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.