Optical in-situ analysis of the cavitation damage on technical alloys under repeated single bubbles

随着时间的推移，空化现象——即液体中气泡的形成和破裂——甚至可以破坏高强度材料的表面。材料对空化的阻力通常是用超声声纳电极来研究的。由于声波产生的气泡数量多且具有随机性，因此通常不能将所造成的损伤归为单个气泡事件。然而，通过聚焦激光脉冲，可以产生在位置和时间上精确再现的单个气泡。除了许多关于气泡坍塌的流体动力学的研究外，一些作品还考察了这些单个气泡对可变形固体表面的影响。除了少数例外，样品来自软材料，因此单个气泡产生的变形可以与其动力学相关。然而，目前尚不清楚如何将这些结果转移到更高强度的材料上。拟议的项目旨在解决微观上不受控制的声空化对技术合金的损伤和单个气泡对软材料的精确可复制载荷之间的知识差距。一系列的单气泡将在技术合金表面上提供明确的、重复的应力。通过成像记录每个气泡的动力学，然后在每个气泡破裂后，通过光学显微镜原位记录损伤的增量增加，并辅以高分辨率的非原位技术，以获得损伤演变的详细图像。这使得能够以前所未有的时空分辨率检查孵化和侵蚀阶段的损害发展，以及将崩塌过程的细节与材料变化联系起来。与此同时，用声纳电极进行标准测试，将这种损伤与通常已知的材料抗空化性联系起来。首先，将显微镜光学集成到现有的单泡实验中，并与气泡的高速可视化同步。使用来自软材料的样品，这种原位成像通过非原位共聚焦显微镜进行验证。在第二步中，对NiAl青铜和316L钢进行了一系列测试，以开发半自动方法来有效地评估非常大的数据集，并将原位和非原位显微镜相关联。最后，将这些方法应用于两种材料的实验中，实验中气泡的数量、与样品表面的距离和气泡直径都发生了变化。采用高分辨率电子显微镜对技术合金的损伤机理进行了详细分析，并与ASTM G32超声空化实验样品进行了对比。在这个项目中生成的大量数据将被公开访问，特别是对于项目之外的建模工作。

With time, cavitation – that is, the formation and collapse of gas bubbles in liquids – can damage the surface even of high-strength materials. The resistance of materials to cavitation is typically investigated using ultrasound sonotrodes. Due to the large number and stochastic nature of the acoustically generated bubbles, in general the resulting damage cannot be assigned to individual bubble events. However, by means of focused laser pulses it is possible to generate individual bubbles that are precisely reproducible in location and time. Besides many studies on the fluid dynamics of the collapsing bubble, some works examine the effect of such individual bubbles on a deformable solid surface. With few exceptions, the sample is from a soft material, such that the deformation resulting from a single bubble can be correlated with its dynamics. However, it is unclear how these results can be transferred to higher-strength materials.The proposed project aims at this knowledge gap between microscopically uncontrolled damage to technical alloys by acoustic cavitation on one hand and precisely reproducible loading on soft materials by individual bubbles on the other. Series of single bubbles will provide defined, repeated stress on surfaces of technical alloys. The dynamics of each bubble are recorded by imaging, then after each bubble collapse the incremental increase in damage is recorded in situ by optical microscopy, and this is complemented by high-resolution ex-situ techniques to yield a detailed picture of the damage evolution. This enables examining the development of damage during the incubation and erosion phases in unprecedented spatio-temporal resolution, as well as correlating the details of the collapse processes with material changes. In parallel, standard tests with a sonotrode are performed to put this damage into context with what is generally known about the materials’ cavitation resistance.First, microscopy optics are integrated into an existing single-bubble experiment and synchronized with high-speed visualization of the bubbles. Using samples from a soft material, this in-situ imaging is validated by ex-situ confocal microscopy. In a second step, series of tests are carried out on NiAl bronze and 316L steel in order to develop semi-automated methods to efficiently evaluate the very large data sets and to correlate in-situ and ex-situ microscopy. Finally, these methods are used in experiments on both materials, in which the number of bubbles, their distance from the sample surface, and the bubble diameter are varied. Ex-situ, the damage mechanisms in the technical alloys are analyzed in detail by high-resolution electron microscopy and compared with samples from experiments with ultrasonic cavitation according to ASTM G32. The extensive data generated in this project will be made publicly accessible, in particular for modeling work outside of the project.