Cracking and fine grain area formation in a bimodal titanium alloy under very high cycle fatigue loading

• Systematic microstructural characterization works were conducted to bridge the multiscale microstructural evolution mechanisms involved in FGA formation down to the atomic scale. • Grain rotation principle of α structure during crack initiation and extension is elucidated. • Geometric necessary dislocation density threshold in generating the disorientated grain boundary to separate the coarser grain into finer α and β structures is determined. Identifying the multiscale microstructural evolution of materials subjected to very high cycle fatigue (VHCF) loading is essential for advancing the understanding of long-term durability in metallic materials, but has rarely been investigated on the evolution of functional structures reaching very high cycle integrity down to nanoscale and atomic levels. Focusing on the microstructural mechanisms governing the formation of fine grain area (FGA) in a titanium alloy, which has long been a critical concern in VHCF, systematic microstructural characterization work was conducted to bridge microstructural evolution mechanisms across multiple length scales by leveraging the transmission Kikuchi technique, and aberration-corrected scanning transmission electron microscopy. This study elucidates the crack nucleation mechanism, grain rotation and the associated geometrically necessary dislocation density threshold required to generate the disorientated grain boundary that separates the coarser grain into finer ones, and atom-scale strain relaxation during the grain refinement process.