A Moving Cracking Story: Designing against Hydrogen Embrittlement in Titanium

据估计，全球每年与腐蚀相关的损害成本为1.9万亿GB(占GDP的3.4%)，腐蚀每年造成英国约800亿GB的损失。氢致应力腐蚀脆化是一类重要的环境退化现象。直到60年代末，钛合金还被认为通过与水蒸气反应而不会发生应力腐蚀脆化，但后来的经验证明了这一假设是错误的。因此，如果能够通过材料设计来了解和缓解钛合金中与氢相关的降解，则可以为英国带来实质性的工业和安全好处。由于氢在世界上的普遍存在，氢相关的破裂在材料科学中是一个巨大的挑战；从陶瓷到钙钛矿型太阳能电池，氢相关的降解机制对包括金属在内的许多材料的使用寿命至关重要。我们的策略将是通过限制脆化物种的进入或通过在材料内提供陷阱来为材料提供耐氢性，在材料中，这些物种可以被以某种方式失活。氢具有很高的流动性，因此可以浓缩材料中的关键微观和纳米特征并使其脆化，这可能在几分钟或几个小时的过程中发生。然而，一个主要的挑战是检测金属系统中的氢。由于缺乏激发的电子壳层，H不能在电子显微镜和真空系统中测量，因此即使是质谱学技术也很难灵敏地测量样品中的H。因此，我们对氢如何在不同材料体系中导致破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。将在低温下制备和处理小样品，以限制氢的迁移率，并将进行元素“原子-原子”映射，以了解氢的迁移率是如何通过捕获到不同的物相、界面和晶体缺陷而变化的。一些钛合金比其他钛合金更耐氢脆和腐蚀，但背后的物理机制尚不清楚。例如，高纯钛几乎不受氢的影响，但如果存在少量杂质，其腐蚀性能会发生巨大变化；一些元素，如Fe，已知会降低腐蚀性能，而另一些元素，包括Mo和Pd，则显著改善腐蚀。然后，我们将通过在不同成分的合金中进行H暴露下的弯曲试验，仔细研究典型合金添加对开裂倾向的影响。将在几个长度尺度上进行详细的微观检查，以了解H诱导失效的机理。工程合金中的氢迁移率和氢相关损伤的预测是复杂的，因为这些材料包含多个相、晶体缺陷和合金元素，这些都影响氢的行为。由于有如此多的交互作用，使用物理上可信的模型和模拟将是将它们完全从彼此中分离出来的关键。因此，我们将开发新的氢在材料内扩散的计算模型，以阐明不同的特性如何影响局部氢的传输和捕获。此外，我们将采用和改进细观力学模拟技术，通过加入新解开的钛的脆化机制的方程，并比较含H合金和无H合金的力学性能。基于这些结果，我们将为合金和工艺设计者制定最佳的材料指南，强调哪些相/合金组合更能抵抗氢诱导的失效。此外，我们还将设计、制造和测试最佳材料，以最终验证我们的概念。

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.