ti, but is a primary problem in zirconium alloys used as cladding material and austenitic stainless steels in pressure vessels; these materials are exposed to highly corrosive environments from high-pressure water steams. Similar to Ti, H-rich Zirconium can fail either by localised pressure of molecular hydrogen in voids or by forming brittle hydride phases, and these mechanisms are essentially controlled by H diffusion. The programme can help to identify optimal microstructures, with their associated processing routes, to prevent/delay embrittlement by controlling H transport, e.g. by producing tortuous trapping landscapes when modifying microstructural arrangements or by preventing H ingress into regions susceptible to embrittlement through microstructural/compositional tailoring.

The work will impact significantly the academic community by providing new insights on the role of microstructure in H diffusion and embrittlement in multi-phase alloys. The correlative characterisation strategy for H trapping proposed here will benefit the Ti community by incorporating new aspects of the microstructure not being considered before, whereas the models will be applicable to different materials without the need of new fitting parameters. Therefore, we will be able to answer grand questions in Ti including, where does hydrogen go and what are the traps in Ti? How does H embrittle, given HELP? Or how do alloying effects work on H embrittlement?