Synergistic influence mechanism of microstructure type and loading mode on the long crack propagation in Ti-55531 alloy

This work aims to reveal the synergistic effect mechanism of microstructure type and loading mode on the long crack propagation in the Ti-55531 alloy. The fracture toughness (K_IC) and fatigue crack propagation rate (da/dN) of the Ti-55531 alloy with lamellar microstructure (LM) and bimodal microstructure (BM) were investigated and compared at room temperature. The results show that both microstructural factors and the loading mode significantly affect the long crack propagation behavior of the alloy. Under static loading, the K_IC of the LM is higher than that of the BM. Under cyclic loading, the da/dN of the BM is lower than that of the LM in the lower stress field intensity factor range (ΔK < 26.47 MPa·m^1/2). However, when ΔK > 26.47 MPa·m^1/2, the da/dN of the BM is higher than that of the LM. The results of the fracture surface morphology and the crack path analysis show that the difference in microstructure characteristics leads to various behaviors of the long crack propagation under both static and cyclic loadings. The key microstructural factors controlling the long crack propagation in the two microstructures under static and cyclic loadings are discussed. The influence of microstructure characteristics on the long crack propagation in different stages of crack growth is also explained. Moreover, a new calculation model of K_IC is proposed. These results could provide support for the reliability design of engineering structures and promote the wider application of the alloy in engineering.