The influence of stress levels on crack initiation and propagation behaviors of an Al-Li alloy under high cycle fatigue

This study investigates the crack initiation and propagation behavior in AA2099 Al-Li alloy under high cycle fatigue (HCF) conditions using SEM, TEM, and EDS. The research focuses on the formation mechanisms of slip bands (SBs) and fatigue striations of small cracks. Results show that crack initiation primarily originates from surface or near-surface regions, particularly near inclusions and Fe-rich particles. Crack initiation tends to be single-source at low stress levels and multi-source at higher stress levels. SBs formation is closely associated with stress levels; as stress increases, SBs formation becomes more pronounced, and band spacing decreases. At low stress levels, significant crack deflection occurs due to single slip mechanism activation, while at high stress levels, multiple slip system activation results in a flatter fatigue crack propagation (FCP) path. Early-stage FCP of Al-Li alloys under HCF exhibits small crack characteristics. The formation of fatigue striations in small cracks is influenced by the degree of work hardening at the crack tip, and the spacing of fatigue striations decreases with increasing stress levels. However, as the crack length increases, the spacing of fatigue striations gradually increases, which is attributed to the development of the crack closure effect during the later stages of FCP.