Investigations of fatigue damage in tempered martensitic steel in the HCF regime

With increasing requirements for high service life and mechanical strength of components undergoing cyclic deformation, identification of quantitative relationships between material properties and its microstructure is of substantial significance. To understand the interaction mechanisms between microstructure, the fatigue crack initiation and the early stage fatigue crack growth fundamental fatigue experiments at 95 Hz and a stress ratio of R = - 1, applied to a 0.5C-1.25Cr-Mo tempered martensitic steel were carried out. The present study is focused on both the observation of the crack initiation and, as main objective, the subsequent short crack growth by insitu observations of a well-selected surface area. With the help of the in-situ crack observations, the correlation with the local microstructure and FIB cross-sections, it was shown that early crack propagation is driven by different damage mechanisms, which take place within martensite blocks as well as on block boundaries. A transgranular crack growth mechanism on the hierarchical martensite block level was correlated to single slip system activities within small blocks or alternating slip system activities within coarse blocks. Interestingly, also intergranular crack propagation on martensite block boundaries was observed within single martensite packets with nearly perpendicular spatial orientation of the crack path to the loading direction. This indicates not only the crack propagation by shear mechanisms, but also brittle crack propagation below the stress intensity threshold value and a high interaction with the underlying microstructure.