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 (α_s) lamellae. Majority of microvoids and microcracks initiate at the interfaces between α_s and residual β matrix (β_r) nearby the notch root, propagating towards the specimen core along α_s/β_r interfaces or passing through α_s lamellae, forming longer microcracks. Moreover, as the width/length ratio of α_s lamella and α colony (d_α and d_c) increases, the cyclic plastic deformation of α_s lamella and α colony intensify significantly. Consequently, numerous fractures occurred in α_s 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 α_s lamellae at smaller microstrutural size (d_α=0.049 and 0.053, d_c=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 α_s/β_r interfaces for NHCF microcrack initiation in this alloy. Furthermore, with an increasing of d_α and d_c, the occurrence of slipping increases, while the occurrences of twins and SFs decrease.