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

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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.