Long Term, In Situ Material Degradation Studies Utilizing High Resolution Laboratory X-ray Tomography

尽管近年来X射线断层扫描技术在非破坏性地观察和量化大量材料中的微观结构和缺陷方面的应用迅速增长，但长期4D（时间和空间分辨率）表征的可用性仍然有限，特别是在降解研究中需要特定环境条件的情况下。虽然同步加速器设施在高分辨率、现场观测和高速采集方面提供了最先进的能力，但分配给个人用户的射束时间（在某些情况下每年最多3-4天）限制了研究真实的系统中缺陷演变的机会。存在对用于长期实验的实验室X射线设施的普遍需求，其不需要同步加速器光源处可用的先进能力。拟议的研究努力建立在英国的研究能力，与亨利莫斯利X射线成像设施在曼彻斯特大学的合作，提供长期的静态和动态负载下的环境退化的原位表征。该项目建立在我们最近的工作，这已经证明了有效的能力，在同步加速器设施的原位测量，表征和量化局部腐蚀和应力腐蚀开裂事件的演变。将通过扩展正在进行的项目来验证环境应力阶段的能力，以解决静态和动态载荷下模拟冷凝蒸汽环境中涡轮机盘钢环境退化的当前关键问题。蒸汽涡轮机是一个特殊的工业实例，其中点蚀和应力腐蚀开裂直接影响结构完整性。为了满足对可靠能源供应日益增长的需求，保持运行设备的可用性已变得具有战略重要性。当务之急是获得相关数据，并制定一个预测框架，结合对损坏机制的理解，使明智的决定，现有工厂的寿命延长和下一代工厂的寿命评估。该小组先前通过非原位microCT实验进行的研究表明，裂纹在500至5000小时的时间尺度内从该系统中的凹坑演变而来。由X射线断层分析提供的证据需要重新评估的早期阶段的应力腐蚀裂纹生长从坑的定量建模，并提出了一个复杂的，可能是棘手的确定性的基础上。依赖经验评估的工程应用似乎是不可避免的，有必要进一步调查的具体性质的早期阶段的裂纹萌生，合并和扩展在整个生命周期的具体坑裂纹事件。通过非原位层析实验不能容易地获得这些信息，因此，原位能力对于提供更稳健的寿命预测策略是必要的。应该强调的是，建模标准的重点是疲劳裂纹和它的使用在这种情况下，并没有特别受到质疑的非原位X射线断层扫描分析应力腐蚀开裂在这个系统。对腐蚀坑产生的腐蚀疲劳裂纹演化的早期阶段进行更详细的探索，也将为该系统提供信息，并将在方案的后期阶段进行。

Although recent years have seen rapid growth in the use of X-ray tomography to non-destructively observe and quantify microstructure and defects within the bulk of materials, the availability of long term 4D (time and spatial resolution) characterisation remains limited especially where specific environmental conditions are needed in degradation studies. While synchrotron facilities provide state-of-the-art capability in terms of high resolution, in situ observations with high speed acquisition, allocation of beam time to individual users (in some cases a maximum of 3-4 days per annum) limits the opportunities to study the evolution of defects in real systems. There exists a general need for laboratory x-ray facilities for long term experiments that do not need the advanced capabilities available at a synchrotron light sources. The proposed research strives to establish research capability within the UK, in collaboration with the Henry Moseley X-ray Imaging Facility at the University of Manchester, to provide for long term in situ characterisation of environmental degradation under static and dynamic load. The project builds on our recent work which has demonstrated effective capability of in situ measurements at synchrotron facilities in characterising and quantifying the evolution of localised corrosion and stress corrosion cracking events. Validation of the capability of the environmental stressing stage will be conducted via the extension of an on-going programme to address current critical issue in environmental degradation of turbine disc steels in a simulated condensed steam environment under static and dynamic loading. Steam turbines are a particular industrial example where pitting and stress corrosion cracking impinge directly on structural integrity. Maintaining availability of operating plant has become strategically important in response to the increasing demand for a reliable energy supply. It is imperative to obtain relevant data and to develop a predictive framework combined with an understanding of damage mechanisms to enable informed decisions concerning life extension of existing plant and life assessment of next generation plant. Previous studies by this group via ex situ microCT experiments indicate cracks evolve from pits in this system over a timescale of 500 to 5000 hours. The evidence provided by the x-ray tomographic analysis necessitates a re-assessment of quantitative modelling of the early stages of stress corrosion crack growth from pits and suggests a complexity that may be intractable on a deterministic basis. A reliance on empirical assessment for engineering application would seem inevitable and there is need for further investigation into the specific character of early stages of crack initiation, coalescence and propagation during the entire life of specific pit-to-crack events. This information cannot be easily obtained via ex situ tomographic experiments and, therefore, in situ capability is necessary to provide more robust life prediction strategies. It should be emphasised that the modelling criteria were focused on fatigue cracks and its use in that context is not specifically questioned by the ex situ x-ray tomography analysis for stress corrosion cracking in this system. More detailed exploration of the early stages of corrosion fatigue crack evolution from corrosion pits would be informative for this system as well and will be performed during the advanced stages of the programme.