13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- However, inclusions with a size up to 20 m did not result in HCF failure. In contrast, artificial micro notches at the surface are the reason for a decline of the fatigue strength in the HCF region for higher ’ martensite volume fractions (figure 3b), which can be ascribed to the higher notch sensitivity of the martensite phase. According to Bowe et al. [18] the threshold value for fatigue crack growth for the fully austenitic condition of a FeNiAl-alloy is five times higher than that for the martensitic condition. It can therefore be assumed, that a higher amount of martensite phase enclosing a surface defect results in an earlier crack initiation and accelerated crack growth. a) b) c) Figure 2: Formation of slip bands (a), ’ martensite in the bulk at slip band intersections (b) and a martensite needle (c) during cyclic loading at = 240 MPa and N = 107. a) b) Figure 3: Crack initiation in the HCF region at a surface inclusion for a specimen with 27% ’ martensite volume fraction (a) and at an artificial micro notch for 30% ’ martensite volume fraction (b). A one-step predeformation leads to a pronounced increase of the cyclic strength for AIS304, but at the same time to failure even in the VHCF regime for an ’ martensite volume fraction of 54%. This change in damage mechanism can be explained by comparing the microstructure of the fully austenitic with the predeformation condition. The major difference lies in the amount and distribution of ’ martensite. For a volume fraction of 27% the ’ martensite is loosely allocated at slip band intersections (figure 4a) and the local notch stress around an inclusion leads to a concentration of the deformation in the softer austenite phase (represented for a fully austenitic condition in figure 4c) and a formation of deformation induced martensite. The localized work hardening and the compression stresses due to phase transformation impede crack initiation at the inclusion. Interconnected clusters of ’ martensite dominate the microstructure for a volume fraction of 56% (figure 4b). Due to the coherency of the martensite clusters, the deformation is no longer compensated by the softer austenite phase and the low ductility of the martensite phase results in crack initiation even at stress amplitudes in the VHCF range. Figure 5 depicts an example for crack initiation at an elongated inclusion for a sample with a volume fraction of martensite > 30%. The crack starts in the austenite phase leading to a brittle fracture of the inclusion before propagating into the notch-sensitive martensite phase. This example demonstrates that an inclusion does not necessarily have to be completely surrounded by the martensite phase. Elastic deformation in the martensitic phase might already be sufficient to have a crack initiating effect in the austenitic notch
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