Microstructural evaluation of the defect tolerance of Cu alloyed steels under cyclic loading

在前期工作中，通过热处理诱导铜沉淀的形成，提高了2wt.%Cu合金钢X0.5CuNi2-2(0.005 wt.-%C-“X0.5”)和X21CuNi2-2(0.21 mA-%C-“X21”)的硬度、抗拉强度、疲劳强度和缺陷容限。所产生的力学性能取决于析出状态，包括析出物的大小、数量、晶格结构和化学成分。铁素体钢X0.5的缺陷容差与析出状态的关系比较明显，而铁素体-珠光体钢X21的缺陷容差与析出状态之间的关系较为复杂，采用了不同的评定方法(如√面积概念、北川-高桥图等)。该项目的总体研究目标是在实际知识状态之外，更深入地了解析出状态与由此产生的疲劳性能之间的关系。应首先对X0.5钢实现这一点。实现这一研究目标的一个基本前提是充分了解铜沉淀与母材之间的共格关系以及相界面的化学成分的影响。上一个项目表明，这一知识对于更好地理解铜的析出状态与力学性能之间的复杂关系，特别是疲劳行为，是必不可少的。目前的研究表明，铜沉淀的稳定性取决于塑性变形量的大小。因此，将通过低循环疲劳试验、六次疲劳试验和真空六次疲劳试验来分析缺陷公差对疲劳状态的依赖关系。在已有的高周疲劳试验结果的基础上，建议的工作将集中在高应变幅值的低周疲劳载荷和以局部循环微塑性为特征的变速循环疲劳状态。将特别注意缺陷容忍度和潜在的微观结构机制。这些研究将通过现场裂纹萌生和扩展实验来扩展，以分析损伤机理。为了彻底了解这些关系，疲劳实验将与高分辨率显微结构分析(透射电子显微镜和3D原子探针)相结合，从而能够研究沉淀和位错之间的相互作用。除对X0.5钢进行检验外，还应澄清先前工作中观察到的对铁素体-珠光体X21钢缺陷公差的局部影响。为此，将进行间断疲劳试验，从而能够定位裂纹萌生位置并定义损伤机制。此外，使用仪器循环纳米压痕测试，将确定铁素体和珠光体颗粒的硬化潜力。结合这些微结构成分中的原子探针断层扫描，可以一致地评估它们各自的沉淀状态。

In the previous project, hardness, tensile and fatigue strength as well as defect tolerance of the 2 wt.% Cu alloyed steels X0.5CuNi2-2 (0.005 wt.-% C – „X0.5“) und X21CuNi2-2 (0.21 Ma-% C – „X21“) was increased by heat-treatment-induced formation of Cu precipitates. The resulting mechanical properties depend on the precipitation state, which includes size, number, lattice structure and chemical composition of the Cu precipitates. While for the ferritic steel X0.5 the influence of the precipitation state on defect tolerance was clearly shown, the relationship between defect tolerance and precipitation state of the ferritic-pearlitic steel X21 is more complex, which was addressed by different evaluation methods (e.g. √area-concept, Kitagawa-Takahashi diagram). The overall research aim of the proposed project is a deeper understanding of the relationship between precipitation state and resulting fatigue properties beyond the actual state-of-knowledge. This shall first be achieved for the steel X0.5. An essential prerequisite to reach this research aim is a sound knowledge of the influence of coherency relationship as well as chemical composition of the phase boundary between Cu precipitate and base material. It was shown in the previous project that this knowledge is essential for a better understanding of the complex relationships between Cu precipitation state and mechanical properties, especially fatigue behavior. Current research shows that the stability of Cu precipitates depends on the applied amount of plastic deformation. Therefore, the dependency of defect tolerance on fatigue regime will be analyzed by LCF, HCF and VHCF fatigue tests. Based on the already existing HCF results, the focus of the proposed work will be on LCF loadings with high strain amplitudes and the VHCF regime characterized by local cyclic microplasticity. Particular attention will be paid to defect tolerance and the underlying microstructural mechanisms. These investigations will be extended by in-situ crack initiation and propagation experiments for analyzing the damage mechanisms. To develop a thorough understanding of these relationships, the fatigue experiments will be combined with high-resolution microstructural analysis (TEM and 3D atom probe), which enables, amongst others, investigating the interaction between precipitates and dislocations. In addition to the examination at the X0.5 steel, the local influences on the defect tolerance of the ferritic-pearlitic steel X21, observed in the previous work, shall be clarified. For this, interrupted fatigue experiments will be conducted, enabling the localization of the crack initiation site and the definition of damage mechanisms. Moreover, using instrumented cyclic nanoindentation tests, the hardening potential of ferrite and pearlite grains will be determined. Combined with atom probe tomography within these microstructural constituents, their individual precipitation states can be consistently evaluated.