Microstructural insights into fatigue short crack propagation behaviour: Secondary crack and the elastic mismatch near twin boundaries

Microstructurally sensitive short crack propagation behaviours in a nickel-based superalloy at room temperature are investigated using in-situ fatigue tests, focusing on the roles of secondary cracks and twin boundaries (TBs). The short cracks predominantly propagate along {1 1 1} slip planes, exhibiting zig-zag paths associated with the activation of multiple slip systems. TBs are found to be preferential sites for the initiation of the TB-parallel secondary cracks, whose subsequent coalescence with the primary cracks leads to sharp increases in crack length. In addition, a comparative analysis of the two cracks penetrating through two twin lamellae with different elastic modulus (along the loading direction) mismatch is carried out within one sample. The results show that TB with high elastic modulus mismatch can lead to a clear retardation in crack propagation rate and eventually result in crack arrest, which is associated with the geometrically necessary dislocation accumulation and the nucleation of TB-parallel cracks.