In Situ Examination of Cyclic Deformation‐Induced Damage Mechanisms in Ti–6Al–4V Alloy under Stress‐Controlled Loading

Because the pressure hull of deep‐diving submersible made of Ti–6Al–4V ELI alloy is often subjected to cyclic high‐pressure seawater during the process of floating and diving, the stress‐controlled low‐cycle fatigue behaviors have garnered significant attention. In order to thoroughly analyze the cyclic deformation damage and microcrack initiation mechanism of Ti–6Al–4V ELI alloy with bimodal structure, an in situ scanning electron microscopy fatigue test coupled with electron backscattered diffraction characterization is employed in this work. It is found that, under a low cyclic stress amplitude, the cyclic deformation is mainly accommodated by basal slips in the α _p grains. Under a high cyclic stress amplitude, the basal slips are activated not only in the α _p grains but also in the β _trans matrix. Correspondingly, the microcrack initiation mechanism changes from the α _p /β _trans boundary cracking under low cyclic stress amplitude to the basal slip bands cracking under high cyclic stress amplitude due to the slip transfer phenomenon. The elucidated transition mechanism of microcrack initiation, contingent upon stress amplitude and mediated through slip transfer behavior, offers theoretical foundations for low‐cycle fatigue performance assessment of Ti–6Al–4V ELI under stress‐controlled conditions.