Influence of multilevel lamellar microstructure on notch high cycle fatigue properties and crack initiation behavior of Ti-55531 alloy

The mechanism of microcrack initiation for notch high cycle fatigue (NHCF) in Ti-55531 alloy with various multilevel lamellar microstructures (MLMs) under the certain notch root radius (R=0.34 mm) was thoroughly investigated. Results indicate that the primary microstructure unit controlling fatigue crack initiation is the secondary α (αs) lamellae. Majority of microvoids and microcracks initiate at the interfaces between αs and residual β matrix (βr) nearby the notch root, propagating towards the specimen core along αs/βr interfaces or passing through αs lamellae, forming longer microcracks. Moreover, as the width/length ratio of αs lamella and α colony (dα and dc) increases, the cyclic plastic deformation of αs lamella and α colony intensify significantly. Consequently, numerous fractures occurred in αs lamellae, greatly facilitating fatigue microcracks initiation and leading to a severe reduction in both fatigue life and strength of the Ti-55531 alloy. Besides slipping and twinning, a small number of stacking faults (SFs) were also detected in the αs lamellae at smaller microstrutural size (dα=0.049 and 0.053, dc=0.148 and 0.168). Interestingly, the interaction between twins, basal SFs, and dislocation slip could be another significant mechanism that promotes the cracking of αs/βr interfaces for NHCF microcrack initiation in this alloy. Furthermore, with an increasing of dα and dc, the occurrence of slipping increases, while the occurrences of twins and SFs decrease. •The primary microstructure unit controlling fatigue crack initiation is the αs lamellae.•Twins and SFs also play a crucial role in fatigue cracks initiation.•With an increase of dα and dc, dislocation slipping increases but twins and SFs decrease.