A Moving Cracking Story: Designing against Hydrogen Embrittlement in Titanium

据估计，全球每年与腐蚀相关的损失为1.9万亿英镑（占GDP的3.4%），英国每年的腐蚀成本约为800亿英镑。氢相关应力腐蚀脆化是一类重要的环境退化。直到60年代末，人们还认为钛合金不会因与水蒸气反应而产生应力腐蚀脆化，但后来的经验证明这一假设是错误的。因此，如果可以通过材料设计来理解和减轻钛合金中h相关的降解，那么英国可以获得实质性的工业和安全效益。由于氢裂解在世界范围内的普遍性，氢裂解是材料科学领域的一个重大挑战；从陶瓷到钙钛矿太阳能电池，氢相关的降解机制对包括金属在内的许多材料的使用可行性至关重要。我们的策略是为材料提供h耐受性，要么通过限制易脆物种的进入，要么通过在材料内部设置陷阱，在那里这些物种可以以某种方式失效。氢是高度可移动的，因此可以在材料中集中和脆化关键的微观和纳米级特征，这可能在几分钟或几小时内发生。然而，一个主要的挑战是探测金属系统中的H。由于缺乏激发的电子壳层，氢不能在电子显微镜和真空系统中测量，因此即使是质谱技术也难以灵敏地测量样品中的氢。因此，我们对氢气如何导致不同材料系统开裂的理解比我们可能愿意承认的要有限得多。我们将开发原子尺度实验测量的新方法，以确定氢在材料中的位置。将制备小样品并在低温下处理以限制H的迁移率，并进行元素“原子对原子”的映射，以了解H的迁移率如何通过捕获不同的材料相，界面和晶体缺陷而变化。有些钛合金比其他合金更耐氢脆和腐蚀，但其背后的物理机制尚不清楚。例如，高纯度的钛几乎不受H的影响，但如果存在少量杂质，其腐蚀性能就会发生巨大变化；一些元素，如铁，已知会降低腐蚀性能，而其他元素，包括Mo和Pd，则会显著提高腐蚀性能。然后，我们将使用不同成分的合金在H暴露下的弯曲试验，仔细检查典型合金添加物对开裂倾向的影响。将在几个长度尺度上进行详细的显微检查，以了解h诱导破坏的机制。工程合金中H迁移率和H相关损伤的预测是复杂的，因为这些材料包含多种相、晶体缺陷和合金元素，这些都影响H的行为。有这么多相互作用的影响，使用物理上忠实的模型和模拟将是至关重要的，以充分解开它们彼此之间的纠缠。因此，我们将开发新的氢在材料内扩散的计算模型，以阐明不同的特征如何影响局部氢输运和捕获。此外，我们将采用并改进微观力学建模技术，通过纳入新揭示的Ti脆机制方程，并将含h合金与不含h合金的力学性能进行比较。基于这些结果，我们将为合金和工艺设计人员制定最佳材料指南，强调哪些相/合金组合更能抵抗h诱导的失效。此外，将设计、制造和测试最佳材料，以提供我们概念的最终验证。

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.