Understanding stress-oxidation interaction on grain boundary failure: in situ microscale crack propagation experiments

工程材料的氧化失效和应力腐蚀开裂(SCC)是材料科学和工程应用中的一个长期问题，例如高温下的能量转换。本项目旨在了解热、力学和环境因素对NiFe二元合金(Ni-40at.%Fe合金，面心立方，伽马相)晶界的相互作用。为了实现这一目标，我计划通过微悬臂梁弯曲和断裂实验来变形双晶NiFe合金中具有代表性的晶界。不同的悬臂梁尺寸(尺寸效应)、加载速率(扩散时间尺度)、加载条件(应力效应)和环境(对比真空和氧化)将被用来研究应力-氧化相互作用和微观结构的演变：1)我通过在固定温度下不同环境下机械加载和不加载的情况下，通过制造微悬臂梁来研究应力对氧化物生长速率的影响。比较了微悬臂梁内部的拉伸和压缩对扩散和氧化物形成的影响，从而对微悬臂梁失效的影响。2)在恒温下的原位细观断裂实验中，采用不同的加载速率和不同的试件尺寸，研究了扩展裂纹尖端的扩散和氧化物的形成。新方法将进一步加深对氧化物形成过程中裂纹扩展的理解。3)通过微悬臂梁断裂试验对国标断裂韧度进行了量化。断裂韧性是氧化物侵入长度的函数，氧化物侵入长度与几何形状、加载速率、应力状态和GB类型有关。4)通过高分辨率扫描电子显微镜和透射电子显微镜观察断口，我可以评估氧化诱导断裂的机制：i)氧化诱导的动态脆化；ii)应力辅助晶界氧化引起的断裂。有限元模拟将用于计算氧化物的侵入长度、应力状态和几何效应。将断裂韧性的实验结果与模拟结果进行了对比，验证了模拟的正确性，并量化了氧化物的影响。所建立的模型可用于改进抗氧化金属的材料设计。

Oxidation-induced failure and stress corrosion cracking (SCC) of engineering materials has been a long-standing issue in material science and engineering application, for instance, energy conversion at elevated temperatures. This project aims to understand the interplay of thermal, mechanical, and environmental factors on the grain boundaries (GBs) in a NiFe binary alloy (Ni-40at.%Fe alloy, fcc, gamma-phase).To achieve this goal, I plan to deform representative GBs in bi-crystal NiFe alloy via micro-cantilever bending and fracture experiments. Various cantilever sizes (size effect), loading rates (diffusion time scale), loading conditions (stress effect), and environments (vacuum and oxidation for comparison) will be considered to investigate the stress-oxidation interaction and the microstructure evolution:1) I study the stress effect on the oxide growth rate by producing micro-cantilevers by quantifying the oxide intrusion length along the GBs with and without mechanical loading in different environments at fixed temperature. I compare the effects of tension and compression within the micro-cantilevers on diffusion and oxide formation, and thus on the micro-cantilever failure.2) I study the diffusion and oxide formation at the evolving crack tip along the GBs by using multiple loading rates and different sample sizes during in situ micromechanical fracture experiments at fixed temperature. New method will further the understanding of crack propagation with oxide formation. 3) I quantify the GB fracture toughness via fracture tests of micro-cantilevers. This fracture toughness is studied in function of the oxide intrusion length that relates to geometry, loading rates, stress states and GB type. Then I determine the most robust GB type with the highest resistance to oxidation and fracture.4) By examining the fractured sample using high resolution scanning electron microscopy and transmission electron microscopy, I can evaluate the mechanisms which govern the oxide-induced fracture: i) oxide induced dynamic embrittlement; ii) fracture induced by stress-aided grain boundary oxidation.Finite element modelling will be used to calculate the oxide intrusion length, stress states, and geometry effect. Experimental results of the fracture toughness are compared to the modeling to validate the simulations and quantify the influence of the oxide. The developed models can be used to improve the material design of oxidation resistant metals.